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Kawakami T, Yu LJ, Liang T, Okazaki K, Madigan MT, Kimura Y, Wang-Otomo ZY. Crystal structure of a photosynthetic LH1-RC in complex with its electron donor HiPIP. Nat Commun 2021; 12:1104. [PMID: 33597527 PMCID: PMC7889895 DOI: 10.1038/s41467-021-21397-9] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2020] [Accepted: 01/26/2021] [Indexed: 11/12/2022] Open
Abstract
Photosynthetic electron transfers occur through multiple components ranging from small soluble proteins to large integral membrane protein complexes. Co-crystallization of a bacterial photosynthetic electron transfer complex that employs weak hydrophobic interactions was achieved by using high-molar-ratio mixtures of a soluble donor protein (high-potential iron-sulfur protein, HiPIP) with a membrane-embedded acceptor protein (reaction center, RC) at acidic pH. The structure of the co-complex offers a snapshot of a transient bioenergetic event and revealed a molecular basis for thermodynamically unfavorable interprotein electron tunneling. HiPIP binds to the surface of the tetraheme cytochrome subunit in the light-harvesting (LH1) complex-associated RC in close proximity to the low-potential heme-1 group. The binding interface between the two proteins is primarily formed by uncharged residues and is characterized by hydrophobic features. This co-crystal structure provides a model for the detailed study of long-range trans-protein electron tunneling pathways in biological systems. The high potential iron-sulfur (HiPIP) proteins are direct electron donors to the light-harvesting-reaction center complexes (LH1-RC) in photosynthetic β- and γ-Proteobacteria. Here, the authors present the 2.9 Å crystal structure of the HiPIP-bound LH1-RC complex from the thermophilic purple sulfur bacterium Thermochromatium tepidum and discuss mechanistic implications for the electron transfer pathway.
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Affiliation(s)
| | - Long-Jiang Yu
- Photosynthesis Research Center, Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing, China.
| | - Tai Liang
- Faculty of Science, Ibaraki University, Mito, Japan
| | | | - Michael T Madigan
- Department of Microbiology, Southern Illinois University, Carbondale, IL, USA
| | - Yukihiro Kimura
- Department of Agrobioscience, Graduate School of Agriculture, Kobe University, Nada, Kobe, Japan.
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Burggraf F, Koslowski T. Charge transfer through a cytochrome multiheme chain: Theory and simulation. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2014; 1837:186-92. [DOI: 10.1016/j.bbabio.2013.09.005] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/05/2013] [Revised: 08/22/2013] [Accepted: 09/10/2013] [Indexed: 10/26/2022]
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Collins AM, Kirmaier C, Holten D, Blankenship RE. Kinetics and energetics of electron transfer in reaction centers of the photosynthetic bacterium Roseiflexus castenholzii. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2010; 1807:262-9. [PMID: 21126505 DOI: 10.1016/j.bbabio.2010.11.011] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/24/2010] [Revised: 11/18/2010] [Accepted: 11/19/2010] [Indexed: 10/18/2022]
Abstract
The kinetics and thermodynamics of the photochemical reactions of the purified reaction center (RC)-cytochrome (Cyt) complex from the chlorosome-lacking, filamentous anoxygenic phototroph, Roseiflexus castenholzii are presented. The RC consists of L- and M-polypeptides containing three bacteriochlorophyll (BChl), three bacteriopheophytin (BPh) and two quinones (Q(A) and Q(B)), and the Cyt is a tetraheme subunit. Two of the BChls form a dimer P that is the primary electron donor. At 285K, the lifetimes of the excited singlet state, P*, and the charge-separated state P(+)H(A)(-) (where H(A) is the photoactive BPh) were found to be 3.2±0.3 ps and 200±20 ps, respectively. Overall charge separation P*→→ P(+)Q(A)(-) occurred with ≥90% yield at 285K. At 77K, the P* lifetime was somewhat shorter and the P(+)H(A)(-) lifetime was essentially unchanged. Poteniometric titrations gave a P(865)/P(865)(+) midpoint potential of +390mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of +85 and +265mV were measured, likely reflecting a pair of low-potential hemes and a pair of high-potential hemes, respectively. The time course of electron transfer from reduced Cyt to P(+) suggests an arrangement where the highest potential heme is not located immediately adjacent to P. Comparisons of these and other properties of isolated Roseiflexus castenholzii RCs to those from its close relative Chloroflexus aurantiacus and to RCs from the purple bacteria are made.
