Cytochrome c3 (M(r) 26,000) isolated from sulfate-reducing bacteria and its relationships to other polyhemic cytochromes from Desulfovibrio

New model for electron flow for sulfate reduction in Desulfovibrio alaskensis G20

To understand the energy conversion activities of the anaerobic sulfate-reducing bacteria, it is necessary to identify the components involved in electron flow. The importance of the abundant type I tetraheme cytochrome c3 (TpIc3) as an electron carrier during sulfate respiration was questioned by the previous isolation of a null mutation in the gene encoding TpIc3, cycA, in Desulfovibrio alaskensis G20. Whereas respiratory growth of the CycA mutant with lactate and sulfate was little affected, growth with pyruvate and sulfate was significantly impaired. We have explored the phenotype of the CycA mutant through physiological tests and transcriptomic and proteomic analyses. Data reported here show that electrons from pyruvate oxidation do not reach adenylyl sulfate reductase, the enzyme catalyzing the first redox reaction during sulfate reduction, in the absence of either CycA or the type I cytochrome c3:menaquinone oxidoreductase transmembrane complex, QrcABCD. In contrast to the wild type, the CycA and QrcA mutants did not grow with H2 or formate and sulfate as the electron acceptor. Transcriptomic and proteomic analyses of the CycA mutant showed that transcripts and enzymes for the pathway from pyruvate to succinate were strongly decreased in the CycA mutant regardless of the growth mode. Neither the CycA nor the QrcA mutant grew on fumarate alone, consistent with the omics results and a redox regulation of gene expression. We conclude that TpIc3 and the Qrc complex are D. alaskensis components essential for the transfer of electrons released in the periplasm to reach the cytoplasmic adenylyl sulfate reductase and present a model that may explain the CycA phenotype through confurcation of electrons.

Hemoproteins in dissimilatory sulfate- and sulfur-reducing prokaryotes

Dissimilatory sulfate and sulfur reduction evolved billions of years ago and while the bacteria and archaea that use this unique metabolism employ a variety of electron donors, H(2) is most commonly used as the energy source. These prokaryotes use multiheme c-type proteins to shuttle electrons from electron donors, and electron transport complexes presumed to contain b-type hemoproteins contribute to proton charging of the membrane. Numerous sulfate and sulfur reducers use an alternate pathway for heme synthesis and, frequently, uniquely specific axial ligands are used to secure c-type heme to the protein. This review presents some of the types and functional activities of hemoproteins involved in these two dissimilatory reduction pathways.

The bacterial metallome: composition and stability with specific reference to the anaerobic bacterium Desulfovibrio desulfuricans

In bacteria, the intracellular metal content or metallome reflects the metabolic requirements of the cell. When comparing the composition of metals in phytoplankton and bacteria that make up the macronutrients and the trace elements, we have determined that the content of trace elements in both of these microorganisms is markedly similar. The trace metals consisting of transition metals plus zinc are present in a stoichometric molar formula that we have calculated to be as follows: Fe(1)Mn(0.3)Zn(0.26)Cu(0.03)Co(0.03)Mo(0.03). Under conditions of routine cultivation, trace metal homeostasis may be maintained by a series of transporter systems that are energized by the cell. In specific environments where heavy metals are present at toxic levels, some bacteria have developed a detoxification strategy where the metallic ion is reduced outside of the cell. The result of this extracellular metabolism is that the bacterial metallome specific for trace metals is not disrupted. One of the microorganisms that reduces toxic metals outside of the cell is the sulfate-reducing bacterium Desulfovibrio desulfuricans. While D. desulfuricans reduces metals by enzymatic processes involving polyhemic cytochromes c3 and hydrogenases, which are all present inside the cell; we report the presence of chain B cytochrome c nitrite reductase, NrfA, in the outer membrane fraction of D. desulfuricans ATCC 27774 and discuss its activity as a metal reductase.

