詳細

佐賀　佳央 （サガ　ヨシタカ）

  • 理工学部 理学科　教授
Last Updated :2024/04/25

コミュニケーション情報 byコメンテータガイド

  • コメント

    光生体超分子の超分子構造と機能について研究しています。また、光生体超分子の構成成分についても研究を行っております。

研究者情報

学位

  • 博士（工学）（東京大学）

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J-Global ID

研究キーワード

  • 生物有機化学   人工光合成   カロテノイド   光合成   クロロフィル   

現在の研究分野（キーワード）

    光生体超分子の超分子構造と機能について研究しています。また、光生体超分子の構成成分についても研究を行っております。

研究分野

  • ナノテク・材料 / 機能物性化学
  • ライフサイエンス / 機能生物化学
  • ナノテク・材料 / 生物分子化学
  • ライフサイエンス / 生物物理学

経歴

  • 2017年04月 - 現在  近畿大学理工学部理学科化学コース教授
  • 2018年06月 - 2018年12月  アムステルダム自由大学客員研究員（兼任）
  • 2014年10月 - 2018年03月  科学技術振興機構さきがけ研究者（兼任）
  • 2010年04月 - 2017年03月  近畿大学理工学部理学科化学コース准教授
  • 2005年04月 - 2010年03月  近畿大学理工学部理学科化学コース講師
  • 2003年01月 - 2005年03月  日本学術振興会特別研究員（SPD）
  • 2002年04月 - 2002年12月  日本学術振興会特別研究員（PD）
  • 1999年04月 - 2002年03月  立命館大学総合理工学研究機構ポストドクトラルフェロー

学歴

  • 1996年04月 - 1999年03月   東京大学   大学院工学系研究科   化学生命工学専攻 博士課程
  • 1994年04月 - 1996年03月   東京大学   大学院工学系研究科   化学生命工学専攻 修士課程
  • 1990年04月 - 1994年03月   東京大学   工学部

所属学協会

  • 日本ペプチド学会   国際ポルフィリン・フタロシアニン学会   日本光合成学会   日本生物物理学会   光化学協会   日本化学会   アメリカ生物物理学会   

研究活動情報

論文

  • Shota Kawato; Shinichi Sato; Hirotaka Kitoh-Nishioka; Yoshitaka Saga
    Photochemical & Photobiological Sciences 2024年04月
  • Masayuki Morimoto; Haruna Hirao; Masaharu Kondo; Takehisa Dewa; Yukihiro Kimura; Zheng-Yu Wang-Otomo; Hitoshi Asakawa; Yoshitaka Saga
    Photosynthesis Research 157 13 - 20 2023年03月 [査読有り]
     
    Structural information on the circular arrangements of repeating pigment-polypeptide subunits in antenna proteins of purple photosynthetic bacteria is a clue to a better understanding of molecular mechanisms for the ring-structure formation and efficient light harvesting of such antennas. Here, we have analyzed the ring structure of light-harvesting complex 2 (LH2) from the thermophilic purple bacterium Thermochromatium tepidum (tepidum-LH2) by atomic force microscopy. The circular arrangement of the tepidum-LH2 subunits was successfully visualized in a lipid bilayer. The average top-to-top distance of the ring structure, which is correlated with the ring size, was 4.8 ± 0.3 nm. This value was close to the top-to-top distance of the octameric LH2 from Phaeospirillum molischianum (molischianum-LH2) by the previous analysis. Gaussian distribution of the angles of the segments consisting of neighboring subunits in the ring structures of tepidum-LH2 yielded a median of 44°, which corresponds to the angle for the octameric circular arrangement (45°). These results indicate that tepidum-LH2 has a ring structure consisting of eight repeating subunits. The coincidence of an octameric ring structure of tepidum-LH2 with that of molischianum-LH2 is consistent with the homology of amino acid sequences of the polypeptides between tepidum-LH2 and molischianum-LH2.
  • Yoshitaka Saga; Kohei Hamanishi; Tetsuya Yamamoto; Kazuki Hinago; Yutaka Nagasawa
    The Journal of Physical Chemistry B 127 12 2683 - 2689 2023年03月 [査読有り]
  • Yoshitaka Saga; Kohei Hamanishi
    ACS Omega 7 33 29231 - 29235 2022年08月 [査読有り]
  • Yusuke Takashima; Yoshitaka Saga
    Photochemical & Photobiological Sciences 21 7 1193 - 1199 2022年03月 [査読有り]
  • Yukihiro Kimura; Michie Imanishi; Li Yong; Yuki Yura; Takashi Ohno; Yoshitaka Saga; Michael T Madigan; Zheng-Yu Wang-Otomo
    The Journal of Chemical Physics 2022年02月 [査読有り]
  • Masayuki Tanaka; Aiko Tanaka; Yoshitaka Saga
    Journal of Porphyrins and Phthalocyanines 26 273 - 278 2022年01月 [査読有り]
     
    C132-[Formula: see text]-epimers of chlorophyll (Chl) molecules are important cofactors in the photosystem I reaction centers in oxygenic photosynthetic organisms; however, their production mechanism is still unclear. The reaction properties of Chl epimerization are helpful for a better understanding of the molecular mechanism of the in vivo formation of Chl C132-[Formula: see text]-epimers. We report herein the kinetic properties of the epimerization of formylated Chl molecules, Chl [Formula: see text] and Chl [Formula: see text], by use of triethylamine. Both Chl [Formula: see text] and Chl [Formula: see text] performed faster epimerization kinetics than Chl [Formula: see text], indicating that the electron-withdrawing ability of the formyl groups directly linked to the chlorin macrocycle is responsible for acceleration of the epimerization. Comparing the rate constants of the two mono-formylated Chl molecules indicated that the epimerization of Chl [Formula: see text] was faster than that of Chl [Formula: see text]. This difference is rationalized by invoking a combination of the inductive effects of the C3- and C7-substituents in Chls; the sums of Hammett [Formula: see text] parameters of the C3- and C7-substituents exhibited high correlations with the epimerization rate constants of Chls [Formula: see text], [Formula: see text], and [Formula: see text].
  • Yoshitaka Saga; Aiko Tanaka; Madoka Yamashita; Toshiyuki Shinoda; Tatsuya Tomo; Yukihiro Kimura
    Photochemistry and Photobiology 2021年09月 [査読有り]
     
    The interactions of chlorophyll (Chl) and bacteriochlorophyll (BChl) pigments with the polypeptides in photosynthetic light-harvesting proteins are responsible for controlling the absorption energy of (B)Chls in protein matrixes. The binding pocket of B800 BChl a in LH2 proteins, which are peripheral light-harvesting proteins in purple photosynthetic bacteria, is useful for studying such structure-property relationships. We report the reconstitution of Chl f, which has the formyl group at the 2-position, in the B800 cavity of LH2 from the purple bacterium Rhodoblastus acidophilus. The Q(y) absorption band of Chl f in the B800 cavity was shifted by 14 nm to longer wavelength compared to that of the corresponding five-coordinated monomer in acetone. This redshift was larger than that of Chl a and Chl b. Resonance Raman spectroscopy indicated hydrogen bonding between the 2-formyl group of Chl f and the LH2 polypeptide. These results suggest that this hydrogen bonding contributes to the Q(y) redshift of Chl f. Furthermore, the Q(y) redshift of Chl f in the B800 cavity was smaller than that of Chl d. This may have arisen from the different patterns of hydrogen bonding between Chl f and Chl d and/or from the steric hindrance of the 3-vinyl group in Chl f.
  • Eduard Elias; Nicoletta Liguori; Yoshitaka Saga; Judith Schäfers; Roberta Croce
    Biomacromolecules 22 8 3313 - 3322 2021年08月 [査読有り]
  • Yoshitaka Saga; Yuji Otsuka; Aiko Tanaka; Yuto Masaoka; Tsubasa Hidaka; Yutaka Nagasawa
    The Journal of Physical Chemistry B 125 6830 - 6836 2021年06月 [査読有り]
  • Y. Kimura; T. Yamashita; R. Seto; M. Imanishi; M. Honda; S. Nakagawa; Y. Saga; S. Takenaka; L.-J. Yu; M. T. Madigan; Z.-Y. Wang-Otomo
    Photosynthesis Research 148 1-2 77 - 86 2021年05月 [査読有り]
  • Yoshitaka Saga; Madoka Yamashita; Yuto Masaoka; Tsubasa Hidaka; Michie Imanishi; Yukihiro Kimura; Yutaka Nagasawa
    The Journal of Physical Chemistry B 125 8 2009 - 2017 2021年03月 [査読有り]
  • Yoshitaka Saga; Yuji Otsuka; Daichi Funakoshi; Yuto Masaoka; Yu Kihara; Tsubasa Hidaka; Hiroka Hatano; Hitoshi Asakawa; Yutaka Nagasawa; Hitoshi Tamiaki
    Scientific Reports 10 1 19383  2020年11月 [査読有り]
     
    Abstract Natural chlorophylls have a D-ring reduced chlorin π-system; however, no naturally occurring photosynthetically active B-ring reduced chlorins have been reported. Here we report a B-ring reduced chlorin, 17,18-didehydro-bacteriochlorophyll (BChl) a, produced by in situ oxidation of B800 bacteriochlorophyll (BChl) a in a light-harvesting protein LH2 from a purple photosynthetic bacterium Phaeospirillum molischianum. The regioselective oxidation of the B-ring of B800 BChl a is rationalized by its molecular orientation in the protein matrix. The formation of 17,18-didehydro-BChl a produced no change in the local structures and circular arrangement of the LH2 protein. The B-ring reduced 17,18-didehydro-BChl a functions as an energy donor in the LH2 protein. The photoactive B-ring reduced Chl isomer in LH2 will be helpful for understanding the photofunction and evolution of photosynthetic cyclic tetrapyrrole pigments.
  • Shin‐ichi Sasaki; Yuki Hashimoto; Yusuke Kinoshita; Hitoshi Tamiaki; Shengnan Duan; Xiao‐Feng Wang; Yoshitaka Saga; Hiroaki Yamamoto; Toshitaka Ikeuchi; Nobue Shishioh
    ChemPhotoChem 4 12 5399 - 5407 2020年07月 [査読有り]
  • Yoshitaka Saga; Kanji Miyagi; Hiroki Sato; Chiasa Uragami; Hideki Hashimoto
    Journal of Photochemistry and Photobiology A: Chemistry 398 112593 - 112593 2020年07月 [査読有り]
  • Yoshitaka Saga; Shiori Nakagawa
    Journal of Porphyrins and Phthalocyanines 24 04 499 - 504 2020年04月 [査読有り]
     
    Chlorophyll (Chl) and bacteriochlorophyll (BChl) pigments, which are crucial cyclic tetrapyrroles in photosynthesis, generally have a chiral center in their exo-cyclic five-membered E-ring. Although [Formula: see text]-epimers (primed-type) of (B)Chl pigments are rarely present in photosynthetic organisms, they play key roles in photosynthetic reaction center complexes. The epimerization mechanism of (B)Chl pigments in vivo has not been unraveled. The structural effects on the physicochemical properties of (B)Chl epimerization reactions provide useful information to tackle this question. We analyzed epimerization of three pigments, BChl [Formula: see text], Chl [Formula: see text], and 3-acetyl Chl [Formula: see text], to elucidate the structural factors that are responsible for epimerization reactions. We compared the epimerization kinetics of the three pigments and concluded that the bacteriochlorin skeleton (7,8,17,18-tetrahydroporphyrin) significantly retarded the epimerization kinetics. Thus, BChl [Formula: see text] exhibited slower epimerization kinetics than Chl [Formula: see text] in spite of the presence of the electron-withdrawing 3-acetyl group that accelerates epimerization. In contrast to the large structural effects of (B)Chl molecules on epimerization kinetics, the thermodynamic properties at equilibrium in the epimerization of the three pigments were barely influenced by their molecular structures. This study also demonstrates that a semi-synthetic pigment, 3-acetyl Chl [Formula: see text], is appropriate for comparative analyses of the structural effects of BChl [Formula: see text] and Chl [Formula: see text] on their physicochemical properties.
  • Yoshitaka Saga; Madoka Yamashita; Michie Imanishi; Yukihiro Kimura; Yuto Masaoka; Tsubasa Hidaka; Yutaka Nagasawa
    ACS Omega 5 12 6817 - 6825 2020年 [査読有り]
     
    The manipulation of B800 bacteriochlorophyll (BChl) a in light-harvesting complex 2 (LH2) from the purple photosynthetic bacterium Phaeospirillum molischianum (molischianum-LH2) provides insight for understanding the energy transfer mechanism and the binding of cyclic tetrapyrroles in LH2 proteins since molischianum-LH2 is one of the two LH2 proteins whose atomic-resolution structures have been determined and is a representative of type-2 LH2 proteins. However, there is no report on the substitution of B800 BChl a in molischianum-LH2. We report the reconstitution of 3-acetyl chlorophyll (AcChl) a, which has a 17,18-dihydroporphyrin skeleton, to the B800 site in molischianum-LH2. The 3-acetyl group in AcChl a formed a hydrogen bond with beta'-Thr23 in essentially the same manner as native B800 BChl a, but this hydrogen bond was weaker than that of B800 BChl a. This change can be rationalized by invoking a small distortion in the orientation of the 3-acetyl group in the B800 cavity by dehydrogenation in the B-ring from BChl a. The energy transfer from AcChl a in the B800 site to B850 BChl a was about 5-fold slower than that from native B800 BChl a by a decrease of the spectral overlap between energy-donating AcChl a and energy-accepting B850 BChl a.
  • In situ Conversion of Chlorophyll b Reconstituted into Photosynthetic Protein LH2
    Yoshitaka Saga; Madoka Yamashita; Shiori Nakagawa
    Chemistry Letters 48 1270 - 1273 2019年10月 [査読有り]
  • Michie Imanishi; Mizuki Takenouchi; Shinichi Takaichi; Shiori Nakagawa; Yoshitaka Saga; Shinji Takenaka; Michael T. Madigan; Jörg Overmann; Zheng-Yu Wang-Otomo; Yukihiro Kimura
    Biochemistry 58 25 2844 - 2852 American Chemical Society ({ACS}) 2019年06月 [査読有り]
     
