Mechanism of electron transport during thiosulfate oxidation in an obligately mixotrophic bacterium Thiomonas bhubaneswarensis strain S10 (DSM 18181T)

This study describes the thiosulfate-supported respiratory electron transport activity of Thiomonas bhubaneswarensis strain S10 (DSM 18181^T). Whole-genome sequence analysis revealed the presence of complete sox (sulfur oxidation) gene cluster (soxCDYZAXB) including the sulfur oxygenase reductase (SOR), sulfide quinone reductase (SQR), sulfide dehydrogenase (flavocytochrome c (fcc)), thiosulfate dehydrogenase (Tsd), sulfite dehydrogenase (SorAB), and intracellular sulfur oxidation protein (DsrE/DsrF). In addition, genes encoding respiratory electron transport chain components viz. complex I (NADH dehydrogenase), complex II (succinate dehydrogenase), complex III (ubiquinone-cytochrome c reductase), and various types of terminal oxidases (cytochrome c and quinol oxidase) were identified in the genome. Using site-specific electron donors and inhibitors and by analyzing the cytochrome spectra, we identified the shortest thiosulfate-dependent electron transport chain in T. bhubaneswarensis DSM 18181^T. Our results showed that thiosulfate supports the electron transport activity in a bifurcated manner, donating electrons to quinol (bd) and cytochrome c (Caa ₃ ) oxidase; these two sites (quinol oxidase and cytochrome c oxidase) also showed differences in their phosphate esterification potential (oxidative phosphorylation efficiency (P/O)). Further, it was evidenced that the substrate-level phosphorylation is the major contributor to the total energy budget in this bacterium.

     

Participation of plastoquinone,cytochrome c 553 and ferrodoxin-NADP+ oxido-reductase in both photosynthesis and respiration in Spirulina maxima

Dark and light oxidation of NADPH was measured in Spirulina maxima thylakoid membranes. The dark reaction was more cyanide sensitive than the light reaction. In light, 83% of the electrons from NADPH produced H₂O₂ on reducing oxygen, whereas in the dark this number was only 36%. These results are explained by assuming the presence of an electron transport segment common to the photosynthetic and the respiratory chains, so that electrons flowing through the cyanide sensitive oxidase in the dark are diverted to the photosytem (PS) I reaction center (P700). In addition, cytochrome (cyt) c ₅₅₃ was found to be an electron donor for both cyt oxidase and P700. Half maximum reduction rates were obtained with 7 M cyt c ₅₅₃. The intrathylakoidal concentration of cyt c ₅₅₃ was determined to be 83 M. About 60% of the respiratory NADPH oxidation activity was lost by extracting the membranes with pentane and was restored by adding plastoquinone (the main photosythetic quinone). NADPH oxidation activity was also inhibited upon washing the membranes with a low salt buffer. This activity was restored by adding partially purified ferredoxin-NADP⁺ oxido-reductase (FNR). A model for the electron transport in thylakoids, in which cyt c ₅₅₃, plastoquinone and FNR participate in both photosynthesis and respiration is proposed.     

Mislocalization of Rieske Protein PetA Predominantly Accounts for the Aerobic Growth Defect of tat Mutants in Shewanella oneidensis

Shewanella oneidensis exhibits a remarkable versatility in respiration, which largely relies on its various respiratory pathways. Most of these pathways are composed of secretory terminal reductases and multiple associated electron transport proteins that contain cofactors such as Fe-S, molybdopterin, and NiFe. The majority of these cofactors are inserted enzymatically in the cytoplasm, and thus are substrates of the twin-arginine translocation (Tat) protein export system, which transports fully folded proteins. Using genomic array footprinting, we discovered that loss of TatA or TatC caused a reduction in the growth rate of S. oneidensis under aerobic conditions. Mutational analysis of the predicted Tat substrates revealed that PetA, the Rieske Fe-S subunit of the ubiquinol-cytochrome c reductase, predominantly dictates the aerobic growth defect of tat mutants in S. oneidensis. In addition, evidence is presented that the signal sequence in PetA appears to be resistant to cleavage after the protein is inserted into the cytoplasmic membrane.     