Heterologous expression of dodecaheme “nanowire” cytochromes c from Geobacter sulfurreducens

Multiheme cytochromes c are difficult to produce in heterologous systems. The genome of delta-proteobacterium Geobacter sulfurreducens contains more than a hundred genes coding for c-type cytochromes. Among those are two dodecaheme cytochromes c representing a new class of multiheme cytochromes, whose putative structure is a one-dimensional array of small highly homologous domains that contain three hemes and are covalently bound by short linkers. They are likely to form "nanowires" that are part of the electron transfer chain. We cloned the genes coding for the two cytochromes into a vector we developed for ligation-independent cloning of proteins targeted to the Escherichia coli periplasmic space. We expressed the proteins in E. coli co-transformed with a plasmid harboring the cytochrome c maturation genes. Expression levels were optimized by varying IPTG concentrations used for induction. Although both proteins appeared insoluble or strongly associated with cell membranes, they were solubilized using 0.5 M sodium chloride which was more selective than conventional solubilizing agents, such as HEGA-10 or beta-octylglucoside. The solubilized proteins were dialyzed and purified by cation exchange chromatography followed by gel filtration. Mass-spectrometry analysis confirmed that both purified proteins contained the complete set of covalently attached hemes, 12 per molecule. Their visible spectra were typical of c-type cytochromes. Both proteins were successfully crystallized.

Sulphate respiration from hydrogen in Desulfovibrio bacteria: a structural biology overview

Sulphate-reducing organisms are widespread in anaerobic enviroments, including the gastrointestinal tract of man and other animals. The study of these bacteria has attracted much attention over the years, due also to the fact that they can have important implications in industry (in biocorrosion and souring of oil and gas deposits), health (in inflamatory bowel diseases) and the environment (bioremediation). The characterization of the various components of the electron transport chain associated with the hydrogen metabolism in Desulfovibrio has generated a large and comprehensive list of studies. This review summarizes the more relevant aspects of the current information available on the structural data of various molecules associated with hydrogen metabolism, namely hydrogenases and cytochromes. The transmembrane redox complexes known to date are also described and discussed. Redox-Bohr and cooperativity effects, observed in a few cytochromes, and believed to be important for their functional role, are discussed. Kinetic studies performed with these redox proteins, showing clues to their functional inter-relationship, are also addressed. These provide the groundwork for the application of a variety of molecular modelling approaches to understanding electron transfer and protein interactions among redox partners, leading to the characterization of several transient periplasmic complexes. In contrast to the detailed understanding of the periplasmic hydrogen oxidation process, very little is known about the cytoplasmic side of the respiratory electron transfer chain, in terms of molecular components (with exception of the terminal reductases), their structure and the protein-protein interactions involved in sulphate reduction. Therefore, a thorough understanding of the sulphate respiratory chain in Desulfovibrio remains a challenging task.

Multi-heme cytochromes--new structures, new chemistry

Heme is one of the most pervasive cofactors in nature and the c-type cytochromes represent one of the largest families of heme-containing proteins. Recent progress in bacterial genomic analysis has revealed a vast range of genes encoding novel c-type cytochromes that contain multiple numbers of heme cofactors. The genome sequence of Geobacter sulfurreducens, for example, includes some one hundred genes encoding c-type cytochromes, with around seventy of these containing two, or more, heme groups and with one protein containing an astonishing twenty seven heme groups. This wealth of cytochromes is of great significance in the respiratory flexibility shown by bacteria such as Geobacter. In addition, we are now discovering that many of these multi-heme cytochromes have associated enzymatic activities and in some cases this is revealing new chemistries. The purpose of this perspective is to describe recent progress in the structural and functional analyses of these new multi-heme cytochromes. To illustrate this we have chosen to focus on three of these cytochromes which exhibit catalytic activities; nitrite reductase, hydroxylamine oxidoreductase and tetrathionate reductase. In addition we consider the multi-heme cytochromes from Geobacter and Desulfovibrio species. Finally, we consider and contrast the repeating structural modules found in these multi-heme cytochromes.