    The light-harvesting 1 reaction center (LH1-RC) complex in the purple sulfur bacterium Thiorhodovibrio (Trv.) strain 970 cells exhibits its LH1 Q(y) transition at 973 nm, the lowest-energy Q(y) absorption among purple bacteria containing bacteriochlorophyll a (BChl a). Here we characterize the origin of this extremely red-shifted Qy transition. Growth of Try. strain 970 did not occur in cultures free of Ca2+, and elemental analysis of Ca2+-grown cells confirmed that purified Try. strain 970 LH1-RC complexes contained Ca2+. The LH1 Qy band of Try. strain 970 was blue-shifted from 959 to 875 nm upon Ca2+ depletion, but the original spectral properties were restored upon Ca2+ reconstitution, which also occurs with the thermophilic purple bacterium Thermochromatium (Tch.) tepidum. The amino acid sequences of the LH1 alpha- and beta-polypeptides from Try. strain 970 closely resemble those of Tch. tepidum; however, Ca2+ binding in the Try. strain 970 LH1-RC occurred more selectively than in Tch. tepidum LH1-RC and with a reduced affinity. Ultraviolet resonance Raman analysis indicated that the number of hydrogen-bonding interactions between BChl a and LH1 proteins of Try. strain 970 was significantly greater than for Tch. tepidum and that Ca2+ was indispensable for maintaining these bonds. Furthermore, perfusion-induced Fourier transform infrared analyses detected Ca2+-induced conformational changes in the binding site closely related to the unique spectral properties of Try. strain 970. Collectively, our results reveal an ecological strategy employed by Try. strain 970 of integrating Ca2+ into its LH1-RC complex to extend its light-harvesting capacity to regions of the near-infrared spectrum unused by other purple bacteria.
  • Yoshitaka Saga; Kiyoshiro Kawano; Yuji Otsuka; Michie Imanishi; Yukihiro Kimura; Sayaka Matsui; Hitoshi Asakawa
    Scientific Reports 9 3636  2019年03月 [査読有り]
     
    Engineering chlorophyll (Chl) pigments that are bound to photosynthetic light-harvesting proteins is one promising strategy to regulate spectral coverage for photon capture and to improve the photosynthetic efficiency of these proteins. Conversion from the bacteriochlorophyll (BChl) skeleton (7,8,17,18-tetrahydroporphyrin) to the Chl skeleton (17,18-dihydroporphyrin) produces the most drastic change of the spectral range of absorption by light-harvesting proteins. We demonstrated in situ selective oxidation of B800 BChl a in light-harvesting protein LH2 from a purple bacterium Rhodoblastus acidophilus by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The newly formed pigment, 3-acetyl Chl a, interacted with the LH2 polypeptides in the same manner as native B800. B850 BChl a was not oxidized in this reaction. CD spectroscopy indicated that the B850 orientation and the content of the α-helices were unchanged by the B800 oxidation. The nonameric circular arrangement of the oxidized LH2 protein was visualized by AFM; its diameter was almost the same as that of native LH2. The in situ oxidation of B800 BChl a in LH2 protein with no structural change will be useful not only for manipulation of the photofunctional properties of photosynthetic pigment-protein complexes but also for understanding the substitution of BChl to Chl pigments in the evolution from bacterial to oxygenic photosynthesis.
  • Yoshitaka Saga; Madoka Yamashita; Michie Imanishi; Yukihiro Kimura
    Chemistry Letters 47 8 1071 - 1074 2018年08月 [査読有り]
     
    Chlorophyll (Chl) d, which is a major pigment in an oxygenic photosynthetic organism Acaryochloris marina, was almost fully inserted into the circularly arranged binding sites of B800 bacteriochlorophyll (BChl) a in a bacterial light-harvesting protein, LH2. The 3-formyl group in Chl d was hydrogen-bonded with the polypeptides in essentially the same manner as the 3-acetyl group in BChl a in native LH2.
  • Yoshitaka Saga; Keiya Hirota; Sayaka Matsui; Hitoshi Asakawa; Hiroshi Ishikita; Keisuke Saito
    Biochemistry 57 21 3075 - 3083 2018年05月 [査読有り]
     
    The selective removal of B800 bacteriochlorophyll (BChl) a from light-harvesting complex 2 (LH2) in purple photosynthetic bacteria is a clue about elucidation of the mechanism for the transfer of energy from these pigments to B850 BChl a and their roles in the LH2 protein structure. We demonstrated that the kinetics of the removal of B800 BChl a from two representative LH2 proteins derived from Phaeospirillum molischianum and Rhodoblastus acidophilus differed significantly, in contrast to the calculated binding enthalpy. These results may be interpreted as changes in the local structure near B800 BChl a with respect to the geometries of the original crystal structures upon removal of B800 BChl a. Despite the difficulty of removing B800 BChl a from molischianum-LH2, we prepared the molischianum-LH2 protein lacking B800 BChl a by combination of two detergents, n-dodecyl β-d-maltoside and n-octyl β-d-glucoside, under acidic conditions. Spectral and atomic force microscopy analyses indicated that the absence of B800 BChl a had little effect on the local structure in the vicinity of B850 BChl a and the circular arrangement in this protein. These results suggest that the hydrophobic domain near B850 BChl a is rigid and plays a major role in the structural formation of molischianum-LH2.
  • Yoshitaka Saga; Kanji Miyagi
    Photochemistry and Photobiology 94 698 - 704 Wiley 2018年04月 [査読有り]
  • Yoshitaka Saga; Kenta Amari; Kanji Miyagi
    Journal of Photochemistry and Photobiology A: Chemistry 353 591 - 596 2018年02月 [査読有り]
     
    The LH2 protein has two types of bacteriochlorophyll (BChl) a termed B800 and B850 BChl a. Substitution of B800 BChl a with chlorophyll (Chl) a derivatives in this protein has considerable attention from the viewpoints of elucidation of the energy transfer mechanism and developments of artificial photosynthetic devices based on the LH2 structure. Herein we examine reconstitution properties of Chl a and 132-demethoxycarbonyl Chl a (pyroChl a) into the B800 binding sites in LH2 from the purple photosynthetic bacterium Rhodoblastus (Rbl.) acidophilus. Both the Chl a derivatives were successfully inserted into the B800 binding sites in the LH2 protein whose B800 BChl a was removed (B800-free LH2), and the energy transfer from these derivatives to B850 BChl a was observed in the reconstituted LH2 proteins. Extraction of chlorophyllous pigments from the reconstituted LH2 proteins allows us to estimate the occupancy of Chl a and pyroChl a in the nine B800 binding sites from the molar ratio of (pyro)Chl a to residual B850 BChl a: the occupancy of Chl a was 72% and was 1.2-times larger than that of pyroChl a. These results indicate that Chl a and pyroChl a can be accommodated in the B800 binding sites in the LH2 protein derived from this bacterium, but the affinity of pyroChl a would be slightly smaller than that of Chl a. The binding properties of Chl a derivatives into the B800 binding sites in LH2 will be useful for manipulation of light-harvesting abilities in photosynthetic antenna proteins.
  • Yoshitaka Saga; Naoya Takahashi; Tomohiro Miyatake; Hitoshi Tamiaki
    Journal of Photochemistry and Photobiology A: Chemistry 353 612 - 617 2018年02月 [査読有り]
     
    Green photosynthetic bacteria have extramembranous light-harvesting complexes called chlorosomes. Light-energy captured by bacteriochlorophyll (BChl) self-aggregates inside chlorosomes is transferred to BChl a-peptide complexes called baseplates on the chlorosome surface. We substitute the baseplate complex in chlorosomes from a green sulfur photosynthetic bacterium Chlorobaculum limnaeum with amphiphilic Zn BChl a derivatives. Zn BChl a derivatives 1–3, which were esterified with polyethylene glycol (PEG) 600, PEG400, and diethylene glycol, respectively, were successfully hybridized with chlorosomes without baseplates. These derivatives in the hybridized chlorosomes emitted by predominant excitation of BChl e self-aggregates at 460 nm, indicating that they were able to accept the excitation energy from BChl e aggregates inside chlorosomes. Zn BChl a derivatives 1 and 2 emitted at 787 nm by the excitation energy transfer. In contrast, fluorescence from 3 esterified with a short hydrophilic chain, diethylene glycol, was broadened around 785 and 820 nm. The broad emission bands would consist of at least two bands, which were ascribable to the monomeric and aggregated forms. These results indicate that PEG600 and PEG400 provide appropriate amphiphilicity to Zn BChl a derivatives for dispersion in the membranous region of chlorosomes. The present study will be helpful for not only elucidation of the energy transfer mechanism in chlorosomes but also development of artificial photofunctional supramolecular complexes by hybridization of natural photosynthetic systems with synthetic molecules.
  • Yoshitaka Saga; Hayato Yamashita
    JOURNAL OF BIOSCIENCE AND BIOENGINEERING 124 4 408 - 413 2017年10月 [査読有り]
     
    Metabolic substitution of the esterifying chain in bacteriochlorophyll (BChl) c in green photosynthetic bacteria grown by supplementation of exogenous alcohols has attracted attentions to study supramolecular structures and biogenesis of major antenna complexes chlorosomes in these bacteria as well as BChl pigment biosynthesis. Actual substrates in the enzymatic attachment of the esterifying moieties to the precursor of BChl c, namely bacteriochlorophyllide (BChlide) c, in these bacteria are believed to be diphosphate esters of alcoholic substrates, although only intact alcohols have so far been supplemented in the bacterial cultures. We report herein BChl c compositions in the green sulfur photosynthetic bacterium Chlorobaculum tepidum by supplementation with geranyl and geranylgeranyl diphosphates. The supplementation of these diphosphates hardly produced BChl c derivatives esterified with geraniol and geranylgeraniol in Cba. tepidum, whereas these BChl c derivatives were accumulated by supplementation of intact geraniol and geranylgeraniol. The sharp contrast of the incorporation efficiency of the supplemental isoprenoid moieties in BChl c using the isoprenoid diphosphates to that by the isoprenoid alcohols was mainly ascribable to less penetration abilities of the diphosphate substrates into Cba. tepidum cells because of their anionic and polar diphosphate moiety. (C) 2017, The Society for Biotechnology, Japan. All rights reserved.
  • Yoshitaka Saga; Keiya Hirota; Hitoshi Asakawa; Kazufumi Takao; Takeshi Fukuma
    BIOCHEMISTRY 56 27 3484 - 3491 2017年07月 [査読有り]
     
    Light-harvesting complex 2 (LH2) is an integral membrane protein in purple photosynthetic bacteria. This protein possesses two types of bacteriochlorophyll (BChl) a, termed B800 and B850, which exhibit lowest-energy absorption bands (Q(y), bands) around 800 and 850 nm. These BChl a pigments in the LH2 protein play crucial roles not only in photosynthetic functions but also in folding and maintaining its protein structure. We report herein the reversible structural changes in the LH2 protein derived from a purple photo synthetic bacterium, Rhodoblastus acidophilus, induced by the removal of B800 BChl a (denoted as B800-free LH2) and the reconstitution of exogenous BChl a. Atomic force microscopy observation clearly visualized the nonameric ring structure of the B800-free LH2 with almost the same diameter as the native LH2. Size exclusion chromatography measurements indicated a considerable decrease in the size of the protein induced by the removal of B800 BChI a. The protein size was almost recovered by the insertion of BChl a pigments into the B800 binding sites. The decrease in the LH2 size would mainly originate from the shrinkage of the B800 binding sites perpendicular to the macrocycle of B800 BChI a without deformation of the circular arrangement. The reversible changes in the LH2 structure induced by the removal and reconstitution of B800 BChI a will be helpful for understanding the structural principle and the folding mechanism of photosynthetic pigment protein complexes.
  • Yoshitaka Saga; Nozomi Yoshida; Shota Yamada; Tadashi Mizoguchi; Hitoshi Tamiaki
    Biochemistry and Biophysics Reports 9 42 - 46 2017年03月 [査読有り]
     
    Unnatural glycolipids possessing the diyne moiety in their acyl groups were successfully biosynthesized in the green sulfur photosynthetic bacterium Chlorobaculum (Cba.) tepidum by cultivation with supplementation of 10,12-heptadecadiynic acid. Monogalactosyldiacylglycerol (MGDG) and rhamnosylgalactosyldiacylglycerol (RGDG) esterified with one 10,12-heptadecadiynic acid were primarily formed in the cells, and small amounts of glycolipids esterified with the two unnatural fatty acids can also be detected. The relative ratio of these unnatural glycolipids occupied in the total glycolipids was estimated to be 49% based on HPLC analysis using a evaporative light scattering detector. These results indicate that the acyl groups in glycolipids, which play important roles in the formation of extramembranous antenna complexes called chlorosomes, can be modified in vivo by cultivation of green sulfur photosynthetic bacteria with exogenous synthetic fatty acids. Visible absorption and circular dichroism spectra of Cba. tepidum containing the unnatural glycolipids demonstrated the formation of chlorosomes, indicating that the unnatural glycolipids in this study did not interfere with the biogenesis of chlorosomes.
  • Yoshitaka Saga; Hayato Yamashita; Keiya Hirota
    BIOORGANIC & MEDICINAL CHEMISTRY 24 18 4165 - 4170 2016年09月 [査読有り]
     
    The green sulfur photosynthetic bacterium Chlorobaculum (Cba.) tepidum was grown in liquid cultures containing perfluoro-1-decanol, 1H, 1H, 2H, 2H-heptadecafluoro-1-decanol [CF3(CF2)(7)(CH2)(2)OH] or 1H, 1H-nonadecafluoro-1-decanol [CF3(CF2)(8)CH2OH], to introduce rigid and fluorophilic chains into the esterifying moiety of light-harvesting bacteriochlorophyll (BChl) c. Exogenous 1H, 1H, 2H, 2H-heptadecafluoro-1-decanol was successfully attached to the 17(2)-carboxy group of bacteriochlorophyllide (BChlide) c in vivo: the relative ratio of the unnatural BChl c esterified with this perfluoroalcohol over the total BChl c was 10.3%. Heat treatment of the liquid medium containing 1H, 1H, 2H, 2H-heptadecafluoro- 1-decanol with beta-cyclodextrin before inoculation increased the relative ratio of the BChl c derivative esterified with this alcohol in the total BChl c in Cba. tepidum. In a while, 1H, 1H-nonadecafluoro-1-decanol was not attached to BChlide c in Cba. tepidum, which was grown by its supplementation. These results suggest that the rigidity close to the hydroxy group of the esterifying alcohol is not suitable for the recognition by the BChl c synthase called BchK in Cba. tepidum. The unnatural BChl c esterified with 1H, 1H, 2H, 2H-heptadecafluoro-1-decanol participated in BChl c self-aggregates in chlorosomes. (C) 2016 Elsevier Ltd. All rights reserved.
  • Yoshitaka Saga; Keiya Hirota
    ANALYTICAL SCIENCES 32 7 801 - 804 2016年07月 [査読有り]
     
    The molar extinction coefficients of light-harvesting complex 2 (LH2) have been ambiguous in spite of its fame and wide utilization. Herein we determine the molar extinction coefficients of the LH2 proteins derived from the three purple photosynthetic bacteria Rhodoblastus acidophilus, Rhodobacter sphaeroides and Phaeospirillum molischianum at 298 K by direct extraction of bacteriochlorophyll (BChl) a from the lyophilized proteins, followed by estimation of BChl a amounts from their electronic absorption spectra.
  • Yoshitaka Saga; Keisuke Hayashi; Keiya Hirota; Jiro Harada; Hitoshi Tamiaki
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY 313 44 - 51 2015年12月 [査読有り]
     
