A Membrane-Bound Cytochromec3: A Type II Cytochromec3 fromDesulfovibrio vulgaris Hildenborough

A new tetraheme cytochrome c(3) was isolated from the membranes of Desulfovibrio vulgaris Hildenborough (DvH). This cytochrome has a molecular mass of 13.4 kDa and a pl 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 c(3) 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 c(3) proteins, such as a solvent-exposed heme 1 surrounded by an acidic surface area, and a heme 4 which locks most of the surface lysine patch proposed to be the site of hydrogenase interaction in other cytochrome c(3) proteins. Furthermore, in contrast to previously discovered cytochrome c(3) 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 port of membrane-bound oxidoreductase complexes. Altogether these observations suggest that the DvH and Da cytochromes are a new type of cytochrome c(3) proteins (Type II: TpII-c(3)) with different redox partners and physiological function than the other cytochrome c(3) proteins (Type I: TpI-c(3)). The DvH TpII-c(3) is reduced at considerable rates by the two membrane-bound [NiFe] and [NiFeSe] hydrogenases, but catalytic amounts of TpI-c(3) increase these rates two- and fourfold, respectively. With the periplasmic [Fe] hydrogenase TpII-c(3), is reduced much slower than TpI-c(3), and no catalytic effect of Tpl-c(3) is observed.