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

Abstract

Three c-type cytochromes were isolated and characterized from the sulfate reducer Desulfovibrio africanus. A basic tetraheme cytochrome c 3 of molecular mass 16 kDa was previously described and we have extended its characterization. Two other c 3-type cytochromes, not previously observed, have also been characterized. These include an acidic tetraheme cytochrome c 3 of molecular mass 15 kDa and an octaheme dimeric cytochrome c 3 with a native size of 35 kDa. This is the first report of the presence of two distinct tetraheme cytochromes c 3 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 c 3 family. The acidic tetraheme cytochrome c 3 exhibited similar midpoint potential values for all four hemes ( E m1 = −210 mV; E m2 = −240 mV E m3 = −260 mV E m4 = −270 mV) whereas in the basic tetraheme cytochrome c 3 one heme had a much more positive potential than the others ( E m1 = −90 mV E m2 = −260 mV E m3 = −280 mV E m4 = −290 mV The acidic tetraheme cytochrome c 3 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 c 3. The weak reactivity of the acidic tetraheme cytochrome c 3 towards hydrogenase has been correlated with its low content of basic residues.