    Green sulfur photosynthetic bacteria Chlorobaculum (Cba.) tepidum and Cba. limnaeum were cultivated with the supplementation of 9-decyn-1-ol, 9-decen-1-ol, and decan-1-ol by two methods to biosynthesize unnatural chlorosomal bacteriochlorophyll (BChl) pigments possessing an (un)saturated bond at the terminus of the esterifying chain. The supplementation of the acetone solutions of the exogenous alcohols every 24 h to Cba. tepidum (denoted as Method A) successfully produced unnatural BChls c esterified with the alcohols in vivo. The ratios of unnatural BChls c esterified with 9-decyn-1-ol, 9-decen-1-ol, and decan-1-ol over the total BChls c in Cba. tepidum were not so different from each other, indicating that terminal unsaturated moieties in the exogenous alcohols barely affected the last reaction in the BChl c biosynthesis catalyzed by the BChl c synthase. Cba. tepidum grown in a liquid culture that initially contained 9-decyn-1-ol and 9-decen-1-ol (denoted as Method B) also accumulated BChls c esterified with these alcohols; the relative ratios of the unnatural BChls c were analogous to those obtained by Method A at the corresponding final concentrations. In contrast, Cba. limnaeum did not synthesize BChl e esterified with the three exogenous alcohols. The incorporation of unsaturated bonds into the esterifying moieties of BChl c by a biosynthetic reaction will be useful as structural probes for the investigation of pigment interactions and the biogenesis of chlorosomes as well as the scaffolds for bioorthogonal modifications. (C) 2015 Elsevier B.V. All rights reserved.
  • Yoshitaka Saga; Keiya Hirota; Jiro Harada; Hitoshi Tamiaki
    BIOCHEMISTRY 54 32 4998 - 5005 2015年08月 [査読有り]
     
    The activity of an enzyme encoded by the CT1610 gene in the green sulfur photosynthetic bacterium Chlorobaculum tepidum, which was annotated as bacteriochlorophyll (BChl) a synthase, BchG (denoted as tepBchG), was examined in vitro using the lysates of Escherichia coli containing the heterologously expressed enzyme. BChl a possessing a geranylgeranyl group at the 17-propionate residue (BChl a(GG)) was produced from bacteriochlorophyllide (BChlide) a and geranylgeranyl pyrophosphate in the presence of tepBchG. Surprisingly, tepBchG catalyzed the formation of BChl a bearing a famesyl group (BChl a(F)) as in the enzymatic production of BChl a(GG), indicating loose recognition of isoprenoid pyrophosphates in tepBchG. In contrast to such loose recognition of isoprenoid substrates, BChlide c and chlorophyllide a gave no esterifying product upon being incubated with geranylgeranyl or famesyl pyrophosphate in the presence of tepBchG. These results confirm that tepBchG undoubtedly acts as the BChl a synthase in Cba. tepidum. The enzymatic activity of tepBchG was higher than that of BchG of Rhodobacter sphaeroides at 45 degrees C, although the former activity was lower than the latter below 35 degrees C.
  • Naoya Takahashi; Keiya Hirota; Yoshitaka Saga
    JOURNAL OF PORPHYRINS AND PHTHALOCYANINES 19 4 631 - 637 2015年04月 [査読有り]
     
    Major photosynthetic pigments chlorophyll(Chl) s have a cyclic tetrapyrrole with a fivemembered exocyclic E-ring as photofunctional moieties. Its transformation is one promising methodology to develop functional pigments based on Chl chemistry. This paper report a facile conversion from Chl derivatives lacking the 13(2)-methoxycarbonyl moiety to the corresponding chlorophyllous pigments, whose exocyclic E-ring had two oxo groups at the 13(1)- and 13(2)-positions, by contact with the TiO2 particles in the presence of molecular oxygen in the dark. The conversion proceeded under mild conditions without hydrolysis of the ester group in the 17-propionate residue. This is in sharp contrast to the inevitable ester cleavage in the conventional synthesis of Chl derivatives that possess alpha-diketone in the E-ring. The reaction properties in the present method allowed us to perform a one-step conversion of Chl derivatives esterified with a natural isoprenoid alcohol phytol to the corresponding 13(2)-oxo-pigments. No removal of central Zn from the chlorin macrocycle occurred in the present conversions using Zn pyropheophytin a as a starting material.
  • Yoshitaka Saga; Atsushi Ishitani; Naoya Takahashi; Kenji Kawamura
    BIOORGANIC & MEDICINAL CHEMISTRY LETTERS 25 3 639 - 641 2015年02月 [査読有り]
     
    Incubation of bacteriopheophytin (BPhe) a, which was a demetalated pigment of bacteriochlorophyll a in photosynthetic bacteria, in CH2Cl2 in the presence of TiO2 particles with bubbling O-2 in the dark produced a pigment absorbing 814 nm. Detailed characterization of the novel pigment isolated from the CH2Cl2 suspension revealed that bacteriopurpurin-18 phytyl ester possessing an anhydride-type six-membered exocyclic E-ring was majorly formed by the treatment with TiO2 particles under oxygenic conditions. Oxidation of the bacteriochlorin ring in BPhe a, namely formations of derivatives of 3-acetyl pheophytin a and 3-acetyl protopheophytin a, can barely be detected through the conversion processes. (C) 2014 Elsevier Ltd. All rights reserved.
  • Risato Nishimori; Hitoshi Tamiaki; Shigenori Kashimura; Yoshitaka Saga
    SUPRAMOLECULAR CHEMISTRY 27 1-2 28 - 36 2015年02月 [査読有り]
     
    Esterifying chains of chlorophyllous pigments play important roles in the formation of photosynthetic supramolecules, but their effects have not thoroughly been unraveled yet. Substitution of the esterifying chains in these pigments will be one possible strategy to elucidate this enigma. Recently, unnatural bacteriochlorophylls (BChls) c possessing a hydroxy group at the terminus of the esterifying chains were successfully biosynthesised in the green sulfur bacterium Chlorobaculum (Cba.) tepidum grown by supplementation of alpha,omega-diols. In this paper, in vitro assembling behaviours of unnatural BChls c isolated from Cba. tepidum grown with 1,8-octanediol, 1,12-dodecanediol and 1,16-hexadecanediol were characterised in aqueous Triton X-100 micelles to investigate the effects of the terminal hydroxy group in the esterifying chains of BChls c on self-aggregates such as chlorosomes, major antenna complexes in green photosynthetic bacteria. The bacteriochlorophyll (BChl) c derivatives monoesterified with alpha,omega-diols formed chlorosomal self-aggregates, but their formations were much slower than those of natural BChl c. The Q(y) absorption bands of the residual monomers of these BChl c derivatives were larger than those of natural BChl c. These suggest that the esterifying alpha,omega-diols in these unnatural BChls c somewhat interfered with formation of chlorosome-like aggregates compared with the natural esterifying farnesol.
  • Yoshitaka Saga; Keisuke Hayashi; Tadashi Mizoguchi; Hitoshi Tamiaki
    JOURNAL OF BIOSCIENCE AND BIOENGINEERING 118 1 82 - 87 2014年07月 [査読有り]
     
    The green sulfur photosynthetic bacterium Chlorobaculum tepidum newly produced BChl c derivatives possessing a chlorine or bromine atom at the terminus of the esterifying chain in the 17-propionate residue by cultivation with exogenous omega-halo-1-alkanols. The relative ratios of BChl c derivatives esterified with 8-chloro-1-octanol and omega-chloro-1-decanol were estimated to be 26.5% and 33.3% by cultivation with these omega-chloro-1-alkanols at the final concentrations of 300 and 150 mu M, respectively. In contrast, smaller amounts of unnatural BChls c esterified with w-bromo-1alkanols were biosynthesized than those esterified with omega-chloro-1-alkanols: the ratios of BChl c derivatives esterified with 8-bromo-1-octanol and 10-bromo-1-decanol were 11.3% and 12.2% at the concentrations of 300 and 150 jiM, respectively. These indicate that omega-chloro-1-allcanols can be incorporated into bacteriochlorophyllide c more than (abromo-1 -alkanols in the BChl c biosynthetic pathway. The homolog compositions of the novel BChl c derivatives possessing a halogen atom were analogous to those of coexisting natural BChl c esterified with farnesol. These results demonstrate unique properties of BChl c synthase, BchK, which can utilize unnatural substrates containing halogen in the BChl c biosynthesis of Cba. tepidum. (C) 2014, The Society for Biotechnology, Japan. All rights reserved.
  • Kana Sadaoka; Yoshitaka Saga
    JOURNAL OF PORPHYRINS AND PHTHALOCYANINES 18 6 506 - 512 2014年06月 [査読有り]
     
    The vibrational properties of metal complexes of monoformylated and diformylated chlorophyll derivatives were compared with those of the corresponding free-base chlorins to unravel the effects of the central metal on the carbonyl stretching vibration modes of the peripheral oxygen functional groups in the chlorin macrocycle by means of FTIR spectroscopy. The 3-C=O stretching vibrational bands of a 3-formyl group were shifted to lower wavenumbers by insertion of Zn and Cu into the center of the 3-formyl free-base chlorin. In contrast, the 7- and 8-C=O stretching vibrational bands of the formyl groups linked to the B-ring of the chlorin macrocycle were barely shifted even if 7- and 8-formyl free-base chlorins were metalated. The down-shifts of the 3-C=O and few shifts of the 7-/8C=O vibrational stretching bands were in line with the results of DFT calculations. The difference in the effects of the central metal on the vibrational properties between the formyl group in the A-ring and those in the B-ring is ascribable to the different conjugation manners with the adjacent p-system: the 3-formyl group was connected to the chlorin 18 pi-system, whereas the 7-/8-formyl groups were conjugated to the rather isolated C7-C8 double bond. The 13-C=O stretching vibrational bands were shifted to lower wavenumbers by metalation. These down-shifts can also be rationalized by invoking the conjugation of the 13-keto group with the chlorin 18 pi-system.
  • Yoshitaka Saga; Tatsuya Saiki; Naoya Takahashi; Yutaka Shibata; Hitoshi Tamiaki
    PHOTOCHEMISTRY AND PHOTOBIOLOGY 90 3 552 - 559 2014年05月 [査読有り]
     
    Bacteriochlorophyll (BChl) e was coassembled with BChl c in Triton X-100 micelles in aqueous solutions. The Q(y) absorption bands of the coaggregates were positioned between those of aggregates consisting solely of BChl c or e. The electronic absorption spectra of the coaggregates could not be reproduced by linear combinations of the spectra of the aggregates consisting solely of each pigment, but they were in line with the simulated spectra for the self-aggregates in which both BChls were randomly distributed. These suggest that BChls c and e are not spatially separated; they are homogenously distributed over the self-aggregates to give electronic spectra that are different from those of the aggregate consisting solely of each pigment. Deaggregation of the scrambled self-aggregates by excess Triton X-100 did not produce any spectral components assigned to an aggregate consisting solely of either BChl c or e. Acid-induced decomposition of the scrambled aggregates showed different kinetics from those of the aggregates consisting solely of each pigment. These also support the homogeneous distribution of BChls c and e in the scrambled self-aggregates. These results will be useful to investigate the major light-harvesting antenna systems of green photosynthetic bacteria that contain two kinds of chlorosomal BChls.
  • Risato Nishimori; Aimi Sakamoto; Hitoshi Tamiaki; Shigenori Kashimura; Yoshitaka Saga
    SUPRAMOLECULAR CHEMISTRY 26 10-12 753 - 760 2014年 [査読有り]
     
    Two kinds of zinc 3(1)-hydroxy-13(1)-oxo-chlorins 1 and 2 possessing a pyrenyl group at the 17-propionate residue, of which the linker length between the chlorin and the pyrene moieties was varied, were synthesised from naturally occurring chlorophyll a, and were self-assembled in an aqueous solution. Both zinc chlorins 1 and 2 exhibited Q(y) absorption bands around 720 nm accompanying circular dichroism signals in the Q(y) region, indicating that these zinc chlorins could form self-aggregates like chlorosomes of green photosynthetic bacteria. Addition of gamma-cyclodextrin into an aqueous solution containing the self-aggregates of zinc chlorin 1 esterified with 1-pyrenylmethanol induced the appearance of excimer emission of the pyrene moieties around 480 nm as well as increased the fluorescence intensities of the pyrene monomers at 378 and 396 nm, while only an increase in fluorescence from the monomeric pyrene moiety was observed in the case of 2 esterified with 4-(1-pyrenyl)butanol. Exogenous gamma-cyclodextrin unchanged the spectral features derived from the chlorin moieties of 1 and 2 in the aqueous phase. These suggest that the encapsulation of the pyrenyl groups in the zinc chlorins unchanged their assembling states under the present conditions.
  • Kana Sadaoka; Toru Oba; Hitoshi Tamiaki; Shigenori Kashimura; Yoshitaka Saga
    JOURNAL OF PORPHYRINS AND PHTHALOCYANINES 17 11 1120 - 1128 2013年11月 [査読有り]
     
    Demetalation kinetics of zinc chlorophyll derivative 1 possessing two formyl groups directly linked to the A-and B-rings of the chlorin macrocycle, which was synthesized from chlorophyll b, was examined under acidic conditions and compared with those of Zn chlorins 2 and 3 possessing a single formyl group in the A-and B-ring, respectively, as well as Zn chlorin 4 lacking any formyl group to unravel the substitution effects on demetalation properties of chlorophyllous pigments. Demetalation kinetics of diformylated Zn chlorin 1 was slower than those of monoformylated Zn chlorins 2 and 3, indicating that the effect of the electron-withdrawing formyl group on demetalation kinetics was amplified by introduction of two formyl groups to the chlorin macrocycle. High correlations were observed between demetalation rate constants of Zn chlorins 1-4 and the sum of Hammett sigma parameters of the 3- and 7-substituents on the chlorin macrocycle, indicating that the combination of electron-withdrawing abilities of the substituents linked directly to the cyclic tetrapyrrole was responsible for demetalation properties of zinc chlorophyll derivatives.
  • Kana Sadaoka; Sunao Shoji; Keiya Hirota; Yusuke Tsukatani; Taichi Yoshitomi; Hitoshi Tamiaki; Shigenori Kashimura; Yoshitaka Saga
    BIOORGANIC & MEDICINAL CHEMISTRY 21 22 6915 - 6919 2013年11月 [査読有り]
     
