Cytochrome c interaction with membranes. The enthalpy of oxidation of cytochromes c, c2, and a1,2.

The enthalpy of oxidation of horse-heart cytochrome c bound to phospholipid vesicles was found to be 14.6 ± 0.3 kcal/mole at 25 °C, pH 7.0, equal to the value for oxidation of the free form of the cytochrome. The affinity constants for binding of the reduced and oxidized forms of cytochrome c were the same at 4 °C and 30 °C, indicating that ΔH ° of binding contributes negligibly to the overall enthalpy of oxidation of the bound cytochrome c. The free energy (ΔG °′) of oxidation of the bound cytochrome c was 1.3 kcal/mole smaller than that for the free form, the difference being due to the change in entropy favoring the oxidized state of the cytochrome in the bound state. Measurement of the ΔH °′ for the oxidation of cytochrome a relative to the ferri/ferrocyanide couple shows it to be the same, within the limits of experimental error to that for the oxidation of cytochrome c.