Kinetic advantage of forming respiratory supercomplexes

Components of respiratory chains in mitochondria and some aerobic bacteria assemble into larger, multiprotein membrane-bound supercomplexes. Here, we address the functional significance of supercomplexes composed of respiratory-chain complexes III and IV. Complex III catalyzes oxidation of quinol and reduction of water-soluble cytochrome c (cyt c), while complex IV catalyzes oxidation of the reduced cyt c and reduction of dioxygen to water. We focus on two questions: (i) under which conditions does diffusion of cyt c become rate limiting for electron transfer between these two complexes? (ii) is there a kinetic advantage of forming a supercomplex composed of complexes III and IV? To answer these questions, we use a theoretical approach and assume that cyt c diffuses in the water phase while complexes III and IV either diffuse independently in the two dimensions of the membrane or form supercomplexes. The analysis shows that the electron flux between complexes III and IV is determined by the equilibration time of cyt c within the volume of the intermembrane space, rather than the cyt c diffusion time constant. Assuming realistic relative concentrations of membrane-bound components and cyt c and that all components diffuse independently, the data indicate that electron transfer between complexes III and IV can become rate limiting. Hence, there is a kinetic advantage of bringing complexes III and IV together in the membrane to form supercomplexes.     

Sulfur oxidation in mutants of the photosynthetic green sulfur bacterium Chlorobium tepidum devoid of cytochrome c-554 and SoxB

A mutant devoid of cytochrome c-554 (CT0075) in Chlorobium tepidum (syn. Chlorobaculum tepidum) exhibited a decreased growth rate but normal growth yield when compared to the wild type. From quantitative determinations of sulfur compounds in media, the mutant was found to oxidize thiosulfate more slowly than the wild type but completely to sulfate as the wild type. This indicates that cytochrome c-554 would increase the rate of thiosulfate oxidation by serving as an efficient electron carrier but is not indispensable for thiosulfate oxidation itself. On the other hand, mutants in which a portion of the soxB gene (CT1021) was replaced with the aacC1 cassette did not grow at all in a medium containing only thiosulfate as an electron source. They exhibited partial growth yields in media containing only sulfide when compared to the wild type. This indicates that SoxB is not only essential for thiosulfate oxidation but also responsible for sulfide oxidation. An alternative electron carrier or electron transfer path would thus be operating between the Sox system and the reaction center in the mutant devoid of cytochrome c-554. Cytochrome c-554 might function in any other pathway(s) as well as the thiosulfate oxidation one, since even green sulfur bacteria that cannot oxidize thiosulfate contain a cycA gene encoding this electron carrier.     

Some characteristics of cytochromec-555 isolated from sulfide oxidizingXanthomonas sp. DY44

From a heterotrophic bacterium,Xanthomonas sp. DY44 which was previously reported to oxidize hydrogen sulfide (H₂S) to polysulfide, cytochromec-555 (cyt.c-555) responsible for oxidation of sulfide was purified by DEAE-Toyopearl and Sepadex G-75 column chromatography. Cyt.c-555 with a molecular weight of 12,500 showed maximum absorption at 555 nm (α-peak), 522 nm (β-peak) and 417 nm (γ-peak) for the reduced form which was prepared by addition of Na₂S₂O₄. Cyt.c-555 was also reduced by addition of sulfide (Na₂S and H₂S), and the oxidized products of sulfide by cyt.c-555 was identified as polysulfide. The reduced form of cyt.c-555 was suggested to be oxidized coupled with cyt.c oxidase which is tolerant to sulfide.     