    Pheophytinization of chlorophyll (Chl) c(1), which was isolated from the diatom Chaetoceros gracilis, was kinetically analyzed under weakly acidic conditions, and was compared with that of protochlorophyllide (PChlide) a and chlorophyllide (Chlide) a. Chl c(1) possessing a trans-acrylic acid residue at the 17-position exhibited slower pheophytinization kinetics than PChlide a and Chlide a, both of which possessed a propionic acid residue at the same position. The difference in pheophytinization properties between Chl c(1) and (P) Chlide a was ascribable to the electronegativity of the 17-substituent in Chl c(1) larger than that of (P) Chlide a due to the C17(1)-C17(2) double bond with the conjugated 17(2)-carboxy group in Chl c(1). Demetalation kinetics of PChlide a was slower than that of Chlide a, which originated from the effect of the pi-macrocyclic structures. (C) 2013 Elsevier Ltd. All rights reserved.
  • Naoya Takahashi; Hitoshi Tamiaki; Yoshitaka Saga
    TETRAHEDRON 69 18 3638 - 3645 2013年05月 [査読有り]
     
    Zinc 3(1)-hydroxy-13(1)-oxo-chlorins 1-3 possessing a 12-crown-4, 15-crown-5, and 18-crown-6 ether at the 17-propionate residue were synthesized and self-assembled in aqueous and non-polar organic media. Zn chlorins 1-3 exhibited red-shifted Q(y) absorption bands and large CD signals around 730 nm in an aqueous solution after standing for 2 h or 2 days, indicating the formation of their oligomeric self-aggregates as in chlorosomes of green photosynthetic bacteria. Incubation for 2 weeks decreased the absorption bands around 730 nm, accompanying an appearance of the absorption band around 675 nm. These show that Zn chlorins 1-3 finally formed thermodynamically stable anti-parallel dimers in the aqueous solution. 17(6)-Epimerically pure 1-3 exhibited essentially the same spectral properties as the corresponding epimeric mixtures, but larger CD signals were observed around 730 nm in the spectra of epimerically pure 1 and 2 after standing for 2 h or 2 days than those of their epimeric mixtures. In a non-polar organic solvent, Zn chlorins 1-3 formed less time-dependent chlorosomal self-aggregates that exhibited Q(y) absorption bands around 735 nm. (C) 2013 Elsevier Ltd. All rights reserved.
  • Yoshitaka Saga; Yuki Hirai; Kana Sadaoka; Megumi Isaji; Hitoshi Tamiaki
    PHOTOCHEMISTRY AND PHOTOBIOLOGY 89 1 68 - 73 2013年01月 [査読有り]
     
    Demetalation of chlorophyll (Chl) a and its analogs is an important reaction in oxygenic photosynthetic organisms, which produces the primary electron acceptors in photosystem II reaction centers and is crucial in the Chl degradation. From these viewpoints, demetalation reactions of four Chl a analogs, 3,8-divinyl-Chl a (DV-Chl a), 3-devinyl-3-ethyl-Chl a (mesoChl a), 132-demethoxycarbonyl-Chl a (pyroChl a) and protochlorophyll a (PChl a), were kinetically analyzed under weakly acidic conditions, and were compared with that of Chl a. DV-Chl a exhibited slower demetalation kinetics than did Chl a, whereas demetalation of mesoChl a was faster than that of Chl a. The difference in demetalation kinetics of the three chlorophyllous pigments originates from the electron-withdrawing ability of the vinyl group as the peripheral substituent compared with the ethyl group. Removal of the electron-withdrawing and homoconjugating 132-methoxycarbonyl group in Chl a (Chl a -> pyroChl a) accelerated demetalation kinetics by two-fold. PChl a possessing the porphyrin-type skeleton exhibited slower demetalation kinetics than Chl a. The structure-dependent demetalation properties of Chl a analogs will be useful for understanding in vivo Chl demetalation reactions in oxygenic photosynthetic organisms.
  • Yoshitaka Saga; Yuta Kobashiri; Kana Sadaoka
    INORGANIC CHEMISTRY 52 1 204 - 210 2013年01月 [査読有り]
     
    Removal of the central metal from chlorophyll (Chl) molecules is biologically important in terms of production of the primary electron acceptors in photosystem-II photosynthetic reaction centers and the early stage in Chl degradation. The physicochemical properties on demetalation of chlorophyllous pigments are useful in the understanding of such reaction mechanisms in photosynthetic organisms. Here we analyzed the demetalation kinetics of a series of Zn-Chl derivatives with a systematic variation in the electron-withdrawing and -donating substituents at the 3-position of the chlorin macrocycle under acidic conditions to elucidate thoroughly the substitution effects on the demetalation properties of chlorophyllous pigments. Dehydrogenation of the aliphatic group (CH2CH3 --> CH=CH2 --> C CH) at the 3-position slowed the removal of the central zinc from the chlorin macrocycle. The gradual decrease in the demetalation rate constants of the three zinc chlorins originates from differences in the electron-withdrawing strength of the ethyl, vinyl, and ethynyl groups directly linked to the chlorin pi macrocycle. Reduction of the 3(1)-carbonyl groups significantly increased the demetalation rate constants, and the relative ratios of the demetalation rate constants of the zinc chlorins possessing a carbonyl group to those possessing the corresponding hydroxy group were analogous in the cases of 3-formyl- and 3-acetyl-zinc chlorins. The demetalation rate constants of the seven Zn-Chl derivatives possessing various electron-withdrawing and -donating groups exhibited good correlation with the Hammett sigma parameters of the 3-position substituents.
  • Yoshitaka Saga; Akane Maruko; Kana Sadaoka; Naoya Takahashi
    Chemistry Letters 42 6 672 - 674 2013年 [査読有り]
     
    Zinc methyl 131-deoxopyropheophorbide a, which lacked the 13 1-oxo group in the exo five-membered E-ring, exhibited faster demetalation kinetics than zinc methyl pyropheophorbide a under acidic conditions, indicating that the 13-keto group of chlorophyllous pigments was responsible for resistance against removal of the central metal from the chlorin macrocycle. © 2013 The Chemical Society of Japan.
  • Naoya Takahashi; Sunao Shoji; Hitoshi Tamiaki; Yoshitaka Saga
    BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN 85 9 989 - 994 2012年09月 [査読有り]
     
    Transformation of pigment assembly in photosynthetic supramolecules is an important event in the performance of efficient photosynthetic activities, and model systems mimicking such transformation are useful to elucidate dynamic processes in natural photosynthetic systems. In this paper, we report stimuli-induced self-aggregation of a synthetic zinc bacteriochlorophyll d derivative possessing a triethoxysilyl group at the 17-propionate (Zn chlorin 1) as a model system of chlorosomes, which are extramembranous and major light-harvesting complexes in green photosynthetic bacteria. A mixture of monomeric and dimeric species of epimerically pure Zn chlorin 1 was gradually converted into chlorosomal self-aggregates that exhibited a red-shifted Q(y) absorption band and the CD signal at the corresponding region around 750nm in an aqueous alkaline solution. The finally obtained visible absorption and CD spectra of 1 under the alkaline conditions were almost identical to the spectra of 1 lacking ethoxy groups by alkaline hydrolysis in advance, indicating that the above spectral changes of 1 observed under the alkaline conditions were ascribable to hydrolysis of the ethoxysilyl group in 1. The alteration of assembling states of synthetic zinc chlorin 1 triggered by its chemical modification will be a clue to the construction of model systems for in vivo formation of chlorosomes in green photosynthetic bacteria.
  • Yoshitaka Saga; Hitoshi Tamiaki
    CHEMISTRY & BIODIVERSITY 9 9 1659 - 1683 2012年09月 [査読有り]
     
    Natural chlorophylls (Chls) and bacteriochlorophyll (BChls), which are major pigments in photosynthesis, possess a central Mg (or Zn) in their cyclic tetrapyrrole macrocycles. Removal of the central metal atom from chlorophyllous pigments, called pheophytinization, is a biologically important reaction in that it allows production of the primary electron acceptors in photosystem II(-type) reaction centers and is one of the crucial steps in the Chl degradation pathway. Pheophytinization in processed and postharvest foods derived from vegetables and fruits has attracted considerable attention in agricultural and food chemistry from the viewpoints of maintenance of their color level and evaluation by consumers. In this review, we focus on in vivo demetalation reactions and demetalated products of chlorophyllous pigments. In addition, we summarize kinetic studies on in vitro demetalation of natural (B)Chl molecules and their synthetic analogs under acidic conditions.
  • Yoshitaka Saga; Tae Otono
    CHEMISTRY LETTERS 41 4 360 - 362 2012年04月 [査読有り]
     
    Pheophorbide a methyl ester, which was derived from natural chlorophyll a, was converted to purpurin-18 methyl ester by contact with titanium oxide particles in the presence of oxygen under mild conditions in the dark.
  • Yoshitaka Saga; Ryosuke Miura; Kana Sadaoka; Yuki Hirai
    JOURNAL OF PHYSICAL CHEMISTRY B 115 40 11757 - 11762 2011年10月 [査読有り]
     
    Demetalation of three synthetic zinc cyclic tetrapyrroles that possess identical peripheral substituents, zinc methyl bacteriopyropheophorbide a (zinc bacteriochlorin 1), zinc methyl 3-devinyl-3-acetyl-pyropheophorbide a (zinc chlorin 2), and zinc methyl 3-devinyl-3-acetyl-protopyropheophorbide a (zinc porphyrin 3), was kinetically analyzed under acidic conditions to examine the effects of macrocyclic structures on demetalation without peripheral substitution effects. Zinc bacteriochlorin 1 exhibited much slower demetalation kinetics than zinc chlorin 2 and zinc porphyrin 3. These results indicate that the bacteriochlorin skeleton provides significant resistance to the removal of the central metal from the tetrapyrrole ligand. Comparison of demetalation kinetics of 3-acetyl zinc complexes 2 and 3 with that of 3-vinyl zinc complexes under the same reaction condition demonstrated that the relative ratio (5.0 x 10(-2)) of the demetalation rate constant of the 3-acetyl zinc chlorin 2 to that of the corresponding 3-vinyl zinc chlorin 4 resembled the case of the 3-acetyl zinc porphyrin 3 to the 3-vinyl zinc porphyrin 5 (the relative ratio was 6.8 x 10(-2)). These suggest that the electron-withdrawing 3-acetyl group slows down the demetalation from the tetrapyrrole ligands more than the 3-vinyl group and that the 3-acetyl effect is analogous in both chlorin and porphyrin pi-systems.
  • Nishimori R; Mizoguchi T; Tamiaki H; Kashimura S; Saga Y
    Biochemistry 50 36 7756 - 7764 36 2011年09月 [査読有り]
     
    Unnatural bacteriochlorophyll (BChl) c derivatives possessing a hydroxy group at the terminus of a hydrocarbon chain at the 17-propionate were biosynthesized in the green sulfur photosynthetic bacterium Chlorobaculum tepidum. Addition of exogenous 1,8-octanediol, 1,12-dodecanediol, and 1,16-hexadecanediol in acetone to liquid cultures resulted in accumulation of BChl c monoesterified with the corresponding diols. The relative ratios of the novel BChl c derivatives esterified with 1,8-, 1,12-, and 1,16-diols to totally producing BChl c were 8.2, 50.2, and 57.6% in the cells grown with additive alpha,omega-diols at concentrations of 1.5, 0.06, and 0.06 mM, respectively, at the final concentration. The homologue composition of BChl c derivatives esterified with these alpha,omega-diols was similar to that of original, coexisting BChl c esterified with farnesol (BChl c(F)), suggesting that esterification of alpha,omega-diols occurred at the last step of the BChl c biosynthetic pathway by BChl c synthase, BchK, in the same manner as in BChl c(F). Chlorosomes, which were isolated from cells grown in the presence of exogenous alpha,omega-diols, contained a ratio and a composition of BChl c derivatives esterified with the diols similar to those in the whole cells, indicating that these BChl c derivatives were actually present in chlorosomes. Q(y) absorption bands of C. tepidum cells containing the novel BChl c derivatives were shifted to a shorter wavelength, although their bandwidths were analogous to those of cells obtained by normal cultivation. Circular dichroism spectra of cells that had BChl c derivatives esterified with alpha,omega-diols exhibited S-shaped signals in the Q(y) region, whose polarities were the reverse of those of cells grown in the normal medium and by supplementation with neat acetone as a control experiment. These spectral features of C. tepidum possessing BChl c derivatives esterified with alpha,omega-diols imply that the novel BChl c derivatives possessing a hydroxy group at the terminus of a hydrocarbon chain affect their self-assembly in chlorosomes.
  • Kana Sadaoka; Shigenori Kashimura; Yoshitaka Saga
    BIOORGANIC & MEDICINAL CHEMISTRY 19 13 3901 - 3905 2011年07月 [査読有り]
     
    Reduction of the 7-formyl groups in chlorophyll (Chl) b and its demetalated compound pheophytin (Phe) b was kinetically analyzed by using tert-butylamine-borane complex (t-BuNH(2)center dot BH(3)), and was compared with that of the 3-formyl groups in Chl d and Phe d. Reduction kinetics of the 7-formyl group in Chl b was similar to that in Phe b in dichloromethane containing 5 mM t-BuNH(2)center dot BH(3). Little difference of the reduction kinetics of the 7-formyl groups between Chl b and Phe b was in sharp contrast to the reduction kinetics of the 3-formyl groups in Chl d and Phe d: the 3-formyl group in Phe d was reduced 5.3-fold faster than that in Chl d. The 7-formyl groups in Chl b and Phe b were reduced more slowly than the 3-formyl groups in Chl d and Phe d, respectively. The difference of the reactivity between the 3- and 7-formyl groups was in line with (13)C NMR measurements of chlorophyllous pigments, in which the chemical shifts of carbon atoms in the 7-formyl groups of Chl b and Phe b were high-field shifted compared with those in the 3-formyl groups of Chl d and Phe d, respectively. These indicate that the 7-formyl groups in chlorophyllous pigments were less reactive for reduction to the corresponding hydroxymethyl groups than the 3-formyl groups due to the difference in electronic states of the formyl groups in the A-and B-rings of the chlorin macrocycle. (C) 2011 Elsevier Ltd. All rights reserved.
  • Yuki Hirai; Shigenori Kashimura; Yoshitaka Saga
    PHOTOCHEMISTRY AND PHOTOBIOLOGY 87 2 302 - 307 2011年03月 [査読有り]
     
    Conversion of the formyl group at the 7-position in chlorophyll (Chl) b to the methyl group via the hydroxymethyl group is biologically important in Chl b degradation. To clarify the effects of the 7-substituents on demetalation properties of chlorophyllous pigments in the early process of Chl b degradation, we report demetalation kinetics of the zinc Chl derivative possessing a 7-hydroxymethyl group, which is a good model compound of the intermediate molecule in the early process of Chl b degradation, under acidic conditions, and compare its properties with those of zinc Chl derivatives possessing a methyl and a formyl group, which are model compounds of Chls a and b, respectively. Demetalation rate constants of 7-hydroxymethyl zinc chlorin were much larger than those of 7-formyl zinc chlorin, but were slightly smaller than those of 7-methyl zinc chlorin. The activation energy for demetalation reaction of 7-formyl zinc chlorin was larger than those of other derivatives. Demetalation rate constants of 7-deformyl-7-hydroxymethyl Chl b were also larger than those of Chl b, and were similar to those of Chl a. These indicate that the 7-hydroxymethyl group in the chlorin macrocycle has a smaller effect on demetalation compared with the 7-formyl group.
  • Yuki Hirai; Shin-ichi Sasaki; Hitoshi Tamiaki; Shigenori Kashimura; Yoshitaka Saga
    JOURNAL OF PHYSICAL CHEMISTRY B 115 12 3240 - 3244 2011年03月 [査読有り]
     
