Molecular Oxygen Modulates Cytochrome c Oxidase Function*

Abstract

This study sought to determine whether molecular oxygen interacts with cytochrome c oxidase to modify its catalytic activity. Such an interaction could explain the observation that mitochondria incubated under low O₂ concentrations exhibit a reversible suppression of State 3 respiration. Oxidized bovine heart cytochrome c oxidase was incubated in oxygen concentrations of <50 μM for 4 h. The enzyme exhibited a reversible decrease in V_max after incubation, compared with control enzyme incubated at higher oxygen concentrations. This change was accompanied by a small increase in the apparent K_m of the enzyme for both cytochrome c and oxygen, although the optical absorption spectra of oxidized, cycling, or reduced enzyme were not affected. Spectroscopy studies after 4 h of incubation revealed that heme a₃ was 33% reduced during cycling at [O₂] = 25 μM whereas enzyme at [O₂] = 135 μM was only 18% reduced, suggesting that the site of inhibition occurred at the electron transfer step between heme a₃ and O₂. These results provide a mechanistic explanation for the observation that intact cells or mitochondria exhibit a reversible inhibition of respiration during prolonged exposure to [O₂] <25 mM, by demonstrating that the catalytic activity of cytochrome c oxidase function is similarly inhibited, possibly through an allosteric effect of molecular O₂ on the enzyme.