Flash‐induced membrane potential generation by cytochrome c oxidase

Abstract

Flash-induced single-electron reduction of cytochrome c oxidase. Compound F (oxoferryl state) by Ru^II(2,2'-bipyridyl)²⁺ ₃ [Nilsson (1992) Proc. Natl. Acad. Sci. USA 89, 6497-6501] gives rise to three phases of membrane potential generation in proteoliposomes with τ values and contributions of ca. 45 μs (20%), 1 ms (20%) and 5 ms (60%). The rapid phase is not sensitive to the binuclear centre ligands, such as cyanide or peroxide, and is assigned to vectorial electron transfer from Cu_A to heme a. The two slow phases kinetically match reoxidation of heme a, require added H₂O₂ or methyl peroxide for full development, and are completely inhibited by cyanide; evidently, they are associated with the reduction of Compound F to the Ox state by heme a. The charge transfer steps associated with the F to Ox conversion are likely to comprise (i) electrogenic uptake of a ‘chemical’ proton from the N phase required for protonation of the reduced oxygen atom and (ii) electrogenic H⁺ pumping across the membrane linked to the F to Ox transition. Assuming heme a ‘electrical location’ in the middle of the dielectric barrier, the ratio of the rapid to slow electrogenic phase amplitudes indicates that the F to Ox transition is linked to transmembrane translocation of 1.5 charges (protons) in addition to an electrogenic uptake of one ‘chemical’ proton required to form Fe³⁺-OH⁻ from Fe⁴⁺ = O²⁻. The shortfall in the number of pumped protons and the biphasic kinetics of the millisecond part of the electric response matching biphasic reoxidation of heme a may indicate the presence of 2 forms of Compound F, reduction of only one of which being linked to full proton pumping.