    Effects of peripheral subtituents in the A- and B-rings of the chlorin macrocycle on demetalation of zinc chlorophyll derivatives were examined. The demetalation rate constants of zinc 8-formyl-chlorin (1) were comparable to those of zinc 3-formyl-chlorin (2). Both 1 and 2 exhibited significantly slower demetalation kinetics than the corresponding zinc chlorin 3, which had no formyl group. Comparison of demetalation kinetics between zinc 3-ethyl-chlorin (3) and zinc 3-vinyl-chlorin (4) indicated that the substitution at the 3-position of the chlorin macrocycle from a vinyl to an ethyl group accelerated removal of the central zinc. Activation energies of demetalation of zinc chlorins 1, 2, and 5 possessing a formyl group in the A- or B-ring of the chlorin macrocycle were larger than those of zinc chlorins 3 and 4 lacking a formyl group. These results indicate that the formyl groups in the A- or B-ring provide tolerance to demetalation of chlorin molecules due to their electron-withdrawing effects.
  • Yoshitaka Saga; Sayaka Hojo; Yuki Hirai
    BIOORGANIC & MEDICINAL CHEMISTRY 18 15 5697 - 5700 2010年08月 [査読有り]
     
    Metalloporphyrins and metallochlorins are important biological molecules in nature. To examine the effect of macrocyclic structures on removal of central metals from these cyclic tetrapyrrole molecules without peripheral substitution effects, demetalation of zinc methyl pyropheophorbide a (zinc chlorin 1) and zinc methyl protopyropheophorbide a (zinc porphyrin 2) was kinetically analyzed under acidic conditions. Both 1 and 2 exhibited gradual spectral changes from zinc complexes to free-base forms with several isosbectic points in acetone/water (10: 1) at the proton concentration of 6.1 x 10(-3) M. Demetalation of zinc porphyrin 2 was slower than that of zinc chlorin 1 in the temperature range between 15 and 35 degrees C. This indicates that the porphyrin macrocycle provides tolerance to removal of the central metal from cyclic tetrapyrrole ligands compared with the chlorin macrocycle. (C) 2010 Elsevier Ltd. All rights reserved.
  • Kana Sadaoka; Yuki Hirai; Shigenori Kashimura; Yoshitaka Saga
    CHEMISTRY LETTERS 39 6 567 - 569 2010年06月 [査読有り]
     
    Reduction of formyl groups in natural chlorophyll (Chl) d purified from a cyanobacterium, Acaryochloris marina, and its demetalation product, pheophytin (Phe) d, was kinetically analyzed. Chl d exhibited slower reduction kinetics of the formyl group to the hydroxymethyl group by tert-butylamine borane complex than Phe d.
  • Yoshitaka Saga; Yutaka Shibata; Hitoshi Tamiaki
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY C-PHOTOCHEMISTRY REVIEWS 11 1 15 - 24 2010年03月 [査読有り][招待有り]
     
    Photosynthetic light-harvesting complexes play important roles in the early events in photosynthesis in nature, and have been studied by various spectroscopic methodologies. Single molecule spectroscopy is one of the recent powerful methods to study fine electronic structures of photosynthetic light-harvesting complexes without averaging over an ensemble of the complexes. In this review, we focus on spectroscopic properties of single light-harvesting complexes in anoxygenic photosynthetic bacteria. In single supramolecule spectroscopy of light-harvesting complexes of purple photosynthetic bacteria, information on excitonic (de)localization on the arrangements of bacteriochlorophyll (BChl) a molecules, which is hidden in conventional measurements, has been obtained in detail. Single supramolecule spectroscopy has been applied to major antenna complexes (=chlorosomes) in green photosynthetic bacteria, revealing the spectral properties in relation to anisotropy and excitation energy transfer in single chlorosomes, their heterogeneity, and the number of BChl molecules per chlorosome. (C) 2010 Elsevier B.V. All rights reserved.
  • Yoshitaka Saga; Toshihiro Nakagawa; Tomohiro Miyatake; Hitoshi Tamiaki
    CHEMISTRY LETTERS 38 9 882 - 883 2009年09月 [査読有り]
     
    Visible absorption and circular dichroism spectra of zinc chlorophyll derivatives possessing an oligo(oxyethylene) moiety at the 17-propionate in aqueous media were drastically changed by treatment with dichloromethane or chloroform, indicating that dimeric and monomeric species were converted into chlorosome-like large self-aggregates.
  • Yuki Hirai; Hitoshi Tamiaki; Shigenori Kashimura; Yoshitaka Saga
    PHOTOCHEMISTRY AND PHOTOBIOLOGY 85 5 1140 - 1146 2009年09月 [査読有り]
     
    Demetalation kinetics of bacteriochlorophylls (BChls) c, d and e from green sulfur photosynthetic bacteria were studied under weakly acidic conditions. Demetalation rate constants of BChl e possessing a formyl group at the 7-position were significantly smaller than those of BChls c and d, which had a methyl group at this position. The activation energy of demetalation of 3(1)R-8,12-diethyl([E,E])-BChl e was 1.5-times larger than that of 3(1)R-[E,E]-BChl c. (15)N-labeled 3(1)R-[E,E]-BChls c and e were purified from cells of green sulfur bacteria grown in a medium containing (15)NH(4)Cl, and their (15)N NMR spectra were measured. The chemical shifts of N(21), N(22) and N(23) atoms of 3(1)R-[E,E]-BChl e were lower-field shifted than those of 3(1)R-[E,E]-BChl c, respectively, and especially the difference in chemical shifts of N(22) was significantly large. These results suggest that the electron-withdrawing formyl group at the 7-position of BChl e affected an electronic state of the chlorin macrocycle and caused BChl e to be more tolerant for removal of the central magnesium compared with BChls c and d.
  • Yutaka Shibata; Yoshitaka Saga; Hitoshi Tamiaki; Shigeru Itoh
    PHOTOSYNTHESIS RESEARCH 100 2 67 - 78 2009年05月 [査読有り]
     
    The polarization anisotropy of fluorescence spectra from single chlorosomes isolated from a green sulfur bacterium, Chlorobium (Cb.) tepidum, was observed at 13 K. As the polarizer was rotated, the intensities of the fluorescence bands of both bacteriochlorophyll (BChl)-c self-aggregates and BChl-a in baseplate proteins showed clear oscillations. From the oscillation, the values of the degree of polarization (DP) and the phase shift (PS) between the BChl-c and BChl-a bands were determined for each single chlorosome. The DP versus PS plot for Cb. tepidum chlorosomes showed linear correlations between the PS and the DP values for both BChl-c and BChl-a fluorescence bands. This tendency could be explained from a simulation assuming a random orientation of chlorosomes and a triaxial orientation distribution of emitting transition dipoles within a single chlorosome. The intensity ratios among the X-/Y-/Z-principal transition dipoles were estimated to be 0.3/0.5/1 and 1/0.6/0.1 for the BChl-c and BChl-a fluorescence bands, respectively. Here, the X-, Y-, and Z-axes are perpendicular, parallel to the cytoplasmic membrane, and parallel to the chlorosome long axis, respectively. A theoretical calculation based on the exciton theory was conducted to reproduce the observed triaxial orientation distribution of emitting transition dipoles. The simulation revealed that a deformation introduced to the circular cross section of the rod-shaped BChl-c self-aggregates could qualitatively reproduce results of this study.
  • Yoshitaka Saga; Yuichi Nakai; Hitoshi Tamiaki
    SUPRAMOLECULAR CHEMISTRY 21 8 738 - 746 2009年 [査読有り]
     
    Easy to prepare fluoride ion sensors bearing OH and NH groups as binding sites and nitrophenyl group as a chromophore [2-[(4-nitrophenylimino)methyl]-4-methylphenol (1), 2-[(4-nitrophenylamino)methyl]-4-methylphenol (2), 2-[(2,4-dinitrophenylimino)methyl]-4-methylphenol (3) and 2-[(2,4-dinitrophenylamino)methyl]-4-methylphenol (4)] were synthesised and characterised. The visible colour changes, the UV-vis and 1H NMR spectral changes towards anions were studied. The sensing ability of the receptor depends on the presence of electron-withdrawing group attached to the receptors and extended conjugation in the receptor framework and the order of the sensing ability is found to be 3142 based on binding constant.
  • Yuki Hirai; Hitoshi Tamiaki; Shigenori Kashimura; Yoshitaka Saga
    PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES 8 12 1701 - 1707 2009年 [査読有り]
     
    Demetalation kinetics of natural chlorophyll (Chl) d purified from Acaryochloris marina was first studied and compared with those of Chls a and b. The demetalation rate constant of Chl d, which possessed a formyl group at the 3-position, was five-fold smaller than that of Chl a possessing a vinyl group at the same position in aqueous acetone at the proton concentration of 1.2 x 10(-3) M at 25 degrees C. In contrast, the demetalation rate constant of Chl b possessing a formyl group at the 7-position was 26 times smaller than that of Chl a. The activation energy of demetalation reaction of Chl d was larger than that of Chl a, but smaller than that of Chl b. These indicate that the substitution effect of 3-formyl group on the acidic removal of central magnesium in Chls was smaller than that of 7-formyl group.
  • Yoshitaka Saga; Jiro Harada; Hiromitsu Hattori; Kanako Kaihara; Yuki Hirai; Hirozo Oh-oka; Hitoshi Tamiaki
    PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES 7 10 1210 - 1215 2008年10月 [査読有り]
     
    The effects of exogenous vitamin B12 on the green sulfur photosynthetic bacterium Chlorobium (Chl.) tepidum were examined. Wild-type cells and mutant cells lacking a gene CT0388 (denoted as VB0388) of Chl. tepidum were grown in liquid cultures containing different concentrations of vitamin B-12. The VB0388 cells hardly grew in vitaminB(12)-limited media, indicating that the product of CT0388 actually played an important role in vitamin B,, biosynthesis in Chl. tepidum. Both wild-type and VB0388 cells in vitamin B-12-limited media exhibited absorption bands and CD signals at the Q(y) region that were shifted to a shorter wavelength than those of cells grown in normal media. BChl c isomers that had S-stereochemistry at the T-position tended to increase in Chl. tepidum grown in vitamin B-12-limited media.
  • Yoshitaka Saga; Yutaka Shibata; Shigeru Ltoh; Hitoshi Tamiaki
    JOURNAL OF PHYSICAL CHEMISTRY B 111 43 12605 - 12609 2007年11月 [査読有り]
     
    The number of pigments in single light-harvesting complexes (chlorosomes) were calculated by imaging single chlorosomes in a frozen buffer at cryogenic temperature with a confocal laser fluorescence microscope and pigment extraction. Chlorosomes were isolated from two types of green photosynthetic bacteria Chlorobium (Chl.) tepidunt and Chlorojflexus (Cfl.) aurantiacus and were individually imaged in the frozen medium. Each fluorescence spot observed mainly came from a single chlorosome and was ascribable to self-aggregates of bacteriochlorophyll (BChl) c molecules as core parts of chlorosomes. A three-dimensional distribution of fluorescence of single chlorosomes was analyzed, and the number of chlorosomes in a volume of 54 000 mu M3 was counted directly. On the basis of the results, averaged numbers of the BChl c molecules contained in a single chlorosome of Chl. tepiduin and Cfl. aurantiacus were determined to be 1.4 x 10(5) and 9.6 x 10(4), respectively. The present numbers are almost comparable to those estimated by other methods (MartinezPlanells et al., Photosynth. Res. 2002, 71, 83 and Montano et al., Biophys. J. 2003, 85, 2560).
  • Yutaka Shibata; Yoshitaka Saga; Hitoshi Tamiaki; Shigeru Itoh
    BIOCHEMISTRY 46 23 7062 - 7068 2007年06月 [査読有り]
     
    The polarization anisotropy of fluorescence from single chlorosomes isolated from a green filamentous bacterium, Chloroflexus aurantiacus, was measured using a confocal laser microscope at 13 K. Each single chlorosome that is floating in a frozen solvent exhibited strong polarization anisotropy of fluorescence. We calculated the degrees of fluorescence polarization for 51 floating single chlorosomes. The value ranged from 0.1 to 0.76 for the BChl-c aggregate in the core chlorosomes and from 0 to 0.4 for the energy acceptor BChl-a in the baseplate protein in the outer membrane. The shifts in polarization angles between the two emission bands were distributed over all the possible values with a sharp peak around 90 degrees, suggesting the perpendicular orientation between the transition dipoles of the fluorescence emission from the BChl-c aggregate and that from BChl-a. A simulation assuming a random orientation of chlorosomes reproduced the experimental results exactly. The analysis further indicated the appreciable contribution of the transition dipole of BChl-c that has an orientation perpendicular to the major polarization axis in each chlorosome. Small values of the degrees of polarization implied the BChl-a transition dipole to be somewhat tilted with respect to the normal of the cytoplasmic membrane to which chlorosomes are attached. These conclusions can be obtained only by observing the fluorescence of single chlorosomes.
  • Yoshitaka Saga; Yuki Hirai; Hitoshi Tamiaki
    FEBS LETTERS 581 9 1847 - 1850 2007年05月 [査読有り]
     
    Substituent-dependent demetalation kinetics of natural bacteriochlorophyll (BChl) c and e homologs purified from two green sulfur photosynthetic bacteria was first studied. Separated BChl e homologs, which possessed a formyl group at the 7-position of their chlorin macrocycles, exhibited a significantly slow removal of central magnesium to free-base bacteriopheophytins in acidic aqueous acetone compared with the corresponding BChl c homologs, which possessed a methyl group at the 7-position. Additional methyl groups at the 8(2)-position of both BChl c and e molecules had little effect on the demetalation kinetics. (C) 2007 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.
  • Yutaka Shibata; Yoshitaka Saga; Hitoshi Tamiaki; Shigeru Itoh
    BIOPHYSICAL JOURNAL 91 10 3787 - 3796 2006年11月 [査読有り]
     