Interaction and electron transfer between the high molecular weight cytochrome and cytochrome c3 from Desulfovibrio vulgaris Hildenborough: Kinetic, microcalorimetric, EPR and electrochemical studies

The complex formation between the tetraheme cytochrome c3 and hexadecaheme high molecular weight cytochrome c (Hmc), the structure of which has recently been resolved, has been characterized by cross-linking experiments, EPR, electrochemistry and kinetic analysis, and some key parameters of the interaction were determined. The analysis of electron transfer between [Fe] hydrogenase, cytochrome c3 and Hmc demonstrates a redox-shuttling role of cytochrome c3 in the pathway from hydrogenase to Hmc, and shows an effect of redox state on the interaction between the two cytochromes. The role of polyheme cytochromes in electron transfer from periplasmic hydrogenase to membrane redox proteins is assessed. A model with cytochrome c3 as an intermediate between hydrogenase and various polyheme cytochromes is proposed and its physiological consequences are discussed.

Remediation and Liquid-Liquid Phase Transfer Extraction of Chromium(VI). A Review

Chromium(VI) is a widely used industrial chemical, and is generally considered to pose a greatest human health risk because it is more toxic, more soluble, and more mobile than chromium(III). Workers in industries that use chromium, in particular stainless steel welding, chromate production, chromium plating, and chrome pigment industries, where exposure via inhalation of aerosols is primarily to hexavalent chromium, are at increased risk of chromium effects. In this article we demonstrate various studies regarding remediation methods in particular liquid-liquid extraction of chromate and dichromate anions with various functionalized calixarenes. This review article briefly discusses various molecular designs of calixarene-type macrocycles for chromium(VI) oxoanion recognition, and gives examples on the relationship between structure and selectivity. The article does not, however, attempt to cover all of the different approaches to chromium(VI) extraction. A review with 73 references.

Role of the aromatic ring of Tyr43 in tetraheme cytochrome c(3) from Desulfovibrio vulgaris Miyazaki F

Tyrosine 43 is positioned parallel to the fifth heme axial ligand, His34, of heme 1 in the tetraheme cytochrome c(3). The replacement of tyrosine with leucine increased the redox potential of heme 1 by 44 and 35 mV at the first and last reduction steps, respectively; its effects on the other hemes are small. In contrast, the Y43F mutation hardly changed the potentials. It shows that the aromatic ring at this position contributes to lowering the redox potential of heme 1 locally, although this cannot be the major contribution to the extremely low redox potentials of cytochrome c(3). Furthermore, temperature-dependent line-width broadening in partially reduced samples established that the aromatic ring at position 43 participates in the control of the kinetics of intramolecular electron transfer. The rate of reduction of Y43L cytochrome c(3) by 5-deazariboflavin semiquinone under partially reduced conditions was significantly different from that of the wild type in the last stage of the reduction, supporting the involvement of Tyr43 in regulation of reduction kinetics. The mutation of Y43L, however, did not induce a significant change in the crystal structure.

Biochemical and genetic characterization of PpcA, a periplasmic c-type cytochrome in Geobacter sulfurreducens

