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.