Energy-dependent reversal of the cytochrome oxidase reaction.

Abstract

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 from fully oxidized to 2 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 2 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/Cu center. Titrations with redox potential indicate that each transition is a 1-electron step, a finding that would identify the 2nd 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 (2 electrons) and O2. On the basis of these data a catalytic scheme of O2 reduction is proposed for the heme a3/Cu center of cytochrome oxidase.