A 9.6 kDa periplasmic c -type cytochrome, designated PpcA, was purified from the Fe(III)-reducing bacterium Geobacter sulfurreducens and characterized. The purified protein is basic (pI 9.5), contains three haems and has an N-terminal amino acid sequence closely related to those of the previously described trihaem c (7) cytochromes of Geobacter metallireducens and Desulfuromonas acetoxidans. The gene encoding PpcA was identified from the G. sulfurreducens genome using the N-terminal sequence, and encodes a protein of 71 amino acids (molecular mass 9.58 kDa) with 49% identity to the c (7) cytochrome of D. acetoxidans. In order to determine the physiological role of PpcA, a knockout mutant was prepared with a single-step recombination method. Acetate-dependent Fe(III) reduction was significantly inhibited in both growing cultures and cell suspensions of the mutant. When ppcA was expressed in trans, the full capacity for Fe(III) reduction with acetate was restored. The transfer of electrons from acetate to anthraquinone 2,6-disulphonate (AQDS; a humic acid analogue) and to U(VI) was also compromised in the mutant, but acetate-dependent reduction of fumarate was not altered. The rates of reduction of Fe(III), AQDS, U(VI) and fumarate were also the same in the wild type and ppcA mutant when hydrogen was supplied as the electron donor. When taken together with previous studies on other electron transport proteins in G. sulfurreducens, these results suggest that PpcA serves as an intermediary electron carrier from acetate to terminal Fe(III) reductases in the outer membrane, and is also involved in the transfer of electrons from acetate to U(VI) and humics.

The crystal structure of the hexadeca-heme cytochrome Hmc and a structural model of its complex with cytochrome c(3)

Sulfate-reducing bacteria contain a variety of multi-heme c-type cytochromes. The cytochrome of highest molecular weight (Hmc) contains 16 heme groups and is part of a transmembrane complex involved in the sulfate respiration pathway. We present the 2.42 A resolution crystal structure of the Desulfovibrio vulgaris Hildenborough cytochrome Hmc and a structural model of the complex with its physiological electron transfer partner, cytochrome c(3), obtained by NMR restrained soft-docking calculations. The Hmc is composed of three domains, which exist independently in different sulfate-reducing species, namely cytochrome c(3), cytochrome c(7), and Hcc. The complex involves the last heme at the C-terminal region of the V-shaped Hmc and heme 4 of cytochrome c(3), and represents an example for specific cytochrome-cytochrome interaction.

Thermodynamic and kinetic characterization of trihaem cytochrome c3 from Desulfuromonas acetoxidans

Trihaem cytochrome c3 (also known as cytochrome c551.5 and cytochrome c7) is isolated from the periplasmic space of Desulfuromonas acetoxidans, a sulfur-reducing bacterium. Thermodynamic and kinetic data for the trihaem cytochrome c3 are presented and discussed in the context of the possible physiological implications of its functional properties with respect to the natural habitat of D. acetoxidans, namely as a symbiont with green sulfur bacteria working as a mini-sulfuretum. The thermodynamic properties were determined through the fit of redox titration data, followed by NMR and visible spectroscopy, to a model of four functional centres that describes the network of cooperativities between the three haems and one protolytic centre. The kinetics of trihaem cytochrome c3 reduction by sodium dithionite were studied using the stopped-flow technique and the data were fitted to a kinetic model that makes use of the thermodynamic properties to obtain the rate constants of the individual haems. This analysis indicates that the electrons enter the cytochrome mainly via haem I. The reduction potentials of the haems in this cytochrome show little variation with pH within the physiological range, and the kinetic studies show that the rates of reduction are also independent of pH in the range studied. Thus, although the trihaem cytochrome c3 is readily reduced by hydrogenases from Desulfovibrio sp. and its haem core is similar to that of the homologous tetrahaem cytochromes c3, its physico-chemical properties are quite different, which suggests that these multihaem cytochromes with similar structures perform different functions.

Production and preliminary characterization of a recombinant triheme cytochrome c(7) from Geobacter sulfurreducens in Escherichia coli