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Affiliation(s)
- Aaron M Collins
- Department of Chemistry, Washington University in St. Louis, St. Louis, MO 63130, USA
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Pottosin II, Chamorovsky CS, Chamorovsky SK. Cooperative interaction of high-potential hemes in the cytochrome subunit of the photosynthetic reaction center of bacterium Ectothiorhodospira shaposhnikovii. BIOCHEMISTRY (MOSCOW) 2008; 72:1254-60. [PMID: 18205609 DOI: 10.1134/s0006297907110120] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
Cooperative interaction of the high-potential hemes (C(h)) in the cytochrome subunit of the photosynthesizing bacterium Ectothiorhodospira shaposhnikovii was studied by comparing redox titration curves of the hemes under the conditions of pulse photoactivation inducing single turnover of electron-transport chain and steady-state photoactivation, as well as by analysis of the kinetics of laser-induced oxidation of cytochromes by reaction center (RC). A mathematical model of the processes of electron transfer in cytochrome-containing RC was considered. Theoretical analysis revealed that the reduction of one heme C(h) facilitated the reduction of the other heme, which was equivalent to a 60 mV positive shift of the midpoint potential. In addition, reduction of the second heme C(h) caused a three- to four-fold acceleration of the electron transfer from the cytochrome subunit to RC.
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Affiliation(s)
- I I Pottosin
- Centro Universitario de Investigaciones Biomedicas, Universidad de Colima, Colima, Mexico
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Dahl C. Inorganic Sulfur Compounds as Electron Donors in Purple Sulfur Bacteria. SULFUR METABOLISM IN PHOTOTROPHIC ORGANISMS 2008. [DOI: 10.1007/978-1-4020-6863-8_15] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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6
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Alric J, Tsukatani Y, Yoshida M, Matsuura K, Shimada K, Hienerwadel R, Schoepp-Cothenet B, Nitschke W, Nagashima KVP, Verméglio A. Structural and Functional Characterization of the Unusual Triheme Cytochrome Bound to the Reaction Center of Rhodovulum sulfidophilum. J Biol Chem 2004; 279:26090-7. [PMID: 15069076 DOI: 10.1074/jbc.m400361200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The cytochrome bound to the photosynthetic reaction center of Rhodovulum sulfidophilum presents two unusual characteristics with respect to the well characterized tetraheme cytochromes. This cytochrome contains only three hemes because it lacks the peptide motif CXXCH, which binds the most distal fourth heme. In addition, we show that the sixth axial ligand of the third heme is a cysteine (Cys-148) instead of the usual methionine ligand. This ligand exchange results in a very low midpoint potential (-160 +/- 10 mV). The influence of the unusual cysteine ligand on the midpoint potential of this distal heme was further investigated by site-directed mutagenesis. The midpoint potential of this heme is upshifted to +310 mV when cysteine 148 is replaced by methionine, in agreement with the typical redox properties of a His/Met coordinated heme. Because of the large increase in the midpoint potential of the distal heme in the mutant, both the native and modified high potential hemes are photooxidized at a redox poise where only the former is photooxidizable in the wild type. The relative orientation of the three hemes, determined by EPR measurements, is shown different from tetraheme cytochromes. The evolutionary basis of the concomitant loss of the fourth heme and the down-conversion of the third heme is discussed in light of phylogenetic relationships of the Rhodovulum species triheme cytochromes to other reaction center-associated tetraheme cytochromes.
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Affiliation(s)
- Jean Alric
- Laboratoire de Genetique et Biophysique des Plantes, UMR 6191 CNRS-CEA-CNRS-Aix-Marseille II, 163 avenue de Luminy, Marseille, France
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Brugna M, Nitschke W, Asso M, Guigliarelli B, Lemesle-Meunier D, Schmidt C. Redox components of cytochrome bc-type enzymes in acidophilic prokaryotes. II. The Rieske protein of phylogenetically distant acidophilic organisms. J Biol Chem 1999; 274:16766-72. [PMID: 10358018 DOI: 10.1074/jbc.274.24.16766] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The Rieske proteins of two phylogenetically distant acidophilic organisms, i.e. the proteobacterium Thiobacillus ferrooxidans and the crenarchaeon Sulfolobus acidocaldarius, were studied by EPR. Redox titrations at a range of pH values showed that the Rieske centers of both organisms are characterized by redox midpoint potential-versus-pH curves featuring a common pK value of 6.2. This pK value is significantly more acidic (by almost 2 pH units) than that of Rieske proteins in neutrophilic species. The orientations of the Rieske center's g tensors with respect to the plane of the membrane were studied between pH 4 and 8 using partially ordered samples. At pH 4, the Sulfolobus Rieske cluster was found in the "typical" orientation of chemically reduced Rieske centers, whereas this orientation changed significantly on going toward high pH values. The Thiobacillus protein, by contrast, appeared to be in the "standard" orientation at both low and high pH values. The results are discussed with respect to the molecular parameters conveying acid resistance and in light of the recently demonstrated long-range conformational movement of the Rieske protein during enzyme turnover in cytochrome bc1 complexes.
