Molecular oxygen modulates cytochrome c oxidase function.
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 microM for 4 h. The enzyme exhibited a reversible decrease in Vmax after incubation, compared with control enzyme incubated at higher oxygen concentrations. This change was accompanied by a small increase in the apparent Km 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 microM whereas enzyme at [O2] = 135 microM 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.
Chandel
N S
NS
Department of Medicine, University of Chicago, Chicago, Illinois 60637, USA.
Budinger
G R
GR
Schumacker
P T
PT
D000818:Animals; D001205:Ascorbic Acid; D002384:Catalysis; D002417:Cattle; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008929:Mitochondria, Heart / Q000201:enzymology; D010100:Oxygen / Q000378:metabolism*; D010101:Oxygen Consumption; D013053:Spectrophotometry; D013772:Tetramethylphenylenediamine
D003576:Electron Transport Complex IV; D013772:Tetramethylphenylenediamine; D001205:Ascorbic Acid; D010100:Oxygen