Zinc ions inhibit oxidation of cytochrome c oxidase by oxygen

Cytochrome c oxidase is a membrane-bound enzyme that catalyses the reduction of O-2 to H2O and uses part of the energy released in this reaction to pump protons across the membrane, We have investigated the effect of addition of Zn2+ on the kinetics of two reaction steps in cytochrome c oxidase that are associated with proton pumping; the peroxy to oxo-ferryl (P-r --> F) and the oxo-ferryl to oxidised (F --> O) transitions. The Zn2+ binding resulted in a decrease of the F --> O rate from 820 s(-1) (no Zn2+) to a saturating value of similar to 360 s(-1) with an apparent K-D of similar to2.6 muM. The P-r --> F rate (similar to 10(4) s(-1) before addition of Zn2+) decreased more slowly with increasing Zn2+ concentration and a K-D of similar to 120 muM was observed. The effects on both kinetic phases were fully reversible upon addition of EDTA. Since both the P-r --> F and F --> O transitions are associated with proton uptake through the D-pathway, a Zn2+-binding site is likely to be located at the entry point of this pathway. where several carboxylates and histidine residues are found that may co-ordinate Zn2+. (C) 2001 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.