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Affiliation(s)
- M Brugna
- Laboratoire de Bioénergétique et Ingénierie des Protéines (UPR 9036), CNRS, Institut de Biologie Structurale et Microbiologie, 31 chemin Joseph Aiguier, 13402 Marseille Cedex 20, France
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8
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Vassiliev IR, Yu J, Jung YS, Schulz R, Ganago AO, McIntosh L, Golbeck JH. The cysteine-proximal aspartates in the Fx-binding niche of photosystem I. Effect of alanine and lysine replacements on photoautotrophic growth, electron transfer rates, single-turnover flash efficiency, and EPR spectral properties. J Biol Chem 1999; 274:9993-10001. [PMID: 10187775 DOI: 10.1074/jbc.274.15.9993] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The FX electron acceptor in Photosystem I (PS I) is a highly electronegative (Em = -705 mV) interpolypeptide [4Fe-4S] cluster ligated by cysteines 556 and 565 on PsaB and cysteines 574 and 583 on PsaA in Synechocystis sp. PCC 6803. An aspartic acid is adjacent to each of these cysteines on PsaB and adjacent to the proline-proximal cysteine on PsaA. We investigated the effect of D566PsaB and D557PsaB on electron transfer through FX by changing each aspartate to the neutral alanine or to the positively charged lysine either singly (D566APsaB, D557APsaB, D566KPsaB, and D557KPsaB) or in pairs (D557APsaB/D566APsaB and D557KPsaB/D566APsaB). All mutants except for D557KPsaB/D566APsaB grew photoautotrophically, but the growth of D557KPsaB and D557APsaB/D566APsaB was impaired under low light. The doubling time was increased, and the chlorophyll content per cell was lower in D557KPsaB and D557APsaB/D566APsaB relative to the wild type and the other mutants. Nevertheless, the rates of NADP+ photoreduction in PS I complexes from all mutants were no less than 75% of that of the wild type. The kinetics of back-reaction of the electron acceptors on a single-turnover flash showed efficient electron transfer to the terminal acceptors FA and FB in PS I complexes from all mutants. The EPR spectrum of FX was identical to that in the wild type in all but the single and double D566APsaB mutants, where the high-field resonance was shifted downfield. We conclude that the impaired growth of some of the mutants is related to a reduced accumulation of PS I rather than to photosynthetic efficiency. The chemical nature and the charge of the amino acids adjacent to the cysteine ligands on PsaB do not appear to be significant factors in the efficiency of electron transfer through FX.
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Affiliation(s)
- I R Vassiliev
- Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, Pennsylvania 16802, USA
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Agalidis I, Othman S, Boussac A, Reiss-Husson F, Desbois A. Purification, redox and spectroscopic properties of the tetraheme cytochrome c isolated from Rubrivivax gelatinosus. EUROPEAN JOURNAL OF BIOCHEMISTRY 1999; 261:325-36. [PMID: 10103066 DOI: 10.1046/j.1432-1327.1999.00277.x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The tetraheme cytochrome c subunit of the Rubrivivax gelatinosus reaction center was isolated in the presence of octyl beta-D-thioglucoside by ammonium sulfate precipitation and solubilization at pH 9 in a solution of Deriphat 160. Several biochemical properties of this purified cytochrome were characterized. In particular, it forms small oligomers and its N-terminal amino acid is blocked. In the presence or absence of diaminodurene, ascorbate and dithionite, different oxidation/reduction states of the isolated cytochrome were studied by absorption, EPR and resonance Raman spectroscopies. All the data show two hemes quickly reduced by ascorbate, one heme slowly reduced by ascorbate and one heme only reduced by dithionite. The quickly ascorbate-reduced hemes have paramagnetic properties very similar to those of the two low-potential hemes of the reaction center-bound cytochrome (gz = 3.34), but their alpha band is split with two components peaking at 552 nm and 554 nm in the reduced state. Their axial ligands did not change, being His/Met and His/His, as indicated by the resonance Raman spectra. The slowly ascorbate-reduced heme and the dithionite-reduced heme are assigned to the two high-potential hemes of the bound cytochrome. Their alpha band was blue-shifted at 551 nm and the gz values decreased to 2.96, although the axial ligations (His/Met) were conserved. It was concluded that the estimated 300 mV potential drop of these hemes reflected changes in their solvent accessibility, while the reduction in gz indicates an increased symmetry of their cooordination spheres. These structural modifications impaired the cytochrome's essential function as the electron donor to the photooxidized bacteriochlorophyll dimer of the reaction center. In contrast to its native state, the isolated cytochrome was unable to reduce efficiently the reaction center purified from a Rubrivivax gelatinosus mutant in which the tetraheme was absent. Despite the conformational changes of the cytochrome, its four hemes are still divided into two groups with a pair of low-potential hemes and a pair of high-potential hemes.
