A mechanistic principle for proton pumping by cytochrome c oxidase

Pump it up The nature of proton pumping by cytochrome c oxidase, last link in the electron transfer chain in mitochondria and many bacteria, has been a reliable source of controversy over the years. Its mechanism of action is still something of a mystery, but a new study of proton pumping events in lipid vesicles containing a single molecule of the oxidase suggests a mechanism of action that could be generalized to all membrane-bound ion transporters. In aerobic organisms, cellular respiration involves electron transfer to oxygen through a series of membrane-bound protein complexes. The process maintains a transmembrane electrochemical proton gradient that is used, for example, in the synthesis of ATP. In mitochondria and many bacteria, the last enzyme complex in the electron transfer chain is cytochrome c oxidase (Cyt c O), which catalyses the four-electron reduction of O 2 to H 2 O using electrons delivered by a water-soluble donor, cytochrome c 1 , 2 , 3 , 4 , 5 , 6 , 7 . The electron transfer through Cyt c O, accompanied by proton uptake to form H 2 O drives the physical movement (pumping) of four protons across the membrane 8 per reduced O 2 . So far, the molecular mechanism of such proton pumping driven by electron transfer has not been determined in any biological system. Here we show that proton pumping in Cyt c O is mechanistically coupled to proton transfer to O 2 at the catalytic site, rather than to internal electron transfer. This scenario suggests a principle by which redox-driven proton pumps might operate and puts considerable constraints on possible molecular mechanisms by which Cyt c O translocates protons.