Equilibrium and kinetic measurements of the redox potentials of cytochromes c2 in vitro and in vivo.

The equilibrium oxidation-reduction mipoint potential (Em) of isolated Rhodopseudomonas sphaeroides cytochrome c2 exhibits a pH-dependent behavior which can be ascribed to a pK on the oxidized form at pH 8.0 (Pettigrew et al. (1975) Biochim. Biophys. Acta 430, 197-208). However, as with mammalian cytochrome c (Brandt, K.G. Parks, P.C., Czerlinski, G.H. and Hess, G.P. (1966) J. Biol. Chem. 241, 4180-4185) this pK can more properly be attributed to the combination of a pK beyond pH 11, and a slow conformational change of the ferricytochrome. This has been demonstrated by resolving the Em of cytochrome c2 before and after the conformational change. The Em of the unaltered form is essentially pH independent between pH 7 and 11.5, and the lower equilibrium Em is due solely to the conformational change. In vivo the conformational change is prevented by the binding of the cytochrome c2 to the photochemical reaction center, and the cytochrome exhibits an essentially pH-independent Em from pH 5 to 11. The alkaline transition thus has little physiological significance, and it is unlikely that the redox reactions of cytochrome c2 in vivo involve protons.