Thermodynamics of the P-type Ferryl Form of Bovine Cytochrome c Oxidase

AbstractSeveral ferryl states of the catalytic hemea3-CuB center of the respiratory cytochromec oxidases (CcOs) are observed during the reduction of O2 to H2O. One of theP-type ferryl forms,PM, is produced by the reaction of the two-electron reduced CcO with O2. In this state, the hemea3 iron is in the ferryl state and a free radical should be also present at the catalytic center. However, the energetics of thePM formation has not been experimentally established yet. Here, the generation ofPM by the reaction of oxidized bovine CcO (O) with one molecule of H2O2 was investigated by the isothermal titration calorimetry and UV-Vis absorption spectroscopy. Two kinetic phases, corresponding to the formation ofPM and its endogenous conversion back toO, were resolved by both methods. The ΔH of the entire process (–66 kcal/mol H2O2) was larger than the heat ( –50.8 kcal/mol O2) liberated during O2 reduction by ferrocytochromec (pH 8, 25 °C). Interestingly, ΔH of the first phase (–32 kcal/mol ferryl state) far exceeds the enthalpy of thePM production. The data indicate that during the first phase, the radical inPM is quenched and spectrally similar secondP-type ferryl form (PR) is produced. Additionally, it was shown that the entropy contribution to the Gibbs energy change ( ΔG = –46 kcal/mol O2) during the catalytic reduction of O2 by ferrocytochromec is negligible ( –0.7 cal·mol–1·K–1).
Source: Biochemistry (Moscow) - Category: Biochemistry Source Type: research