Multiheme cytochromes c have been found in a number of sulfate- and metal ion-reducing bacteria. Geobacter sulfurreducens is one of a family of microorganisms that oxidize organic compounds, with Fe(III) oxide as the terminal electron acceptor. A triheme 9.6 kDa cytochrome c(7) from G. sulfurreducens is a part of the metal ion reduction pathway. We cloned the gene for cytochrome c(7) and expressed it in Escherichia coli together with the cytochrome c maturation gene cluster, ccmABCDEFGH, on a separate plasmid. We designed two constructs, with and without an N-terminal His-tag. The untagged version provided a good yield (up to 6 mg/l of aerobic culture) of the fully matured protein, with all three hemes attached, while the N-terminal His-tag appeared to be detrimental for proper heme incorporation. The recombinant protein (untagged) is properly folded, it has the same molecular weight and displays the same absorption spectra, both in reduced and in oxidized forms, as the protein isolated from G. sulfurreducens and it is capable of reducing metal ions in vitro. The shape parameters for the recombinant cytochrome c(7) determined by small angle X-ray scattering are in good agreement with the ones calculated from a homologous cytochrome c(7) of known structure.

Resonance Raman study of multihemic c-type cytochromes from Desulfuromonas acetoxidans

Two multihemic cytochromes c from the sulfur reducing bacteria Desulfuromonas acetoxidans have been studied by optical and resonance Raman spectroscopy: cytochrome c551.5, a trihemic cytochrome and cytochrome c Mr 50 000, a recently isolated high molecular mass cytochrome. The redox and Raman characteristics of cytochrome c551.5 are compared to those of the tetrahemic cytochromes c3 from Desulfovibrio. While the redox behavior, followed by spectroelectrochemistry, is similar to that of cytochrome c3, showing the same conformational change after reduction of the highest potential heme, the Raman data show a contribution from a His- form of the axial ligands and lead to the assignment of a band at 218 cm-1 to the Fe(III)-(His)2 stretching vibration. The Raman data on cytochrome c Mr 50 000 are in favor of an entirely low spin species with two different sets of axial ligands. A partially reduced state is easily accessible by ascorbate addition.

A sequential electron transfer from hydrogenases to cytochromes in sulfate-reducing bacteria

A central step in the energy metabolism of sulfate-reducing bacteria is the oxidation of molecular hydrogen, catalyzed by a periplasmic hydrogenase. The resulting electrons are then transferred to various electron transport chains and used for cytoplasmic sulfate reduction. The complex formation between [NiFeSe] hydrogenase and the soluble periplasmic polyheme cytochromes from Desulfomicrobium norvegicum was characterized by cross-linking experiments, BIAcore and kinetics analysis. Analysis of electron transfer between [NiFeSe] hydrogenase and octaheme cytochrome c(3) (M(r) 26¿ omitted¿000) pointed out that this cytochrome is reduced faster in the presence of catalytic amounts of tetraheme cytochrome c(3) (M(r) 13¿ omitted¿000) isolated from the same organism. The activation of the hydrogenase-dependent reduction of polyheme cytochromes by cytochrome c(3) (M(r) 13¿ omitted¿000), which is now described in both Desulfovibrio and Desulfomicrobium, is proposed as a general mechanism. During this process, cytochrome c(3) (M(r) 13¿ omitted¿000) would act as an electron shuttle in between hydrogenase and the polyheme cytochromes and its conductivity appears to be an important factor.

A proton-NMR investigation of the fully reduced cytochrome c7 from Desulfuromonas acetoxidans. Comparison between the reduced and the oxidized forms

The solution structure via 1H NMR of the fully reduced form of cytochrome c7 has been obtained. The protein sample was kept reduced by addition of catalytic amounts of Desulfovibrio gigas iron hydrogenase in H2 atmosphere after it had been checked that the presence of the hydrogenase did not affect the NMR spectrum. A final family of 35 conformers with rmsd values with respect to the mean structure of 8.7 +/- 1.5 nm and 12.4 +/- 1.3 nm for the backbone and heavy atoms, respectively, was obtained. A highly disordered loop involving residues 54-61 is present. If this loop is ignored, the rmsd values are 6.2 +/- 1.1 nm and 10.2 +/- 1.0 nm for the backbone and heavy atoms, respectively, which represent a reasonable resolution. The structure was analyzed and compared with the already available structure of the fully oxidized protein. Within the indetermination of the two solution structures, the result for the two redox forms is quite similar, confirming the special structural features of the three-heme cluster. A useful comparison can be made with the available crystal structures of cytochromes c3, which appear to be highly homologous except for the presence of a further heme. Finally, an analysis of the factors affecting the reduction potentials of the heme irons was performed, revealing the importance of net charges in differentiating the reduction potential when the other parameters are kept constant.