    Fluorescence spectra of single chlorosomes isolated from a green filamentous bacterium (Chloroflexus (C..) aurantiacus) and a green sulfur bacterium (Chlorobium (Cb.) tepidum) were measured by using a confocal laser microscope at 13 K. Chlorosomes were frozen either in a liquid solution (floating chlorosome) or on a quartz plate after being adsorbed (adsorbed chlorosome). Fluorescence peak wavelengths were shorter for the adsorbed single chlorosomes than for the floating ones. Single floating C.. chlorosomes showed a distribution of fluorescence peak positions having a center at 759.0 nm with a full width at half maximum of 6.3 nm. Single floating Cb. chlorosomes showed a 782.7 nm center with a full width at half-maximum of 3.4 nm. The distribution shifted to the blue and became wider with increasing temperature, especially in Cb. chlorosomes, suggesting a large excitonic density of states just above the lowest level. Energy transfer from BChl-c aggregates to BChl-a molecules in the baseplate proteins was observed in the floating chlorosomes but not in the adsorbed ones. A positive correlation was found between the peak wavelength of BChl-c fluorescence and the intensity of BChl-a fluorescence in single C.. chlorosomes. The results suggest that the BChl-c aggregates with longer wavelengths of the fluorescence peaks have a more efficient Forster-type energy transfer to the baseplate BChl-a.
  • Jiro Harada; Shigeaki Osumi; Yoshitaka Saga; Hirozo Oh-oka; Hitoshi Tamiaki
    JOURNAL OF MOLECULAR CATALYSIS B-ENZYMATIC 42 3-4 131 - 131 2006年11月 [査読有り]
  • Yoshitaka Saga; Hitoshi Tamiaki
    JOURNAL OF BIOSCIENCE AND BIOENGINEERING 102 2 118 - 123 2006年08月 [査読有り]
     
    Supramolecular nanostructures of bacteriochlorophyll (BChl) self-aggregates in major light-harvesting complexes (chlorosomes) of green photosynthetic bacteria were successfully observed by freeze-fracture transmission electron microscope. Rod-shaped nanostructures with approximately 10 nm in diameter could be visualized in three types of green sulfur bacteria (Chlorobium). Diameter of the rod-shaped nanostructures in Chlorobium chlorosomes was independent of the molecular structures of their light-harvesting pigments, namely BChl-c or d. In contrast, chlorosomes of the green filamentous bacterium Chloroflexus aurantiacus had rod-shaped nanostructures with approximately 5 nm in diameter. The present results support that BChl self-aggregates in chlorosomes form rod-shaped nanostructures called rod-elements with approximately 10- and 5-nm diameters for Chlorobium and Chloroflexus, respectively.
  • Yoshitaka Saga; Sho Akai; Tomohiro Miyatake; Hitoshi Tamiaki
    BIOCONJUGATE CHEMISTRY 17 4 988 - 994 2006年07月 [査読有り]
     
    Naturally occurring bacteriochlorophyll( BChl) s-c, -d, and -e from green sulfur photosynthetic bacteria were self-assembled in an aqueous solution in the presence of octadecyltriethoxysilane and tetraethoxysilane, followed by polycondensation of the alkoxysilanes by incubation for 50 h at 25 degrees C. The resulting BChl self-assemblies in silicate capsules exhibited visible absorption and circular dichroism spectra similar to the corresponding natural light-harvesting systems (chlorosomes) of green sulfur bacteria. Dynamic light scattering measurements indicated that the silicate capsules had an average hydrodynamic diameter of several hundred nanometers. BChl self-aggregates in silicate capsules were significantly stable to a nonionic surfactant Triton X-100, which was apt to decompose the BChl aggregates to their monomeric form, compared with conventional micelle systems. BChls in silicate capsules were more tolerant to demetalation of the central magnesium under acidic conditions than the natural systems.
  • Kei Wada; Hitomi Yamaguchi; Jiro Harada; Keiko Niimi; Shigeaki Osumi; Yoshitaka Saga; Hirozo Oh-oka; Hitoshi Tamiaki; Keiichi Fukuyama
    JOURNAL OF MOLECULAR BIOLOGY 360 4 839 - 849 2006年07月 [査読有り]
     
    BchU plays a role in bacteriochlorophyll c biosynthesis by catalyzing methylation at the C-20 position of cyclic tetrapyrrole chlorin using S-adenosylmethionine (SAM) as a methyl source. This methylation. causes red-shifts of the electronic absorption spectrum of the light-harvesting pigment, allowing green photosynthetic bacteria to adapt to low-light environments. We have determined the crystal structures of BchU and its complex with S-adenosylhomocysteine (SAH). BchU forms a dimer and each subunit consists of two domains, an N-terminal domain and a C-terminal domain. Dimerization occurs through interactions between the N-terminal domains and the residues responsible for the catalytic reaction are in the C-terminal domain. The binding site of SAH is located in a large cavity between the two domains, where SAH is specifically recognized by many hydrogen bonds and a salt-bridge. The electron density map of BchU in complex with an analog of bacteriochlorophyll c located its central metal near the SAH-binding site, but the tetrapyrrole ring was invisible, suggesting that binding of the ring to BchU is loose and/or occupancy of the ring is low. It is likely that His290 acts as a ligand for the central metal of the substrate. The orientation of the substrate was predicted by simulation, and allows us to propose a mechanism for the BchU directed methylation: the strictly conserved Tyr246 residue acts catalytically in the direct transfer of the methyl group from SAM to the substrate through an S(N)2-like mechanism. (c) 2006 Elsevier Ltd. All rights reserved.
  • Yoshitaka Saga; Tetsuichi Wazawa; Yoshiharu Ishii; Toshio Yanagida; Hitoshi Tamiaki
    JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY 6 6 1750 - 1757 2006年06月 [査読有り]
     
    Fluorescence emission properties of intact and alkaline-treated chlorosomes containing bacteriochlorophyll(BChl)-c, d, and e, which were isolated from four species of green sulfur photosynthetic bacteria, were successfully studied at the single-unit level using a total internal reflection fluorescence microscope. Single intact chlorosomes containing BChl-c from Chlorobium (Chl.) tepidum exhibited heterogeneous emission bands of BChl-c self-aggregates. In contrast, fluorescence spectra of chlorosomal BChl self-aggregates in single intact chlorosomes from the other three Chlorobium species were less heterogeneous than those from Chl. tepidum. Removal of energy-accepting BChl-a/protein complexes called baseplates from the intact chlorosomes by treatments with alkaline media hardly changed spectral shapes of BChl aggregates and their peak distributions at the single-chlorosome level. The similarity of spectral properties at the single-unit level between intact and alkaline-treated chlorosomes of four Chlorobium species clearly indicated that the removal of baseplates from intact chlorosomes by the alkali ne-treatment did not affect BChl self-aggregates inside single chlorosomes.
  • Y Saga; TY Kim; T Hisai; H Tamiaki
    THIN SOLID FILMS 500 1-2 278 - 282 2006年04月 [査読有り]
     
    Extramernbranous and peripheral light-harvesting complexes (chlorosomes) of green sulfur photosynthetic bacterium Chlorobium tepidum were electrostatically immobilized on a pyrex glass plate with the help of cationic polylysines. Absorbance of both Soret and Q(y) bands of chlorosomes were linearly increased by repeated manipulation of layer-by-layer adsorption. Spectral features of chlorosome/polylysine layers on the glass plate indicate that the supramolecular structures of self-aggregates of bacteriochlorophyll(BChl)-c in chlorosomes are maintained on the solid surface. BChl-c molecules could be completely extracted from a single chlorosome/polylysine layer on a glass plate as monomers in acetone. Visible absorption spectra of extracted pigments indicate that BChl-c molecules inside chlorosomes were not denatured by electrostatic adsorption of chlorosomes and that the amount of BChl-c molecules in chlorosomes adsorbed on a glass plate was (1.3 +/- 0.05) x 10(-10) Mol/cm(2) (mean standard deviation, three measurements). The estimation of BChl-c amounts suggests that chlorosomes are not so densely immobilized on a polylysine-adsorbed quartz plate. (c) 2005 Elsevier B.V. All rights reserved.
  • Y Saga; S Osumi; H Higuchi; H Tamiaki
    PHOTOSYNTHESIS RESEARCH 86 1-2 123 - 130 2005年11月 [査読有り]
     
    Green sulfur photosynthetic bacteria Chlorobium (Chl.) vibrioforme (DSM 263 strain and NCIB 8327 substrain possessing BChl-c) and Chl. tepidum ( ATCC 49652) were photoautotrophically grown in liquid cultures containing different concentrations of sodium sulfide (Na2S). BChl-c homologs possessing a methyl group at the 12-position tended to increase in cells of the two strains of Chl. vibrioforme cultured under high Na2S concentrations. In contrast, the Na2S concentration in liquid cultures did not affect the relative composition of BChl-c homologs in Chl. tepidum. 8-Propyl-12- methyl([ P, M])-BChl-c homolog, which has been little observed in usual cultivations, could be isolated by reverse-phase high-performance liquid chromatography from the cells of Chl. vibrioforme grown under high Na2S contents. The [P, M]-BChl-c homolog has the R-configuration at the 3(1)-position, which was determined by H-1-NMR analyses.
  • J Harada; Y Saga; H Oh-oka; H Tamiaki
    PHOTOSYNTHESIS RESEARCH 86 1-2 137 - 143 2005年11月 [査読有り]
     
    Two sub-strains of the anoxygenic photosynthetic green sulfur bacterium Chlorobium vibrioforme NCIB 8327 were derived from the same clone and could be discriminated only by their possession of either bacteriochlorophyll (BChl) c or d as the major pigment in the peripheral light-harvesting antenna system, chlorosome (Saga Y et al. (2003) Anal Sci 19: 1575-1579). In the presence of a proper amount of oxygen in the initial culture medium, the BChl d strain showed longer retardation on its growth initiation than the BChl c strain, indicating that the latter was advantageous for survival under aerobic light conditions which produced reactive oxygen species in vivo. The result would be ascribable to the difference of the midpoint potentials between two kinds of chlorosomes formed by self-aggregates of BChl c and d as measured by their fluorescence quenching.
  • Y Saga; H Tamiaki; Y Shibata; S Itoh
    CHEMICAL PHYSICS LETTERS 409 1-3 34 - 37 2005年06月 [査読有り]
     
    The excitation energy transfer from bacteriochlorophyll(BChl)-c self-aggregates to energy-accepting BChl-a in proteins (baseplates) in an individual photosynthetic light-harvesting complex (chlorosome) of a green filamentous photosynthetic bacterium Chloroflextis aurantiacus was successfully observed at cryogenic temperature. The ratio of intensity of the fluorescence peak of BChl-a to that of BChl-c self-aggregates in individual chlorosomes, which demonstrated relative efficiency of the excitation energy transfer, was heterogeneous between 0.09 and 0.72. This suggests that excitonic interaction between BChl-c self-aggregates and BChl-a in baseplates was heterogeneous among individual chlorosomes. (c) 2005 Elsevier B.V. All rights reserved.
  • J Harada; Y Saga; Y Yaeda; H Oh-Oka; H Tamiaki
    FEBS LETTERS 579 9 1983 - 1987 2005年03月 [査読有り]
     
    The activity of a methyltransferase, BchU, which catalyzes methylation at the C-20 position of chlorin ring in the biosynthetic pathway of bacteriochlorophyll c, was investigated in vitro. The bchU gene derived from the photosynthetic green sulfur bacterium, Chlorobium tepidum, was overexpressed in Escherichia coli as a His-tagged protein (HiS(6)-BchU), and the enzyme was purified. In the presence of S-adenosylmethionine, HiS6-BchU methylated zinc bacteriopheophorbide d at the C-20 position to give zinc bacteriopheophorbide c. Metal-free bacteriopheophorbide d could not be methylated by the BchU, indicating that the central metal in the chlorin should be required for the recognition by the BchU. (c) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.
  • 柴田 穣; 佐賀 佳央; 民秋 均
    生物物理 45 S273  一般社団法人 日本生物物理学会 2005年
  • Y Saga; H Tamiaki
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY 75 1-2 89 - 97 2004年07月 [査読有り]
     
    Chlorosomes containing bacteriochlorophyll(BChl)-c and those containing BChl-d were isolated from two substrains of Chlorobium vibrioforme f. sp. thiosulfatophilum NCIB 8327, respectively. The two types of chlorosomes were investigated from the following aspect, what kinds of effects the molecular structure of chlorosomal BChls had on structural and spectroscopic properties of in vivo self-aggregates in chlorosomes without alteration of the other components such as chlorosomal proteins and lipids; both chlorosomes were expected to have the same components except for light-harvesting BChls. In their visible absorption spectra, the differences of Soret and Q(y) peak positions between BChl-c containing and BChl-d containing chlorosomes were similar to the differences between monomeric BChl-c and d. An inverse S-shaped CD signal in the Q(y) region of BChl-d containing chlorosomes was 1.4 times larger than that of BChl-c containing chlorosomes, when the Q(y) absorbance of the two chlorosomes was almost the same. This implies that the excitonic interaction of BChl-d is larger than that of BChl-c in natural chlorosomes. Resonance Raman spectroscopy showed that BChl self-assemblies in both chlorosomes were essentially formed by the same local structural interaction among 3(1)-hydroxy group, 13-keto group, and central magnesium. BChl-d self-aggregates in chlorosomes were more tolerant of 1-hexanol than in vivo BChl-c aggregates, suggesting that the molecular structure of BChl-d provided more stable self-assemblies than BChl-c in natural chlorosomes. (C) 2004 Elsevier B.V. All rights reserved.
  • Y Saga; S Akai; T Miyatake; H Tamiaki
    CHEMISTRY LETTERS 33 5 544 - 545 2004年05月 [査読有り]
     
    Silicate nanocapsules including self-aggregates of zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (zinc chlorin) and methyl bacteriopyropheophorbide-a (bacteriochlorin) were prepared by a sol-gel process. Excitation energy transfer from self-aggregates of zinc chlorins to an energy-accepting bacteriochlorin occurred in the silicate nanocapsule, which was similar to a light-harvesting complex (chlorosome) of green photosynthetic bacteria.
  • Y Saga; H Tamiaki
    JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY 73 1-2 29 - 34 2004年01月 [査読有り]
     
    A chlorophyll analog forming self-assembled monolayers (SAMs) on a gold surface was synthesized for the first time. 13(2)- (Demethoxycarbonyl)pheophorbide-a, which was converted from naturally occurring chlorophyll-a, was condensed with 2-hydroxyethyl disulfide to give a chlorin dyad linked by a disulfide bond. The chlorin analog was spontaneously immobilized on a gold substrate by soaking in an acetone solution of the dyad for 24 h. The resulting gold plate exhibited a visible absorption spectrum with about 420- and 675-nm maxima as the Soret and Q(y) peaks, respectively, indicating that chlorin pi-conjugates were modified on the gold substrate through Au-S bonding. Both visible absorption and fluorescence emission bands of the chlorin chromophores on the gold substrate were red-shifted compared with those of the synthesized chlorin dyad in a homogeneous acetone solution. The measured absorbance at the Soret maximum suggests that the chlorin chromophores on the gold plate were densely packed on a gold surface to form a SAM. Cathodic photocurrents were generated from SAMs of the chlorins on a gold substrate with irradiation of visible-lights above 400 nm. Photoinduced electron transfer from chlorins on the gold substrate to oxygen molecules in an electrolyte solution were attributed to the cathodic photocurrent generation. (C) 2003 Elsevier B.V. All rights reserved.
  • Y Saga; H Tamiaki
    CELL BIOCHEMISTRY AND BIOPHYSICS 40 2 149 - 165 2004年 [査読有り][招待有り]
     
