Proton-transport mechanisms in cytochrome c oxidase revealed by studies of kinetic isotope effects.

Cytochrome c oxidase (CytcO) is a membrane-bound enzyme, which catalyzes the reduction of di-oxygen to water and uses a major part of the free energy released in this reaction to pump protons across the membrane. In the Rhodobacter sphaeroides aa₃ CytcO all protons that are pumped across the membrane, as well as one half of the protons that are used for O₂ reduction, are transferred through one specific intraprotein proton pathway, which holds a highly conserved Glu286 residue. Key questions that need to be addressed in order to understand the function of CytcO at a molecular level are related to the timing of proton transfers from Glu286 to a "pump site" and the catalytic site, respectively. Here, we have investigated the temperature dependencies of the H/D kinetic-isotope effects of intramolecular proton-transfer reactions in the wild-type CytcO as well as in two structural CytcO variants, one in which proton uptake from solution is delayed and one in which proton pumping is uncoupled from O₂ reduction. These processes were studied for two specific reaction steps linked to transmembrane proton pumping, one that involves only proton transfer (peroxy-ferryl, P→F, transition) and one in which the same sequence of proton transfers is also linked to electron transfer to the catalytic site (ferryl-oxidized, F→O, transition). An analysis of these reactions in the framework of theory indicates that that the simpler, P→F reaction is rate-limited by proton transfer from Glu286 to the catalytic site. When the same proton-transfer events are also linked to electron transfer to the catalytic site (F→O), the proton-transfer reactions might well be gated by a protein structural change, which presumably ensures that the proton-pumping stoichiometry is maintained also in the presence of a transmembrane electrochemical gradient. Furthermore, the present study indicates that a careful analysis of the temperature dependence of the isotope effect should help us in gaining mechanistic insights about CytcO.