The structure and function of the cytochrome <Emphasis Type="Italic">c</Emphasis><Subscript>2</Subscript>: reaction center electron transfer complex from <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis>

In the photosynthetic bacterium, Rhodobacter sphaeroides, the mobile electron carrier, cytochrome c₂ (cyt c₂) transfers an electron from reduced heme to the photooxidized bacteriochlorophyll dimer in the membrane bound reaction center (RC) as part of the light induced cyclic electron transfer chain. A complex between these two proteins that is active in electron transfer has been crystallized and its structure determined by X-ray diffraction. The structure of the cyt:RC complex shows the cyt c₂ (cyt c₂) positioned at the center of the periplasmic surface of the RC. The exposed heme edge from cyt c₂ is in close tunneling contact with the electron acceptor through an intervening bridging residue, Tyr L162 located on the RC surface directly above the bacteriochlorophyll dimer. The binding interface between the two proteins can be divided into two regions: a short-range interaction domain and a long-range interaction domain. The short-range domain includes residues immediately surrounding the tunneling contact region around the heme and Tyr L162 that display close intermolecular contacts optimized for electron transfer. These include a small number of hydrophobic interactions, hydrogen bonds and a pi-cation interaction. The long-range interaction domain consists of solvated complementary charged residues; positively charged residues from the cyt and negatively charged residues from the RC that provide long range electrostatic interactions that can steer the two proteins into position for rapid association.     

Sulfide oxidation in the phototrophic sulfur bacterium Chromatium vinosum

Sulfide oxidation in the phototrophic purple sulfur bacterium Chromatium vinosum D (DSMZ 180^T) was studied by insertional inactivation of the fccAB genes, which encode flavocytochrome c, a protein that exhibits sulfide dehydrogenase activity in vitro. Flavocytochrome c is located in the periplasmic space as shown by a PhoA fusion to the signal peptide of the hemoprotein subunit. The genotype of the flavocytochrome-c-deficient Chr. vinosum strain FD1 was verified by Southern hybridization and PCR, and the absence of flavocytochrome c in the mutant was proven at the protein level. The oxidation of thiosulfate and intracellular sulfur by the flavocytochrome-c-deficient mutant was comparable to that of the wild-type. Disruption of the fccAB genes did not have any significant effect on the sulfide-oxidizing ability of the cells, showing that flavocytochrome c is not essential for oxidation of sulfide to intracellular sulfur and indicating the presence of a distinct sulfide-oxidizing system. In accordance with these results, Chr. vinosum extracts catalyzed electron transfer from sulfide to externally added duroquinone, indicating the presence of the enzyme sulfide:quinone oxidoreductase (EC 1.8.5.-). Further investigations showed that the sulfide:quinone oxidoreductase activity was sensitive to heat and to quinone analogue inhibitors. The enzyme is strictly membrane-bound and is constitutively expressed. The presence of sulfide:quinone oxidoreductase points to a connection of sulfide oxidation to the membrane electron transport system at the level of the quinone pool in Chr. vinosum. Received: 5 November 1997 / Accepted: 30 March 1998     

Impacts of Shewanella oneidensis c‐type cytochromes on aerobic and anaerobic respiration

Shewanella are renowned for their ability to utilize a wide range of electron acceptors (EA) for respiration, which has been partially accredited to the presence of a large number of the c-type cytochromes. To investigate the involvement of c-type cytochrome proteins in aerobic and anaerobic respiration of Shewanella oneidensis Mr -1, 36 in-frame deletion mutants, among possible 41 predicted, c-type cytochrome genes were obtained. The potential involvement of each individual c-type cytochrome in the reduction of a variety of EAs was assessed individually as well as in competition experiments. While results on the well-studied c-type cytochromes CymA(SO4591) and MtrC(SO1778) were consistent with previous findings, collective observations were very interesting: the responses of S. oneidensis Mr -1 to low and highly toxic metals appeared to be significantly different; CcoO, CcoP and PetC, proteins involved in aerobic respiration in various organisms, played critical roles in both aerobic and anaerobic respiration with highly toxic metals as EA. In addition, these studies also suggested that an uncharacterized c-type cytochrome (SO4047) may be important to both aerobiosis and anaerobiosis.     

