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In mitochondria and aerobic bacteria energy conservation involves electron transfer through a number of membrane-bound protein complexes to [O.sub.2]. The reduction of [O.sub.2], accompanied by the uptake of substrate protons to form [H.sub.2]O, is catalyzed by cytochrome c oxidase (CcO). This reaction is coupled to proton translocation (pumping) across the membrane such that each electron transfer to the catalytic site is linked to the uptake of two protons from one side and the release of one proton to the other side of the membrane, To address the mechanism of vectorial proton translocation, in this study we have investigated the solvent deuterium isotope effect of proton-transfer rates in CcO oriented in small unilamellar vesicles. Although in [H.sub.2]O the uptake and release reactions occur with the same rates, in [D.sub.2]O the substrate and pumped protons are taken up first ([[tau].sub.D] [congruent to] 200 [micro]s, 'peroxy' to 'ferryl' transition) followed by a significantly slower proton release to the other side of the membrane (To [congruent to] 1 ms). Thus, the results define the order and timing of the proton transfers during a pumping cycle. Furthermore, the results indicate that during CcO turnover internal electron transfer to the catalytic site is controlled by the release of the pumped proton, which suggests a mechanism by which CcO orchestrates a tight coupling between electron transfer and proton translocation. heme-copper oxidases | electron transfer | membrane protein respiratory chain