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Affiliation(s)
- I Agalidis
- Centre de Génétique Moléculaire, CNRS 91198, Gif-sur-Yvette, France
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10
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Guigliarelli B, Bertrand P. Application of EPR Spectroscopy to the Structural and Functional Study of Iron-Sulfur Proteins. ADVANCES IN INORGANIC CHEMISTRY 1999. [DOI: 10.1016/s0898-8838(08)60084-7] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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11
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Vassiliev IR, Jung YS, Yang F, Golbeck JH. PsaC subunit of photosystem I is oriented with iron-sulfur cluster F(B) as the immediate electron donor to ferredoxin and flavodoxin. Biophys J 1998; 74:2029-35. [PMID: 9545061 PMCID: PMC1299543 DOI: 10.1016/s0006-3495(98)77909-3] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
The PsaC subunit of photosystem I (PS I) binds two [4Fe-4S] clusters, F(A) and F(B), functioning as electron carriers between F(X) and soluble ferredoxin. To resolve the issue whether F(A) or F(B) is proximal to F(X), we used single-turnover flashes to promote step-by-step electron transfer between electron carriers in control (both F(A) and F(B) present) and HgCl2-treated (F(B)-less) PS I complexes from Synechococcus sp. PCC 6301 and analyzed the kinetics of P700+ reduction by monitoring the absorbance changes at 832 nm in the presence of a fast electron donor (phenazine methosulfate (PMS)). In control PS I complexes exogenously added ferredoxin, or flavodoxin could be photoreduced on each flash, thus allowing P700+ to be reduced from PMS. In F(B)-less complexes, both in the presence and in the absence of ferredoxin or flavodoxin, P700+ was reduced from PMS only on the first flash and was reduced from F(X)- on the following flashes, indicating lack of electron transfer to ferredoxin or flavodoxin. In the F(B)-less complexes, a normal level of P700 photooxidation was detected accompanied by a high yield of charge recombination between P700+ and F(A)- in the presence of a slow donor, 2,6-dichlorophenol-indophenol. This recombination remained the only pathway of F(A)- reoxidation in the presence of added ferredoxin, consistent with the lack of forward electron transfer. F(A)- could be reoxidized by methyl viologen in F(B)-less PS I complexes, although at a concentration two orders of magnitude higher than is required in wild-type PS I complexes, thus implying the presence of a diffusion barrier. The inhibition of electron transfer to ferredoxin and flavodoxin was completely reversed after reconstituting the F(B) cluster. Using rate versus distance estimates for electron transfer rates from F(X) to ferredoxin for two possible orientations of PsaC, we conclude that the kinetic data are best compatible with PsaC being oriented with F(A) as the cluster proximal to F(X) and F(B) as the distal cluster that donates electrons to ferredoxin.