Mechanism and Consequences of Anaerobic Respiration of Cobalt by Shewanella oneidensis Strain MR-1

Bacteria from the genus Shewanella are the most diverse respiratory organisms studied to date and can utilize a variety of metals and metal(loid)s as terminal electron acceptors. These bacteria can potentially be used in bioremediation applications since the redox state of metals often influences both solubility and toxicity. Understanding molecular mechanisms by which metal transformations occur and the consequences of by-products that may be toxic to the organism and thus inhibitory to the overall process is significant to future applications for bioremediation. Here, we examine the ability of Shewanella oneidensis to catalyze the reduction of chelated cobalt. We describe an unexpected ramification of [Co(III)-EDTA]⁻ reduction by S. oneidensis: the formation of a toxic by-product. We found that [Co(II)-EDTA]²⁻, the product of [Co(III)-EDTA]⁻ respiration, inhibited the growth of S. oneidensis strain MR-1 and that this toxicity was partially abolished by the addition of MgSO₄. We demonstrate that [Co(III)-EDTA]⁻ reduction by S. oneidensis requires the Mtr extracellular respiratory pathway and associated pathways required to develop functional Mtr enzymes (the c-type cytochrome maturation pathway) and ensure proper localization (type II secretion). The Mtr pathway is known to be required for a variety of substrates, including some chelated and insoluble metals and organic compounds. Understanding the full substrate range for the Mtr pathway is crucial for developing S. oneidensis strains as a tool for bioremediation.     

Generation of reducing power in chemosynthesis. VI. Energy-linked reactions in the chemoautotroph,Thiobacillus neapolitanus

Electron donors such as thiosulfate, sulfite, and ascorbate have been shown to enter the respiratory chain ofT. neapolitanus at the level of cytochromec. The enzymatic oxidation of these substrates catalyzed by the cytochrome oxidase (E. C. 1.9.3.1.) ofT. neapolitanus cell-free extracts was coupled to the generation of energy which could be utilized to drive the reverse electron flow from cytochromec to pyridine nucleotides.The reduction of endogenous or added flavin by thiosulfate or ascorbate has been shown to be ATP-dependent; likewise the reduction of cytochromeb by these electron donors also required energy. The rate of ATP-driven reversal of electron transfer from cytochromec to the pyridine nucleotides was much faster compared with the rate of electron reversal catalyzed by the substrate-linked generated energy. The pathway of energy-linked reversal of electron transfer from cytochrome c to pyridine nucleotides involved cytochromeb and flavoproteins.NADH oxidation byT. neapolitanus cell-free extracts is mediated by the flavoprotein and cytochrome systems and this process also appears to be coupled with energy generation. The NADH oxidase (NADH₂: cytochromec oxidoreductase) was partially inhibited by amytal or rotenone, antimycin A or HOQNO, and was relatively insensitive to cyanide or azide.This investigation was supported in part by a National Science Foundation Grant No. GB 6649 and in part by the Department of Interior, Office of Water Resources Research No. A-016-KY.     

Geovibrio ferrireducens,a phylogenetically distinct dissimilatory Fe(III)-reducing bacterium

