Energy-dependent reversal of the cytochrome oxidase reaction

Energization of isolated rat liver mitochondria with ATP under conditions in which cytochrome c is poised in a highly oxidized state shifts the state of cytochrome oxidase (cytochrome c oxidase; ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) from fully oxidized to two new spectroscopically distinguishable states depending on the applied phosphorylation potential and redox potential at cytochrome c. Both new states are spectrally similar or identical to two previously described intermediates in the reaction between reduced enzyme and O2. The data suggest that the energy-dependent transitions are due to reversed electron transfer from water to ferricytochrome c linked to accumulation of intermediates of O2 reduction at the catalytic heme a3/copper center. Titrations with redox potential indicate that each transition is a one-electron step, a finding that would identify the second observed compound as enzyme-bound peroxide or its equivalent. This is consistent with this compound being spectrally identical to "Compound C," previously described as the reaction product between half-reduced oxidase (two electrons) and O2. On the basis of these data a catalytic scheme of O2 reduction is proposed for the heme a3/copper center of cytochrome oxidase.