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Affiliation(s)
- I R Vassiliev
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park 16802, USA
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Nagashima KV, Matsuura K, Shimada K. The nucleotide sequence of the puf operon from the purple photosynthetic bacterium, Rhodospirillum molischianum: Comparative analyses of light-harvesting proteins and the cytochrome subunits associated with the reaction centers. PHOTOSYNTHESIS RESEARCH 1996; 50:61-70. [PMID: 24271822 DOI: 10.1007/bf00018221] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/04/1996] [Accepted: 09/12/1996] [Indexed: 06/02/2023]
Abstract
The nucleotide sequence of the puf operon of the purple bacterium, Rhodospirillum molischianum, was determined. The operon includes genes coding for the β and α subunits of the light-harvesting 1 (LH1) complex and the L, M, and cytochrome subunits of the reaction center complex. As in other purple bacteria, the genes are arranged within the operon in this order. As in Rubrivivax gelatinosus, the deduced amino acid sequence of the cytochrome subunit in Rsp. molischianum contains significant deletions at the attachment site to the M subunit compared with that of Rhodopseudomonas viridis. This suggests that the interaction between the cytochrome subunit and the LM core in Rsp. molischianum and Rvi. gelatinosus is different from that in Rps. viridis. Phylogenetic analysis of the light-harvesting proteins indicated that the LH1 α and β subunits of Rsp. molischianum are included in the lineage of LH1 polypeptides of the purple bacteria, while the LH2 α and β subunits are positioned apart from LH2 polypeptides of the other purple bacteria together with those of Chromatium vinosum. Based on these phylogenetic analyses, the classification of the light-harvesting proteins in purple bacteria is discussed.
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Affiliation(s)
- K V Nagashima
- Department of Biology, Tokyo Metropolitan University, Minamiohsawa 1-1, Hachioji, 192-03, Tokyo, Japan
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Nagashima KV, Shimada K, Matsuura K. Shortcut of the photosynthetic electron transfer in a mutant lacking the reaction center-bound cytochrome subunit by gene disruption in a purple bacterium, Rubrivivax gelatinosus. FEBS Lett 1996; 385:209-13. [PMID: 8647253 DOI: 10.1016/0014-5793(96)00382-1] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
A mutant lacking the reaction center-bound cytochrome subunit was constructed in a purple photosynthetic bacterium, Rubrivivax gelatinosus IL144, by inactivation of the cytochrome gene. Photosynthetic growth of the C244 mutant strain occurred at approximately half the rate of the wild-type strain. Although mutagenesis resulted in a greatly reduced amount of membrane-bound cytochromes c, illumination induced cyclic electron transfer and the generation of membrane potential in the mutant as observed in the wild-type strain. These findings are consistent with previous observations that the cytochrome subunit is absent in the reaction center complex in some species of purple bacteria and that the biochemical removal of the subunit did not significantly affect the in vitro electron transfer from the soluble cytochrome c to the photosynthetic reaction center. These results suggest that the cytochrome subunit in purple bacteria is not essential for photosynthetic electron transfer and growth, even in those species generally containing the subunit.
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Affiliation(s)
- K V Nagashima
- Department of Biology, Tokyo Metropolitan University, Japan
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Kaminskaya O, Bratt PJ, Evans MC. EPR properties of the four hemes in the cytochrome subunit of reaction centres from Rhodopseudomonas viridis: characterization of the individual hemes. Chem Phys 1995. [DOI: 10.1016/0301-0104(95)00020-o] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Kennis JT, Aartsma TJ, Amesz J. Energy trapping in the purple sulfur bacteria Chromatium vinosum and Chromatium tepidum. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 1994. [DOI: 10.1016/0005-2728(94)90046-9] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
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Garcia D, Richaud P, Breton J, Verméglio A. Structure and function of the tetraheme cytochrome associated to the reaction center of Roseobacter denitrificans. Biochimie 1994; 76:666-73. [PMID: 7893818 DOI: 10.1016/0300-9084(94)90143-0] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
We have characterized the tetrahemic RC bound cytochrome isolated from the quasi-photosynthetic bacterium Roseobacter denitrificans in terms of absorption spectrum, redox property and orientation with respect to the membrane plane. The heme, designated H1, which possesses the highest redox midpoint potential (+290 mV), absorbs at 555 nm. Its plane makes an angle of 40 degrees with the membrane plane. The second high potential heme, H2 (+240 mV), peaks at 554 nm and makes a tilt of 55 degrees with the membrane. The two low potential hemes, L1 and L2, present a similar and rather high redox midpoint potential (+90 mV). They absorb at 553 nm and 550 nm. One of these hemes is oriented at 40 degrees while the other makes an angle of 90 degrees with the membrane plane. The soluble cytochrome c551 completes the cyclic electron transfer between the RC and the bc1 complex. Both the oxidation and the re-reduction of cytochrome c551 are diffusible processes. Under semi-aerobic conditions, one of the low potential hemes is photo-oxidized under illumination but only extremely slowly re-reduced. This explains the requirement of high aerobic conditions for growth of Roseobacter denitrificans cells in the light.
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Affiliation(s)
- D Garcia
- CEA, DPVE, SBC, CE de Cadarache, Saint-Paul-lez-Durance, France
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