A new, phylogenetically distinct, dissimilatory, Fe(III)-reducing bacterium was isolated from surface sediment of a hydrocarbon-contaminated ditch. The isolate, designated strain PAL-1, was an obligately anaerobic, non-fermentative, motile, gram-negative vibrio. PAL-1 grew in a defined medium with acetate as electron donor and ferric pyrophosphate, ferric oxyhydroxide, ferric citrate, Co(III)-EDTA, or elemental sulfur as sole electron acceptor. PAL-1 also used proline, hydrogen, lactate, propionate, succinate, fumarate, pyruvate, or yeast extract as electron donors for Fe(III) reduction. It is the first bacterium known to couple the oxidation of an amino acid to Fe(III) reduction. PAl-1 did not reduce oxygen, Mn(IV), U(VI), Cr(VI), nitrate, sulfate, sulfite, or thiosulfate with acetate as the electron donor. Cell suspensions of PAL-1 exhibited dithionite-reduced minus air-oxidized difference spectra that were characteristic of c-type cytochromes. Analysis of the 16S rRNA gene sequence of PAL-1 showed that the strain is not related to any of the described metal-reducing bacteria in the Proteobacteria and, together with Flexistipes sinusarabici, forms a separate line of descent within the Bacteria. Phenotypically and phylogenetically, strain PAl-1 differs from all other described bacteria, and represents the type strain of a new genus and species, Geovibrio ferrireducens. Received: 26 September 1995 / Accepted: 28 February 1996     

Intermediates of denitrification in the chemoautotroph Thiobacillus denitrificans

Summary Intact cells obtained from Thiobacillus denitrificans grown autotrophically with thiosulfate as the oxidizable substrate and nitrate as the final electron acceptor catalyzed the reduction of nitrate, nitrite and nitric oxide stoichiometrically to nitrogen gas with the concomitant oxidation of thiosulfate. In addition, nitrous oxide was also capable of acting as the terminal oxidant of the respiratory chain with thiosulfate as the reductant. The anaerobic oxidation of thiosulfate by NO₃ ^-, NO, and N₂O was sensitive to the flavoprotein inhibitors, antimycin A or NHQNO, and cyanide or azide thus, implicating the participation of flavins, and cytochromes of b-, c-, and a-types in the denitrification process. The nitrite reductase system, however, was not markedly affected by the electron transport chain inhibitors. The experimental observations suggest that the dissimilatory nitrate reduction in the chemoautotroph T. denitrificans involves nitrite, nitric oxide, and nitrous oxide as theintermediates with nitrogen gas as the final reduction product.Non-Standard Abbreviations TTFA Thenoyltrifluoroacetone - NHQNO 2-n-nonyl-4-hydroxyquinoline N-oxide     

Promotion of Iron Oxide Reduction and Extracellular Electron Transfer in Shewanella oneidensis by DMSO

The dissimilatory metal reducing bacterium Shewanella oneidensis MR-1, known for its capacity of reducing iron and manganese oxides, has great environmental impacts. The iron oxides reducing process is affected by the coexistence of alternative electron acceptors in the environment, while investigation into it is limited so far. In this work, the impact of dimethyl sulphoxide (DMSO), a ubiquitous chemical in marine environment, on the reduction of hydrous ferric oxide (HFO) by S. oneidensis MR-1 was investigated. Results show that DMSO promoted HFO reduction by both wild type and ΔdmsE, but had no effect on the HFO reduction by ΔdmsB, indicating that such a promotion was dependent on the DMSO respiration. With the DMSO dosing, the levels of extracellular flavins and omcA expression were significantly increased in WT and further increased in ΔdmsE. Bioelectrochemical analysis show that DMSO also promoted the extracellular electron transfer of WT and ΔdmsE. These results demonstrate that DMSO could stimulate the HFO reduction through metabolic and genetic regulation in S. oneidensis MR-1, rather than compete for electrons with HFO. This may provide a potential respiratory pathway to enhance the microbial electron flows for environmental and engineering applications.     

