Electron transfer in cell respiration is coupled to proton translocation across mitochondrial and bacterial membranes, which is a primary event of biological energy transduction. The resulting electrochemical proton gradient is used to power energy-requiring reactions, such as ATP synthesis. Cytochrome c oxidase is a key component of the respiratory chain, which harnesses dioxygen as a sink for electrons and links O2 reduction to proton pumping1. Electrons from cytochrome c are transferred sequentially to the O2 reduction site of cytochrome c oxidase via two other metal centres, CuA and haem a, and this is coupled to vectorial proton transfer across the membrane by a hitherto unknown mechanism. On the basis of the kinetics of proton uptake and release on the two aqueous sides of the membrane, it was recently suggested that proton pumping by cytochrome c oxidase is not mechanistically coupled to internal electron transfer2. Here we have monitored translocation of electrical charge equivalents as well as electron transfer within cytochrome c oxidase in real time. The results show that electron transfer from haem a to the O2 reduction site initiates the proton pump mechanism by being kinetically linked to an internal vectorial proton transfer. This reaction drives the proton pump and occurs before relaxation steps in which protons are taken up from the aqueous space on one side of the membrane and released on the other2.
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We thank L. Laakkonen for help in preparing Fig. 1, and A. Puustinen and C. Ribacka for providing samples of wild-type and mutant enzyme. This work was supported by grants from the Sigrid Jusélius Foundation, Biocentrum Helsinki and the Academy of Finland (programme 44895).
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Muscle mass and inspired oxygen influence oxygen extraction at maximal exercise: Role of mitochondrial oxygen affinity
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