A membrane-bound cytochrome c(3): A type II cytochrome c(3) from Desulfovibrio vulgaris Hildenborough

A new tetraheme cytochrome c(3) was isolated from the membranes of Desulfov ibrio 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 red uction potentials of - 170 mV -235 mV, -260 mV and -325 mV at pH 7.6. The c omplete 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 struct ure of the DvH cytochrome was built based on the structure of the Da cytoch rome. Both cytochromes share structural features that distinguish them from other cytochrome c(3) proteins, such as a solvent-exposed heme 1 surrounde d by an acidic surface area, and a heme 4 which locks most of the surface l ysine patch proposed to be the site of hydrogenase interaction in other cyt ochrome c(3) proteins. Furthermore, in contrast to previously discovered cy tochrome c(3) proteins, the genes coding for these two cytochromes are adja cent to genes coding for two membrane-associated FeS proteins, which indica tes that they may be port of membrane-bound oxidoreductase complexes. Altog ether 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 amou nts 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.