Cytochrome <Emphasis Type="Italic">c</Emphasis> is rapidly reduced in the cytosol after mitochondrial outer membrane permeabilization

Visible spectroscopy was used to measure real-time changes in the oxidation state of cytochrome c (cyt c) and the a-cytochromes (cyt aa₃) of cytochrome oxidase during mitochondrial outer membrane permeabilization (MOMP) initiated by anisomycin in HL-60 cells. The oxidation state of mitochondrial cyt c was found to be ≈62% oxidized before MOMP and became ≈70% oxidized after MOMP. In contrast, the cytosolic pool of cyt c was found to be almost fully reduced. This oxidation change allows cyt c release to be continuously and quantitatively monitored in real time. Anoxia and antimycin were used to fully reduce and fully oxidize, respectively, the mitochondrial pool of cyt c and it was found that the release of cyt c was independent of it oxidation state consistent with a simple model of cyt c passively diffusing down a concentration gradient through a pore or tear in the outer membrane. After MOMP was complete, the flux of cyt c diffusing back into the mitochondria was measured from the residual mitochondrial oxygen consumption after complete inhibition of the bc₁ with antimycin and myxothiazol. The outer membrane was found to be highly permeable after MOMP implying that the reduction of cyt c in the cytosol must be very rapid. The permeability of the outer membrane measured in this study would result in the release of cyt c with a time constant of less than 1 s.     

Control of pellicle biogenesis involves the diguanylate cyclases PdgA and PdgB,the c-di-GMP binding protein MxdA and the chemotaxis response regulator CheY3 in Shewanella oneidensis

Shewanella oneidensis is an aquatic proteobacterium with remarkable respiratory and chemotactic abilities. It is also capable of forming biofilms either associated to surfaces (SSA-biofilm) or at the air–liquid interface (pellicle). We have previously shown that pellicle biogenesis in S. oneidensis requires the flagellum and the chemotaxis regulatory system including CheA3 kinase and CheY3 response regulator. Here we searched for additional factors involved in pellicle development. Using a multicopy library of S. oneidensis chromosomal fragments, we identified two genes encoding putative diguanylate cyclases (pdgA and pdgB) and allowing pellicle formation in the non-pellicle-forming cheY3-deleted mutant. A mutant deleted of both pdgA and pdgB is affected during pellicle development. By overexpressing phosphodiesterase encoding genes, we confirmed the key role of c-di-GMP in pellicle biogenesis. The mxd operon, previously proposed to encode proteins involved in exopolysaccharide biosynthesis, is also essential for pellicle formation. In addition, we showed that the MxdA protein, containing a degenerate GGDEF motif, binds c-di-GMP and interacts with both CheY3 and PdgA. Therefore, we propose that pellicle biogenesis in S. oneidensis is controlled by a complex pathway that involves the chemotaxis response regulator CheY3, the two putative diguanylate cyclases PdgA and PdgB, and the c-di-GMP binding protein MxdA.     

Hydrogen sulfide-producing kinetics of Shewanella oneidensis in sulfite and thiosulfate respiration

Shewanella oneidensis is a model species for aquatic ecosystems and plays an important role in bioremediation, biofuel cell manufacturing and biogeochemical cycling. S. oneidensis MR-1 is able to generate hydrogen sulfide from various sulfur species; however, its catalytic kinetics have not been determined. In this study, five in-frame deletion mutants of S. oneidensis were constructed and their H₂S-producing activities were analyzed. SirA and PsrA were the two major contributors to H₂S generation under anoxic cultivation, and the optimum SO₃²⁻ concentration for sulfite respiration was approximately 0.8 mM, while the optimum S₂O₃²⁻ concentration for thiosulfate respiration was approximately 0.4 mM. Sulfite and thiosulfate were observed to interfere with each other during respiration, and a high concentration of sulfite or thiosulfate chelated extracellular free-iron but did not repress the expression of sirA or psrA. Nitrite and nitrate were two preferred electron acceptors during anaerobic respiration; however, under energy-insufficient conditions, S. oneidensis could utilize multiple electron acceptors simultaneously. Elucidiating the stoichiometry of H₂S production in S. oneidensis would be helpful for the application of this species in bioremediation and biofuel cell manufacturing, and would help to characterize the ecophysiology of sulfur cycling.     

