The pH dependence of the oxidation-reduction midpoint potential of cytochromes c2 in vivo

A recent report by Pettigrew et al. [Biochim. Biophys. Acta 430, (1976), 197–208] has examined the pH dependence of the oxidation-reduction midpoint potential of cytochromes c₂ in vitro. In media of low ionic strength, these workers identified several pKs on the oxidized forms of the cytochromes, and in some cases there were also pKs on the reduced species. In this work we examine the pH dependence of the midpoint potentials of the cytochromes in situ, attached to the chromatophore membrane. Under these conditions no pK values are detected, and we conclude that in vivo there is no net change in the protonation of cytochrome c₂ during oxidation or reduction.