Molecular oxygen modulates cytochrome c oxidase functions

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 O-2 concentrations exhibit a reversible suppression of State 3 respiration, Oxidized bovine heart cytochrome c oxidase was incubated in oxygen concentrations of < 50 mu M 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 a(3) was 33% reduced during cycling at [O-2] = 25 mu M whereas enzyme at [O-2] = 135 mu M was only 18% reduced, suggesting that the site of inhibition occurred at the electron transfer step between heme a(2) and O-2. These results provide a mechanistic explanation for the observation that intact cells or mitochondria exhibit a reversible inhibition of respiration during prolonged exposure to [O-2] < 25 mM, by demonstrating that the catalytic activity of cytochrome c oxidase function is similarly inhibited, possibly through an allosteric effect of molecular O-2 on the enzyme.