In situ monitoring of the catalytic activity of cytochrome C oxidase in a biomimetic architecture

Cytochrome c oxidase (CcO) from Paracoccus denitrificans was immobilized in a strict orientation via a his-tag attached to subunit I on a gold film and reconstituted in situ into a protein-tethered bilayer lipid membrane. In this orientation, the cytochrome c (cyt c) binding site is directed away from the electrode pointing to the outer side of the protein-tethered bilayer lipid membrane architecture. The CcO can thus be activated by cyt c under aerobic conditions. Catalytic activity was monitored by impedance spectroscopy, as well as cyclic voltammetry. Cathodic and anodic currents of the CcO with cyt c added to the bulk solution were shown to increase under aerobic compared to anaerobic conditions. Catalytic activity was considered in terms of repeated electrochemical oxidation/reduction of the CcO/cyt c complex in the presence of oxygen. The communication of cyt c bound to the CcO with the electrode is discussed in terms of a hopping mechanism through the redox sites of the enzyme. Simulations supporting this hypothesis are included.     

Influence of outer membrane c‐type cytochromes on particle size and activity of extracellular nanoparticles produced by Shewanella oneidensis

The metal‐reducing bacterium Shewanella oneidensis is capable of reducing various metal(loid)s and produces nanoparticles (NPs) extracellularly, in which outer membrane c‐type cytochromes (OMCs) have been suggested to play important roles. The objective of this study was to investigate the influence of the OMCs, that is, MtrC and OmcA, on the size and activity of the extracellular silver NPs (AgNPs) and silver sulfide NPs (Ag₂S NPs) produced by S. oneidensis MR‐1. We found that (i) the lack of OMCs on S. oneidensis cell surface decreased the particle size of the extracellular biogenic AgNPs and Ag₂S NPs; (ii) the biogenic AgNPs from the mutant lacking OMCs showed higher antibacterial activity; and (iii) the biogenic Ag₂S NPs from the mutant lacking OMCs exhibited higher catalytic activity in methylviologen reduction. The results suggest that it may be possible to control particle size and activity of the extracellular biogenic NPs via controlled expression of the genes encoding surface proteins. In addition, we also reveal that in extracellular biosynthesis of NPs the usually neglected non‐cell‐associated NPs could have high catalytic activity, highlighting the need of novel methods that can efficiently retain extracellular NPs in the biosynthesis processes. Biotechnol. Bioeng. 2013; 110: 1831–1837. © 2013 Wiley Periodicals, Inc.     

Crystal Structure of the Electron Carrier Domain of the Reaction Center Cytochrome cz Subunit from Green Photosynthetic Bacterium Chlorobium tepidum

In green sulfur photosynthetic bacteria, the cytochrome c_z (cyt c_z) subunit in the reaction center complex mediates electron transfer mainly from menaquinol/cytochrome c oxidoreductase to the special pair (P840) of the reaction center. The cyt c_z subunit consists of an N-terminal transmembrane domain and a C-terminal soluble domain that binds a single heme group. The periplasmic soluble domain has been proposed to be highly mobile and to fluctuate between oxidoreductase and P840 during photosynthetic electron transfer. We have determined the crystal structure of the oxidized form of the C-terminal functional domain of the cyt c_z subunit (C-cyt c_z) from thermophilic green sulfur bacterium Chlorobium tepidum at 1.3-Å resolution. The overall fold of C-cyt c_z consists of four α-helices and is similar to that of class I cytochrome c proteins despite the low similarity in their amino acid sequences. The N-terminal structure of C-cyt c_z supports the swinging mechanism previously proposed in relation with electron transfer, and the surface properties provide useful information on possible interaction sites with its electron transfer partners. Several characteristic features are observed for the heme environment: These include orientation of the axial ligands with respect to the heme plane, surface-exposed area of the heme, positions of water molecules, and hydrogen-bond network involving heme propionate groups. These structural features are essential for elucidating the mechanism for regulating the redox state of cyt c_z.