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Abstract
A new tetraheme cytochrome c3 was isolated from the membranes of Desulfovibrio vulgaris Hildenborough (DvH). This cytochrome has a molecular mass of 13.4 kDa and a pI of 5.5 and contains four heme c groups with apparent reduction potentials of -170 mV, -235 mV, -260 mV and -325 mV at pH 7.6. The complete sequence of the new cytochrome, retrieved from the preliminary data of the DvH genome, shows that this cytochrome is homologous to the "acidic" cytochrome c3 from Desulfovibrio africanus (Da). A model for the structure of the DvH cytochrome was built based on the structure of the Da cytochrome. Both cytochromes share structural features that distinguish them from other cytochrome c3 proteins, such as a solvent-exposed heme 1 surrounded by an acidic surface area, and a heme 4 which lacks most of the surface lysine patch proposed to be the site of hydrogenase interaction in other cytochrome c3 proteins. Furthermore, in contrast to previously discovered cytochrome c3 proteins, the genes coding for these two cytochromes are adjacent to genes coding for two membrane-associated FeS proteins, which indicates that they may be part of membrane-bound oxidoreductase complexes. Altogether these observations suggest that the DvH and Da cytochromes are a new type of cytochrome c3 proteins (Type II: TpII-c3) with different redox partners and physiological function than the other cytochrome c3 proteins (Type I: TpI-c3). The DvH TpII-c3 is reduced at considerable rates by the two membrane-bound [NiFe] and [NiFeSe] hydrogenases, but catalytic amounts of TpI-c3 increase these rates two- and fourfold, respectively. With the periplasmic [Fe] hydrogenase TpII-c3 is reduced much slower than TpI-c3, and no catalytic effect of TpI-c3 is observed.
Highlights
Sulfate-reducing bacteria are anaerobes that can grow by oxidation of hydrogen or organic compounds using sulfate as terminal electron acceptor
The distribution of a cytochrome between soluble and membrane extracts was previously described for the 16-heme high molecular weight cytochrome c (16Hcc) of Desulfovibrio vulgaris Hildenborough (DvH) and Desulfovibrio gigas (Dg),[9] but these cytochromes are found in much higher amounts in the membrane than in the soluble fraction
In relation to the catalytic effect of the TpI-c3 on the reduction of the TpII-c3 by the [NiFe] and [NiFeSe] hydrogenases, it should be noted that for the Dd27k nine-heme cytochrome c (9Hcc), which presents a similar effect,[8] a high probability specific interaction was observed by modelling studies between the negative heme 1 N-terminal region of 9Hcc and the positive heme 4 region of the TpI-c3.[49]. Since heme 1 of the Dd27k 9Hcc has similar characteristics to that of the TpII-c3s, it is likely that an analogous situation may be present in the interaction between the DvH TpII-c3 and TpI-c3
Summary
Sulfate-reducing bacteria are anaerobes that can grow by oxidation of hydrogen or organic compounds using sulfate as terminal electron acceptor. A characteristic feature of this electron transfer pathway is the involvement of multiheme cytochromes c of low redox potential.[1] Several elements of this family of electron transfer proteins have been identified in Desulfovibrio, the most studied genus of sulfate-reducing bacteria: The tetraheme cytochrome c3 (Mr 13 000)[2] is the most abundant member, and is the only one present in all the species studied so far. In this report we describe the purification and characterisation of the TpII-c3 from the membranes of DvH, as well as a study of its reactivity with the [Fe], [NiFe] and [NiFeSe] hydrogenases from the same organism
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