    Recent spectroscopic studies of photosynthetic light-harvesting supramolecular complexes at the single supramolecule level are reviewed. This report describes the "single-molecule" investigation on light-harvesting complex 2 (LH2) of purple photosynthetic bacteria, phycobiliproteins of cyanobacteria and red algae, light-harvesting complex 2 (LHC2) of higher plants, and chlorosomes of green photosynthetic bacteria. Unique behaviors and spectral features of single light-harvesting apparatus have been unraveled that were hidden by the ensemble averaging of many of the complexes. The information obtained will be useful for understanding the electronic structures and energy-transfer mechanism of photosynthetic light-harvesting supramolecular systems.
  • J Harada; H Oh-oka; Y Saga; H Tamiaki
    PORTABLE SYNCHROTRON LIGHT SOURCES AND ADVANCED APPLICATIONS 716 200 - 200 2004年 [査読有り]
  • Y Saga; H Oh-Oka; T Hayashi; H Tamiaki
    ANALYTICAL SCIENCES 19 12 1575 - 1579 2003年12月 [査読有り]
     
    The light-dependent composition change of light harvesting bacteriochlorophyll(BChl)s in the present culture of a green sulfur photosynthetic bacterium Chlorobium (Chl.) vibrioforme f. sp. thiosulfatophilum strain NCIB 8327 was investigated by visible absorption spectroscopy and HPLC analyses. When the culture was repeatedly grown in liquid media under a low light condition, both the Soret and Q(y) absorption bands of the in vivo spectrum were shifted to longer wavelengths. Analysis of the extracted pigments by HPLC revealed that the ratio of the amount of BChl-c to that of BChl-d molecules gradually increased during repeated cultivation. In contrast, when the culture grown under a low light intensity was transferred to a high light condition and continued to be grown, the absorption bands were shifted to shorter wavelengths and the ratio of BChls-c/d decreased finally to the almost original value. Colonies were prepared on solid agar media from the liquid culture containing both BChls-c and d, which was grown under a low light intensity. Each colony obtained was found to contain either BChl-c or d, but not both of them. Two types of cells isolated in this study were derived from the same clone, judged from their genetic analyses. The variation of pigment composition in our liquid culture observed here could be ascribed to the difference of growth rates between two substrains containing BChl-c and BChl-d, respectively, depending on light conditions.
  • H Tamiaki; M Omoda; Y Saga; H Morishita
    TETRAHEDRON 59 24 4337 - 4350 2003年06月 [査読有り]
     
    Homologously pure methyl bacteriopheophorbides-e and f (BPhes-a/f(M)) were prepared from modification of naturally occurring bacteriochlorophylls-c and d (BChls-c/d), respectively, by transformation of the methyl to formyl group at the 7-position. The absolute configuration of the 1-hydroxyethyl group at the 3-position of (Zn-)BPhes-e/f(M) was determined from comparison with structurally known BChl-c/d epimers. Visible spectra of synthetic (Zn-)BPhe-c/d/e/f(M) showed that the 7(1)-oxidation and the 8(2)/12(1)/20-methylation affected Soret, Q(x) and Q(y) bands of both the monomeric (in a polar organic solvent) and oligomeric species (in a non-polar solvent). (C) 2003 Elsevier Science Ltd. All rights reserved.
  • 柴田 穣; 佐賀 佳央; 民秋 均; 伊藤 繁
    生物物理 43 S201  一般社団法人 日本生物物理学会 2003年
  • T Mizoguchi; Y Saga; H Tamiaki
    PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES 1 10 780 - 787 2002年10月 [査読有り]
     
    Eight bacteriochlorophyll (BChl)-d homologs and epimers were isolated from a strain of the green sulfur bacterium Chlorobium vibrioforme. By a combination of mass spectrometry and H-1-NMR spectroscopy using the chemical shifts of meso- and 3(1)-protons and H-1-H-1 NOE correlations, the molecular structures were determined as (3(1)R)-8-ethyl- 12-methyl, (3(1)R)-8-ethyl-12-ethyl. (3(1)R)-8-propyl-12-methyl, (3(1)S)-8-propyl-12-methyl, (3(1)R)-8-propyl-12-ethyl, (3(1)S)-8-propyl-12-ethyl, (3(1)S)-8-isobutyl-12-methyl and (3(1)S)-8-isobutyl-12-ethyl. The aggregation behavior of the epimerically pure BChls-d in hydrophobic organic solvents was examined to investigate the absolute configuration of the 3-(1-hydroxyethyl) group as well as the bulkiness of the C8 and C12 side-chains by using electronic-absorption and fluorescence-emission spectroscopies. At high concentration of the BChls-d in CH2Cl2, the absolute configuration of the 3-(1-hydroxyethyl) group governed the formation of a subunit as a building block for the subsequent higher assembly. Upon dilution of the resulting Subunit with hexane. the bulkiness of the C8 and C12 side-chains were found to affect the association of the subunits differently: the bulkiness of the C8 side-chain acted as a promoter for the association due to a stabilized hydrophobic interaction among the relevant larger side-chain, whereas the bulkiness of the C12 side-chain acted as an inhibitor for that association due to introduction of a particular steric-hindrance around the side-chain in the aggregates.
  • Y Saga; T Ishikawa; T Watanabe
    BIOELECTROCHEMISTRY 57 1 17 - 22 2002年07月 [査読有り]
     
    The transient photocurrent response from bacteriorhodopsin (bR) on tin oxide electrodes was strongly influenced by metal ions bound to bR molecules. The photocurrent polarity reversal pH, which corresponded to the pH value for the reversal of the proton release/uptake sequence in the bR photocycle, of cation-substituted purple membrane (PM) was shifted to lower pH with the increase in the cation affinities to carboxyl groups and a close correlation was noted between the two values. This suggests that the metal ion present in the extracellular region of a bR molecule modulates the pK(a) of proton release groups of bR by stabilizing the ionized state of the proton-releasing glutamic acids. The behavior of photocurrents at light-off in alkaline media, reflecting the proton uptake by bR, was unchanged by binding monovalent (Na+ and K+) or divalent cations (Mg2+ and Ca2+), but was drastically changed by binding La3+ ions. This can be explained by invoking a substantial slowing of the proton uptake process in the presence of La3+ (C) 2002 Elsevier Science B.V. All rights reserved.
  • Y Saga; T Wazawa; T Mizoguchi; Y Ishii; T Yanagida; H Tamiaki
    PHOTOCHEMISTRY AND PHOTOBIOLOGY 75 4 433 - 436 2002年04月 [査読有り]
     
    The fluorescence emission properties of single chlorosomes from the green sulfur photosynthetic bacterium Chlorobium (Chl.) tepidum are studied for the first time, using a total internal reflection fluorescence microscope. The fluorescence peak positions of bacteriochlorophyll (BChl)-c self-aggregates in a single chlorosome of Chl tepidum were widely distributed in the wavelength region between 750 and 768 nm, and the standard deviation (s.d. = 4.1 nm, n = 51) was larger than that of single chlorosomes of Chloroflexus (Cfl.) (s.d. = 1.9 nm, n = 50). The spectral heterogeneity among single chlorosomes from Chl tepidum was in sharp contrast to those from Cfl. auranthiacus. The difference of chlorosomal spectral properties between Chl tepidum and Cfl. aurantiacus at the single-unit level would be ascribed to the homolog composition of BChl-c-chlorosomes of Chl tepidum have BChl substituted with various alkyl groups at both the 8- and 12-positions, whereas light-harvesting BChl-c molecules in Cfl. chlorosomes have the same substituents at the 8- (ethyl group) and 12- (methyl group) positions.
  • Y Saga; T Miyatake; H Tamiaki
    BIOORGANIC & MEDICINAL CHEMISTRY LETTERS 12 8 1229 - 1231 2002年04月 [査読有り]
     
    Zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (1), a good model compound of light-harvesting pigments of green photosynthetic bacteria. formed self-aggregates in the presence of octadecyltriethoxysilane and tetraethoxysilane in an aqueous solution to exhibit visible absorption spectra similar to the natural antenna. Base-catalyzed cross-linked polymerization of the additive Si-ORs (R = ethyl and H) afforded the formation of a siloxane network (Si-O-Si) on the surface of the self-assemblies of 1. The resulting microcapsules were stable to tolerate the deaggregation to monomeric 1 by addition of surfactant Triton X-100 more largely than the corresponding micelles before polymerization. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • Yoshitaka Saga; Tetsuichi Wazawa; Toshitaka Nakada; Yoshiharu Ishii; Toshio Yanagida; Hitoshi Tamiaki
    Journal of Physical Chemistry B 106 6 1430 - 1433 2002年02月 [査読有り]
     
    The fluorescence emission properties of supramolecular light-harvesting complexes (= chlorosomes) from a green filamentous photosynthetic bacterium (Chloroflexus aurantiacus) on a quartz plate were studied for the first time at the single-unit level with a total internal reflection fluorescence microscope. Atomic force microscope observation evidenced that most chlorosomes were independently situated on the quartz plate. All the fluorescence bands from bacteriochlorophyll-c aggregates of a single chlorosome had a similar peak position and also a similar bandwidth. The conservative spectroscopic properties of each Chloroflexus chlorosome indicate that the presence of stereoisomers at the 31-position of bacteriochlorophyll-c molecules (31-R/S = 2/1) does not provide any heterogeneity of chlorosomal pigment organization among Chloroflexus chlorosomes.
  • Y Saga; T Wazawa; T Nakada; Y Ishii; T Yanagida; H Tamiaki
    JOURNAL OF PHYSICAL CHEMISTRY B 106 6 1430 - 1433 2002年02月 [査読有り]
     
    The fluorescence emission properties of supramolecular light-harvesting complexes (= chlorosomes) from a green filamentous photosynthetic bacterium (Chloroflexus aurantiacus) on a quartz plate were studied for the first time at the single-unit level with a total internal reflection fluorescence microscope. Atomic force microscope observation evidenced that most chlorosomes were independently situated on the quartz plate, All the fluorescence bands from bacteriochlorophyll-c aggregates of a single chlorosome had a similar peak position and also a similar bandwidth. The conservative spectroscopic properties of each Chloroflexus chlorosome indicate that the presence of stereoisomers at the 3(1)-position of bacteriochlorophyll-c molecules (3(1)-R/S = 2/1) does not provide any heterogeneity of chlorosomal pigment organization among Chloroflexus chlorosomes.
  • Y Saga; K Matsuura; H Tamiaki
    PHOTOCHEMISTRY AND PHOTOBIOLOGY 74 1 72 - 80 2001年07月 [査読有り]
     
    Self-aggregation of naturally occurring bacteriochlorophyll (BChl)-e in nonpolar organic solvents was investigated by visible absorption, fluorescence emission and circular dichroism spectra, Cultured brown-colored photosynthetic bacteria have several BChl-e as light-harvesting antenna pigments. Three major BChl-e homologs were separated from the extracts of the culture by reverse-phase high-performance liquid chromatography (HPLC) and characterized by H-1-NMR and fast-atom bombardment mass spectroscopy: 8-ethyl-12-ethyl ([E,E])-, 8-propyl-12-ethyl- and 8-isobutyl-12-ethyl-BChl-e farnesyl esters, All the homologs consisted of a mixture of the 3(1)-epimers, and epimerically pure BChl-e were also given by HPLC separation. All the separated BChl-e epimers, the epimeric mixtures and the homologous mixtures formed self-aggregates in 2% dichloromethane/hexane, giving visible absorption spectra similar to that of the whole cells, which showed two peaks (or shoulders) around 430-450 and 520 nm at the Soret region as well as a red-shifted Q(y) band relative to the monomeric, The spectral properties of the Soret band were basically unchanged among the epimers or epimeric/homologous mixtures. In contrast, the Q(y) band of aggregates of epimeric mixtures (except [E,E]) and homologous mixtures red-shifted and broadened compared with the epimerically pure. The red-shift and broadening of the Q(y) band are advantageous for efficient energy transfer from BChl-e aggregates to BChl-a in a baseplate in chlorosomes because their spectral overlap increases.
  • Y Saga; T Ishikawa; T Watanabe
    CHEMISTRY LETTERS 2 106 - 107 2001年02月 [査読有り]
     
    Lanthanum ions (La3+) drastically changed the pH dependence of the photocurrent responses from bacteriorhodopsin (bR). The pH value of the photocurrent polarity reversal was shifted to lower pH and the ratio of the light-off to the light-on photocurrent peak intensities, I-off/I-on, was significantly decreased, compared with the photoresponse from the wild-type bR. These results are explained by invoking the effect of La3+ on the bR proton pump activity.
  • Y Saga; T Watanabe; K Koyama; T Miyasaka
    ANALYTICAL SCIENCES 15 4 365 - 369 1999年04月 [査読有り]
     
    The photoelectrochemical response of bacteriorhodopsin (bR) was studied in buffered electrolyte solutions. The photocurrent intensity was inversely proportional to the buffer concentration in a wide range, and the response time became shorter with increasing buffer concentration. When plotted as a function of the electrolyte pH, the ratio of the peak photocurrent in the presence of a buffer to that in the absence of the buffer exhibited a minimum at a pH nearly equal to the pK(a)' of the buffer. These results are explained by invoking the buffering capacity, and are in line with the view that the transient bR photoresponse originates in the pH change near the electrode due to proton release/uptake by bR molecules.
  • Y Saga; T Watanabe; K Koyama; T Miyasaka
    JOURNAL OF PHYSICAL CHEMISTRY B 103 1 234 - 238 1999年01月 [査読有り]
     
    Excitation of bacteriorhodopsin (bR) at an electrode-electrolyte interface generates transient photocurrents as evidenced by the turning of an incident light on and off. By use of a gold electrode as substrate, on which an oxide layer can be formed in a controlled manner, we have found two types of photocurrents, both originating in the excitation of bR. For the first type, arising most probably from the pH response of the surface oxide layer due to proton release/uptake by bR, the magnitude of photocurrent well paralleled the amount of surface oxide up to about one monolayer of Au2O3 and reached a maximum roughly equal to those observed on SnO2 electrodes. The second type of photocurrent, being 3- to 4-fold smaller than the first one and essentially potential independent, arises presumably from simple charging, again through proton release/uptake by bR, of the electric double layer at the interface.
  • Y Saga; T Watanabe; K Koyama; T Miyasaka
    CHEMISTRY LETTERS 9 961 - 962 1998年09月 [査読有り]
     
    The transient photocurrent response of bacteriorhodopsin (bR) at electrode-electrolyte interfaces showed a drastic pH dependence, reversing at a low pH. The ratio of the photocurrent by turning the light off to that at the onset of illumination, I-off/I-on, decreased systematically from 0.92 to 0 by raising pH. These results can nearly be correlated with the proton release/uptake sequence in the bR photocycle.
  • M Endo; Y Azuma; Y Saga; A Kuzuya; G Kawai; M Komiyama
    JOURNAL OF ORGANIC CHEMISTRY 62 4 846 - 852 1997年02月 [査読有り]
     
    Phosphoramidite monomers having a benzyl or ethyl ester in the side chain have been synthesized for the facile and selective functionalization of oligonucleotides. Various amino compounds were incorporated into the desired sites in oligonucleotides by aminolysis of the esters. The conjugates, in which oligoamines (catalysts for RNA hydrolysis) were interposed between two oligonucleotides, hydrolyzed RNA substrates at exactly the target sites, which was consistent with the results of a computer-modeling study.
  • M ENDO; Y SAGA; M KOMIYAMA
    TETRAHEDRON LETTERS 35 32 5879 - 5882 1994年08月 [査読有り]
     
    A phosphoramidite monomer, which has a benzyl ester moiety in the side chain and is useful for the site-selective introduction of functional groups into oligonucleotides via various tethers, has been synthesized.

