A detailed comparison of the refined structures of cytochrome c3 molecules from two strains in Desulfovibrio vulgaris: the relationship between the heme structures and their redox properties

Further characterization of the two tetraheme cytochromes c3 from Desulfovibiro africanus: nucleotide sequences, EPR spectroscopy and biological activity

The genes encoding the basic and acidic tetraheme cytochromes c3 from Desulfovibrio africanus have been sequenced. The corresponding amino acid sequences of the basic and acidic cytochromes c3 indicate that the mature proteins consist of a single polypeptide chain of 117 and 103 residues, respectively. Their molecular masses, 15102 and 13742 Da, respectively, determined by mass spectrometry, are in perfect agreement with those calculated from their amino acid sequences. Both D. africanus cytochromes c3 are synthesized as precursor proteins with signal peptides of 23 and 24 residues for the basic and acidic cytochromes, respectively. These cytochromes c3 exhibit the main structural features of the cytochrome c3 family and contain the 16 strictly conserved cysteine + histidine residues directly involved in the heme binding sites. The D. africanus acidic cytochrome c3 differs from all the other homologous cytochromes by its low content of basic residues and its distribution of charged residues in the amino acid sequence. The presence of four hemes per molecule was confirmed by EPR spectroscopy in both cytochromes c3. The g-value analysis suggests that in both cytochromes, the angle between imidazole planes of the axial histidine ligands is close to 90 degrees for one heme and much lower for the three others. Moreover, an unusually high exchange interaction (approximately 10[-2] cm[-1]) was evidenced between the highest potential heme (-90 mV) and one of the low potential hemes in the basic cytochrome c3. The reactivity of D. africanus cytochromes c3 with heterologous [NiFe] and [Fe] hydrogenases was investigated. Only the basic one interacts with the two types of hydrogenase to achieve efficient electron transfer, whereas the acidic cytochrome c3 exchanges electrons specifically with the basic cytochrome c3. The difference in the specificity of the two D. africanus cytochromes c3 has been correlated with their highly different content of basic and acidic residues.

Cytochrome c3 from Desulfovibrio gigas: crystal structure at 1.8 A resolution and evidence for a specific calcium-binding site

Crystals of the tetraheme cytochrome c3 from sulfate-reducing bacteria Desulfovibrio gigas (Dg) (MW 13 kDa, 111 residues, four heme groups) were obtained and X-ray diffraction data collected to 1.8 A resolution. The structure was solved by the method of molecular replacement and the resulting model refined to a conventional R-factor of 14.9%. The three-dimensional structure shows many similarities to other known crystal structures of tetraheme c3 cytochromes, but it also shows some remarkable differences. In particular, the location of the aromatic residues around the heme groups, which may play a fundamental role in the electron transfer processes of the molecule, are well conserved in the cases of hemes I, III, and IV. However, heme II has an aromatic environment that is completely different to that found in other related cytochromes c3. Another unusual feature is the presence of a Ca2+ ion coordinated by oxygen atoms supplied by the protein within a loop near the N-terminus. It is speculated that this loop may be stabilized by the presence of this Ca2+ ion, may contribute to heme-redox perturbation, and might even be involved in the specificity of recognition with its redox partner.

Biochemical studies of the c-type cytochromes of the sulfate reducer Desulfovibrio africanus. Characterization of two tetraheme cytochromes c3 with different specificity

Three c-type cytochromes were isolated and characterized from the sulfate reducer Desulfovibrio africanus. A basic tetraheme cytochrome c3 of molecular mass 16 kDa was previously described and we have extended its characterization. Two other c3-type cytochromes, not previously observed, have also been characterized. These include an acidic tetraheme cytochrome c3 of molecular mass 15 kDa and an octaheme dimeric cytochrome c3 with a native size of 35 kDa. This is the first report of the presence of two distinct tetraheme cytochromes c3 in a Desulfovibrio species. The physico-chemical properties of the three cytochromes, including optical properties, iron content, cysteine and histidine content, N-terminal amino sequence and redox properties, are characteristic of cytochrome c3 family. The acidic tetraheme cytochrome c3 exhibited similar midpoint potential values for all four hemes (Em1 = -210 mV; Em2 = -240 mV; Em3 = -260 mV; Em4 = -270 mV), whereas in the basic tetraheme cytochrome c3 one heme had a much more positive potential than the others (Em1 = -90 mV; Em2 = -260 mV; Em3 = -280 mV; Em4 = -290 mV). The acidic tetraheme cytochrome c3 exhibited unique properties including amino-acid composition and poor reactivity towards hydrogenase. However, it is readily reduced by this enzyme in the presence of the basic cytochrome c3. The weak reactivity of the acidic tetraheme cytochrome c3 towards hydrogenase has been correlated with its low content of basic residues.