Kinetic Treatment of Coupled Electron and Proton Transfer in Flash-Photolysis Experiments on Carbon Monoxide-Inhibited Mixed-Valence Cytochrome c Oxidase

Coupled electron and proton transfer observed in flow-flash experiments on CO-inhibited mixed-valence cytochrome c oxidase is discussed in terms of a model proposed by Brzezinski and co-workers [ J. Bioenerg. Biomembr . 1998, 30, 99−107]. The model includes two redox states of the heme a /heme a 3 pair and two states, protonated and deprotonated, of a redox-linked group L, which is in contact with bulk solution via a proton conducting channel. The proton channel is represented by another protolytic group L‘, which is in equilibrium with bulk solution, but not with group L. The theory reproduces the experimentally observed pH dependence of the slow kinetics of heme a reduction following dissociation of the enzyme−CO complex, and additionally predicts a pH dependence of the fast kinetics due to varying proton equilibrium between group L and bulk solution prior to dissociation. The rates of internal proton transfer between L and L‘ in the reduced and oxidized states, and the bimolecular rate of protonation of L‘ by bulk protons have been evaluated from the present theory and experimental data. The protonation rate of the group L in the reduced state of heme a 3 is = 10 4 s -1 . From the observed pH dependence of the rate constant for the slow kinetic phase of backward electron transfer the rate of L‘ protonation is estimated to be = 5 × 10 11 M -1 s -1 .