MISC

産業財産権

  • 特開2006-055132:メチル基転移酵素、そのDNA、及び特定部位にメチル基が導入されたポルフィリン型化合物の製造方法  2006年03月02日
    民秋 均, 原田 二朗, 佐賀 佳央

受賞

  • 2004年03月 日本化学会 第18回若い世代の特別講演会 講演者
     
    受賞者: 佐賀 佳央

共同研究・競争的資金等の研究課題

  • 日本学術振興会：科学研究費助成事業 基盤研究(C)
    研究期間 : 2021年04月 -2025年03月 
    代表者 : 佐賀 佳央
     
    天然光合成システムにおいて、光収穫タンパク質は光合成反応に必要な光エネルギーを捕集する重要な色素タンパク質である。これらの光収穫タンパク質には、励起子相互作用している色素集団と励起状態がほぼ局在化している色素単量体が共存し、両者の関係性が励起エネルギー移動に重要となっている場合が多い。そこで、紅色光合成細菌の光収穫タンパク質を主な研究ターゲットとして、結合色素の置換によって、環状に配列している色素集団と色素単量体の機能的関係性を改変・制御し、光機能への影響を明らかにするための研究を推進している。 本年度は、紅色光合成細菌の辺縁光収穫タンパク質light-harvesting complex 2（LH2）においてエネルギー供与体として機能する単量体状態の色素（B800バクテリオクロロフィルa）の改変を主に実施した。第一に、紅色光合成細菌から単離精製したLH2タンパク質からB800バクテリオクロロフィルaを選択的に脱離させたのちに、複数種の異種クロロフィル色素をB800結合ポケットに再構成することに成功し、それらの色素の電子状態やタンパク質との相互作用、ならびにエネルギー受容体であるB850バクテリオクロロフィルaへの励起エネルギー移動を解析した。第二に、紅色光合成細菌から単離精製したLH2タンパク質を酸化剤で処理することで、LH2タンパク質に結合した状態のB800バクテリオクロロフィルaを直接的に3-アセチルクロロフィルaに変換する方法論を確立した。この方法論を用いることで、LH2タンパク質に存在する単量体状態のエネルギー供与体を2種類共存させることにも成功し、このような改変LH2タンパク質についてエネルギー移動経路を含めた励起エネルギー移動ダイナミクスを明らかにした。
  • 日本学術振興会：科学研究費助成事業 基盤研究(C)
    研究期間 : 2018年07月 -2022年03月 
    代表者 : 佐賀 佳央
     
    未利用バイオマスの活用は持続可能型社会を形成するうえで重要であると考えられる。それらのバイオマスのなかでも地上に豊富に存在しながら利用があまり進んでいない木質・草木バイオマスの活用は、バイオマス研究分野における重要課題のひとつであると考えられる。この課題に対する大きな障壁のひとつとしてリグニンの難分解性が挙げられ、それに対する取り組みがこれまで進められてきたが、低いリグニン分解効率や大きな環境負荷を伴うプロセスの場合があることがこれらの研究の問題点のひとつであると考えられる。そこで本研究では、植物バイオマスに含まれるクロロフィルを活用した、太陽光による環境低負荷型リグニン分解を目指した研究を推進している。具体的には、複数種の光合成生物から単離した天然クロロフィル類を用いて誘導体化を行い、テトラピロール環の共役系やテトラピロール環に連結している置換基を変化させた。これらによって、吸収可能な波長領域を調節することができた。あわせて、クロロフィル類の分子構造改変を行うときの化学反応性に対する分子構造の要因の解明に関する研究を推進した。また、天然クロロフィル類に配位しているマグネシウムを脱離させたのちに、遷移金属イオンを配位させることで、酸化還元電位や励起三重項状態への項間交差による一重項酸素の発生効率を変化させるための誘導体化を行った。また、得られたクロロフィル誘導体の可視光励起による一重項酸素発生効率を、溶液中やタンパク質に結合した状態で測定し解析を進めた。
  • 日本学術振興会：科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2018年04月 -2020年03月 
    代表者 : 佐賀 佳央
     
    光合成の初期過程で重要な光収穫タンパク質のエネルギーフローに関する構造機能相関の解明には、構造解明されているタンパク質を積極的に改変し、光機能への影響を解析することが有用であると考えられる。そこで、紅色光合成細菌の光収穫タンパク質を主なターゲットとし、バクテリオクロロフィル（BChl）a色素の改変や新たな環状テトラピロール色素の導入を行い、それらの改変タンパク質の機能解析を推進した。 紅色光合成細菌のLH2タンパク質に関する研究では、原子レベルで立体構造が解明されているRhodoblastus acidophilusとPhaeospirillum molischianumの2種類の細菌に由来するLH2タンパク質について、B800 BChl aの結合部位へ側鎖やテトラピロール環骨格が異なった異種クロロフィル色素を再構成することに成功するとともに、再構成色素の分子構造がタンパク質との相互作用や励起エネルギー移動に与える影響を明らかにした。また、BChl bをLH2タンパク質に再構成することでB800 BChl aよりもQy吸収帯を長波長側にシフトさせ、エネルギーアクセプターであるB850 BChl aとのスペクトルの重なりを大きくし、その励起エネルギー移動を解析することにも成功した。あわせて、LH2タンパク質のB800 BChl a結合部位に存在する色素を、タンパク質に結合した状態のままで直接的に分子構造を改変することに成功した。あわせて、2種類の紅色光合成細菌に由来するLH1タンパク質の色素改変に関する研究も推進した。LH2タンパク質に関する研究成果に関しては、論文発表と学会発表を行った。
  • 日本学術振興会：科学研究費助成事業 基盤研究(C)
    研究期間 : 2017年04月 -2020年03月 
    代表者 : 佐賀 佳央
     
    光合成システムにおける異種の色素集積超分子の間でのエネルギー移動の理解は、光生命化学の重要課題である。そこで本研究では、緑色光合成細菌の光捕集システム・クロロゾームにおける内部の色素自己集積体から表面の色素結合膜タンパク質へのエネルギー移動を主なターゲットとした研究を推進した。クロロゾーム表面のエネルギー受容型膜タンパク質を両親媒性の合成色素に置換することに成功し、クロロゾーム内部の色素自己集積体で吸収した光エネルギーを、超分子間励起エネルギー移動によってクロロゾーム膜面の合成色素に集約させることが可能なことを示した。
  • 日本学術振興会：科学研究費助成事業 国際共同研究加速基金(国際共同研究強化)
    研究期間 : 2018年 -2019年 
    代表者 : 佐賀 佳央
     
    光合成アンテナ超分子複合体は光合成機能の効率化のために光エネルギーを集めるとともに過剰なエネルギーは捨てるというインテリジェンスな機能を有する。このような機能の理解を目指して、化学的に改変した光合成超分子複合体を構築し、それらの機能を解析するために、オランダのアムステルダム自由大学との国際共同研究を推進した。光合成アンテナ超分子複合体において重要な役割を果たしているクロロフィル類やカロテノイド類を再構成的方法論によって導入し、得られた光合成アンテナ超分子複合体の分光特性や励起エネルギー移動の特性を明らかにした。
  • 光合成タンパク質における規則的ナノ空隙群の創成
    科学技術振興機構：戦略的創造研究推進事業(さきがけ)
    研究期間 : 2014年10月 -2018年03月 
    代表者 : 佐賀 佳央
  • 緑色光合成細菌の光捕集アンテナ超分子のエネルギー受容膜面の機能化
    文部科学省：科学研究費補助金(新学術領域研究(研究領域提案型))
    研究期間 : 2015年04月 -2017年03月 
    代表者 : 佐賀 佳央
  • 文部科学省：科学研究費補助金(新学術領域研究(研究領域提案型))
    研究期間 : 2013年 -2014年 
    代表者 : 佐賀 佳央
     
    高効率で太陽光エネルギー捕集・伝達を行う天然型の光合成光捕集超分子の良さを生かしながら人工光合成に展開するためには、精密な色素集積超分子構造を保持しつつ、光機能性や安定性の制御や新たな機能性付与を行う方法論が重要である。そこで本研究では、緑色光合成細菌の膜外集光アンテナ超分子・クロロゾームを研究対象とし、構成分子の生合成過程を利用してクロロゾーム改変を行う方法論の開発を目的とした研究を推進した。あわせて、クロロゾーム膜面のエネルギー受容部位へ合成分子を再構成することによる機能化法の開発も推進した。 平成26年度においては、クロロゾーム内部で光機能性部位を形成する集光バクテリオクロロフィル分子の生合成過程の最終反応を利用して、それらの長鎖エステル鎖を改変した。その結果として、緑色硫黄光合成細菌Chlorobaculum tepidumの細胞内でバクテリオクロロフィルcの17位エステル鎖末端にハロゲン原子や不飽和結合を導入することに成功した。このようなエステル鎖を有するバクテリオクロロフィルcは細胞内でクロロゾームの光機能性部位に取り込まれることが示された。また、クロロゾームのバクテリオクロロフィル自己会合体で捕集した光エネルギーをクロロゾーム膜面で受容するベースプレート色素タンパク質の部分を合成機能分子で置き換え天然型の光捕集超分子を合成系と融合させることを目的として、両親媒性の機能分子を合成しベースプレート除去クロロゾームへの再構成を行った。 これらの研究成果は、査読付論文4報、学会発表8件で報告した。
  • 文部科学省：科学研究費補助金(基盤研究(C))
    研究期間 : 2011年 -2013年 
    代表者 : 佐賀 佳央
     
    光化学系II型の光合成反応中心の一次電子受容体の生成やクロロフィル代謝で重要な反応であるクロロフィル類の中心金属の脱離反応（脱金属反応）を生体外で速度論的に解析し、分子構造に依存した脱金属反応特性を明らかにした。また、クロロフィル脱金属反応によって緑色光合成細菌の主要な光捕集超分子であるクロロゾーム型の色素集積超分子の分解を誘起し、その挙動を構成クロロフィル分子の脱金属反応特性に関する情報を取り入れて解析した。
  • 文部科学省：科学研究費補助金(若手研究(B))
    研究期間 : 2008年 -2009年 
    代表者 : 佐賀 佳央
     
    集光バクテリオクロロフィル分子の自己組織化で光機能性部位が構築されている緑色光合成細菌の集光アンテナ超分子・クロロゾームを研究対象とし、緑色光合成細菌の生育環境の変化やクロロゾームへの摂動の印加によるクロロゾームの構成成分や超分子構造の変化を誘起・制御する方法論を開発するとともに、それらによって誘起されるスペクトル特性やエネルギー移動特性などの光合成アンテナ機能の変化を解析し、クロロゾームの光機能性部位の超分子構造と機能の相関を明らかにすることを目指した研究を行った。また、クロロゾームのモデル超分子、およびクロロゾームの構成色素分子を対象とした研究もあわせて行った。緑色硫黄光合成細菌の生育環境の変化によって集光バクテリオクロロフィル異性体組成の変化誘起に成功し、クロロゾーム機能への影響を解析した。また、クロロゾームモデル超分子に摂動を与えることで誘起させる分光特性変化を明らかにするとともに、集光バクテリオクロロフィル分子の酸性条件での脱金属反応特性を解明した。
  • 文部科学省：科学研究費補助金(若手研究(B))
    研究期間 : 2006年 -2007年 
    代表者 : 佐賀 佳央
     
    光合成初期過程における集光アンテナ超分子の高効率光エネルギー捕集・伝達機構の解明は光合成の基礎研究のみならず、超分子化学や光エネルギー変換の研究分野に大きく貢献することが期待できる。そこで、集光バクテリオクロロフィル色素の自己組織化で光機能性部位が構築されている緑色光合成細菌の集光アンテナ・クロロゾームを対象とし、単一超分子分光測定と多数を対象とした物性測定の観点からその励起状態とエネルギー移動機構の解明を目指した研究を行った。二種類の緑色光合成細菌(Chloroflexus aurantiacusとChlorobium tepidum)から単離・精製したクロロゾームを対象として、極低温で共焦点レーザー蛍光顕微鏡による測定を行い,単一超分子レベルでの検出を行うことでひとつのクロロゾームに含まれる集光バクテリオクロロフィルの分子数を推定することに成功した。あわせて、単一クロロゾームレベルでの蛍光異方性の測定に成功し、クロロゾームの超分子構造との関連について議論を行った。また、緑色硫黄光合成細菌のクロロゾームに含まれるバクテリオクロロフィルcとバクテリオクロロフィルeの脱金属反応の速度論解析を行い、バクテリオクロロフィルの分子構造が脱金属反応に与える影響を検討した。
  • 色素集積型光合成アンテナとその人工モデルの単一超分子レベルでの解析
    日本学術振興会：特別研究員奨励費（SPD）
    研究期間 : 2003年04月 -2005年03月 
    代表者 : 佐賀 佳央
  • 光合成アンテナの分光特性と色素組成の単一超分子レベルでの解析
    日本学術振興会：特別研究員奨励費（PD）
    研究期間 : 2002年04月 -2003年03月 
    代表者 : 佐賀 佳央

担当経験のある科目

  • 卒業研究近畿大学
  • 卒業研究ゼミナール近畿大学
  • 超分子化学近畿大学
  • 化学実験近畿大学
  • 教員採用試験対策講座（化学）近畿大学
  • 社会奉仕実習近畿大学
  • インターンシップ近畿大学
  • 基礎ゼミ近畿大学
  • 情報処理近畿大学
  • 科学技術英語近畿大学
  • 生物無機化学近畿大学
  • 分子生物化学近畿大学
  • 基礎生化学近畿大学
  • 生体反応化学特論近畿大学大学院
  • 生体分子化学特論近畿大学大学院

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