On the electron-transfer-coupled proton release of cytochrome c

Horse heart ferrocytochrome c was oxidized by injection of ferrihexacyanide and the amount of protons released was measured quantitatively over the pH range 7–11, leading to the two electron-transfer-linked pK_H's of 9.1 and 10.4. The kinetic result that the proton changes took place more rapidly than the changes in the 695-mμ absorption band for both types of rapid pH changes on oxidized cytochrome c (pH 7 → 10 and pH 10 → 7) led to the proposal of a cyclic reaction scheme. Involved protonreleasing amino acid residues are discussed.