Single-electron reduction of the oxidized state is coupled to proton uptake via the K pathway in Paracoccus denitrificans cytochrome c oxidase

The reductive part of the catalytic cycle of cytochrome c oxidase from Para coccus denitrificans was examined by using time-resolved potential measurem ents on black lipid membranes. Proteoliposomes were adsorbed to the black l ipid membranes and Ru-II(2,2'-bipyridyl)(3)(2+) was used as photoreductant to measure flash-induced membrane potential generation. Single-electron red uction of the oxidized wild-type cytochrome c oxidase reveals two phases of membrane potential generation (tau(1) approximate to 20 mu s and tau(2) ap proximate to 175 mu s) at pH 7.4. The fast phase is not sensitive to cyanid e and is assigned to electron transfer from Cu-A to heme a. The slower phas e is inhibited completely by cyanide and shows a kinetic deuterium isotope effect by a factor of 2-3, Although two enzyme variants mutated in the so-c alled D pathway of proton transfer (D124N and E278Q) show the same time con stants and relative amplitudes as the wild-type enzyme, in the K pathway va riant K354M, tau(2) is increased to 900 mu s. This result suggests uptake o f a proton through the K pathway during the transition from the oxidized to the one-electron reduced state. After the second laser flash under anaerob ic conditions, a third electrogenic phase with a time constant of approxima te to 1 ms appears. The amplitude of this phase grows with increasing flash number. We explain this growth by injection of a second electron into the single-electron reduced enzyme. On multiple flashes, both D pathway mutants behave differently compared with the wild type and two additional slow pha ses of tau(3) approximate to 2 ms and tau(4) approximate to 15 ms are obser ved. These results suggest that the D pathway is involved in proton transfe r coupled to the uptake of the second electron.