The Desulfuromonas acetoxidans triheme cytochrome c7 produced in Desulfovibrio desulfuricans retains its metal reductase activity

Multiheme cytochrome c proteins that belong to class III have been recently shown to exhibit a metal reductase activity, which could be of great environmental interest, especially in metal bioremediation. To get a better understanding of these activities, the gene encoding cytochrome c7 from the sulfur-reducing bacterium Desulfuromonas acetoxidans was cloned from genomic DNA by PCR and expressed in Desulfovibrio desulfuricans G201. The expression system was based on the cyc transcription unit from Desulfovibrio vulgaris Hildenborough and led to the synthesis of holocytochrome c7 when transferred by electrotransformation into the sulfate reducer Desulfovibrio desulfuricans G201. The produced cytochrome was indistinguishable from the protein purified from Desulfuromonas acetoxidans cells with respect to several biochemical and biophysical criteria and exhibited the same metal reductase activities as determined from electrochemical experiments. This suggests that the molecule was correctly folded in the host organism. Desulfovibrio desulfuricans produces functional multiheme c-type cytochromes from bacteria belonging to a different genus and may be considered a suitable host for the heterologous biogenesis of multiheme c-type cytochromes for either structural or engineering studies. This report, which presents the first example of the transformation of a Desulfovibrio desulfuricans strain by electrotransformation, describes work that is the first necessary step of a protein engineering program that aims to specify the structural features that are responsible for the metal reductase activities of multiheme cytochrome c7.

Characterization of the cytochromes C from Desulfovibrio desulfuricans G201

A monoheme cytochrome c553 and a hexadecaheme high molecular weight cytochrome (Hmc) have been isolated and characterized from the sulfate-reducing bacteria Desulfovibrio desulfuricans G201, in addition to the tetraheme cytochrome c3 (Mr 13000) that has been previously described. Both cytochromes are homologous with respect to several biochemical properties to the corresponding cytochromes found in other Desulfovibrio species. However, they are acidic proteins while the corresponding molecules, isolated from other Desulfovibrio species, are relatively more basic. The D. desulfuricans cytochrome content appears identical to that of D. vulgaris Hildenborough. Isolation of these cytochromes from a Desulfovibrio desulfuricans strain is of great interest in order to get more insight on the physiological function of these molecules.

A single mutation in the heme 4 environment of Desulfovibrio desulfuricans Norway cytochrome c3 (Mr 26,000) greatly affects the molecule reactivity

The gene encoding Desulfovibrio desulfuricans Norway cytochrome c3 (Mr 26,000), a dimeric octaheme cytochrome belonging to the polyheme cytochrome c3 superfamily, has been cloned and successfully expressed in another sulfate reducing bacteria, D. desulfuricans G201. The gene, named cycD, is monocistronic and encodes a cytochrome precursor of 135 amino acids with an extension at the NH2 terminus of 24 amino acids. This extension acts as a signal sequence which allows export across the cytoplasmic membrane into the periplasmic space. Tyrosine 73, which is in a close contact with the histidine sixth axial ligand to the heme 4 iron atom, has been replaced by a glutamate residue using site-directed mutagenesis. The cytochrome mutant when expressed in D. desulfuricans G201, is correctly folded and matured. A global increase of the oxidoreduction potentials of about 50 mV is measured for the Y73E cytochrome. The mutation also has a strong influence on the interaction of the cytochrome with its redox partner, the hydrogenase. This suggests, like the tetraheme cytochrome c3 (Mr 13, 000), heme 4 is the interactive heme in the cytochrome-hydrogenase complex and that alteration of the heme 4 environment can greatly affect the electron transfer reaction with its redox partner.

