Maarten Ruitenberg

Reduction of cytochrome c oxidase by a second electron leads to proton translocation

Cytochrome c oxidase, the terminal enzyme of cellular respiration in mitochondria and many bacteria, reduces O2 to water. This four-electron reduction process is coupled to translocation (pumping) of four protons across the mitochondrial or bacterial membrane; however, proton pumping is poorly understood. Proton pumping was thought to be linked exclusively to the oxidative phase, that is, to the transfer of the third and fourth electron. Upon re-evaluation of these data, however, this proposal has been questioned, and a transport mechanism including proton pumping in the reductive phase—that is, during the transfer of the first two electrons—was suggested. Subsequently, additional studies reported that proton pumping during the reductive phase can occur, but only when it is immediately preceded by an oxidative phase. To help clarify the issue we have measured the generation of the electric potential across the membrane, starting from a defined one-electron reduced state. Here we show that a second electron transfer into the enzyme leads to charge translocation corresponding to pumping of one proton without necessity for a preceding turnover.

Zn2+ binding to the cytoplasmic side of Paracoccus denitrificans cytochrome c oxidase selectively uncouples electron transfer and proton translocation1

Using a combination of stopped‐flow spectrophotometric proton pumping measurements and time‐resolved potential measurements on black lipid membranes, we have investigated the effect of Zn2+ ions on the proton transfer properties of Paracoccus denitrificans cytochrome c oxidase. When zinc was enclosed in the interior of cytochrome c oxidase containing liposomes, the H/e stoichiometry was found to gradually decrease with increasing Zn2+ concentration. Half‐inhibition of proton pumping was observed at [Zn2+] i =75 μM corresponding to about 5–6 Zn2+ ions per oxidase molecule. In addition, there was a significant increase in the respiratory control ratio of the proteoliposomes upon incorporation of Zn2+. Time‐resolved potential measurements on a black lipid membrane showed that the electrogenic phases slowed down in the presence of Zn2+ correspond to phases that have been attributed to proton uptake from the cytoplasmic side and to proton pumping. We conclude that Zn2+ ions bind close to or within the two proton transfer pathways of the bacterial cytochrome c oxidase.