Timing of electron and proton transfer in the ba(3) cytochrome c oxidase from Thermus thermophilus.

Heme-copper oxidases are membrane-bound proteins that catalyze the reduction of O(2) to H(2)O, a highly exergonic reaction. Part of the free energy of this reaction is used for pumping of protons across the membrane. The ba(3) oxidase from Thermus thermophilus presumably uses a single proton pathway for the transfer of substrate protons used during O(2) reduction as well as for the transfer of the protons that are pumped across the membrane. The pumping stoichiometry (0.5 H(+)/electron) is lower than that of most other (mitochondrial-like) oxidases characterized to date (1 H(+)/electron). We studied the pH dependence and deuterium isotope effect of the kinetics of electron and proton transfer reactions in the ba(3) oxidase. The results from these studies suggest that the movement of protons to the catalytic site and movement to a site located some distance from the catalytic site [proposed to be a "proton-loading site" (PLS) for pumped protons] are separated in time, which allows individual investigation of these reactions. A scenario in which the uptake and release of a pumped proton occurs upon every second transfer of an electron to the catalytic site would explain the decreased proton pumping stoichiometry compared to that of mitochondrial-like oxidases.