The cytochrome c3 superfamily: amino acid sequence of a dimeric octahaem cytochrome c3 (M(r) 26,000) isolated from Desulfovibrio gigas

Cytochrome c3 (M(r) 26000) isolated from Desulfovibrio gigas is a dimeric cytochrome consisting of two identical subunits of 109 amino acids, each of which contains four haem groups. On the basis of its amino acid sequence, this cytochrome clearly belongs to the cytochrome c3 superfamily, and will be classified in class III of the c-type cytochromes as defined by Ambler [(1980) in From Cyclotrons to Cytochromes (Robinson, A. B. and Kaplan, N. O., eds.), pp. 263-279, Academic Press, London]. It contains ten cysteine and nine histidine residues in each subunit, and eight cysteines and eight histidines linked to the four haem groups were found to be invariant on alignment of all known cytochrome c3 sequences. Two intermolecular disulphide bridges have been determined between cysteine residues 5 and 46 of the two monomers. Cytochrome c3 (M(r) 26,000) from D gigas is clearly different from cytochrome c3 (M(r) 13,000) from the same strain, with which it shows only 27% sequence identity. Compared with cytochrome c3 (M(r) 26,000) from D. desulfuricans Norway, the three-dimensional structure of which has been determined, 26.95% of the residues have been conserved. In the enzyme from D. desulfuricans Norway, hydrophobic interactions have been described across the dimer interface. Residues involved in similar interactions seem to be well conserved in the equivalent D. gigas cytochrome. This sequence provides structural data to allow specification of this new subclass of polyhaem cytochromes. Furthermore, D. gigas cytochrome c3 (M(r) 26,000) is the first polyhaem cytochrome shown to contain two disulphide bridges linking two identical subunits, which could induce more rigid folding. The folding and the evolution of this family of polyhaem cytochromes are discussed.

Crystal structure of a dimeric octaheme cytochrome c3 (M(r) 26,000) from Desulfovibrio desulfuricans Norway

BACKGROUND

The octaheme cytochrome C3 (M(r) 26,000; cc3) from Desulfovibrio desulfuricans Norway is a dimeric cytochrome made up of two identical subunits, each containing four heme groups. It is involved in the redox transfer chain of sulfate-reducing bacteria, which links the periplasmic oxidation of hydrogen to the cytoplasmic reduction of sulfate. The amino-acid sequence of cc3 shows similarities to that of the tetraheme cytochrome c3 (M(r) 13,000; c3) from the same bacteria. Structural analysis of cc3 forms a basis for understanding the precise roles of the multiheme-containing redox proteins and the reason for the presence of several different multiheme cytochromes in one bacterial strain.

RESULTS

The crystal structure of cytochrome cc3 has been determined at 2.16 A resolution. The subunits display the c3 structural fold with significant amino-acid substitutions, relative to the tetraheme cytochromes c3, in the regions of the dimer interface. The identical subunits are related by a crystallographic twofold axis, with one heme of each subunit in close contact. The overall structure and the environments of the different heme groups are compared with those of the tetraheme cytochromes c3.

CONCLUSIONS

A common scheme for interactions between these types of cytochrome and their redox partners involves the interaction of a heme crevice, surrounded by positively charged lysine residues, with acidic residues surrounding the redox partner's functional group. Despite the relatively acidic character of cytochrome cc3, the crevice of one heme is surrounded by a high number of positively charged residues, in the same manner as has been reported for cytochromes c3. The environment of this heme is formed by four flexible surface loops which are variable in length and orientation in the different c3-type cytochromes although the overall structural folds are very similar. It has been proposed that this region, adapted in topology and charge, is the interaction site for physiological partners and is also most likely to be the interaction site in the dimeric cytochrome cc3.