Electron transfer between cytochrome c and the binuclear center of cytochrome oxidase

Abstract

The Minnaert model, which can account for the reaction kinetics between cytochrome oxidase and cytochrome c (Cytc), has been used to justify equal binding rate constants for reduced and oxidized Cytc. Here we extend the model beyond reversible binding of Cytc and its irreversible oxidation to include Cu_A, heme a and the oxidation cycle of the binuclear center. The model reproduces the experimental reduction of Cu_A and heme a during turnover and the low population of the ferryl and ferrous heme a₃. It predicts that the off rate constants for reduced and oxidized Cytc can be unequal and that the non-Nernst response of Cu_A and heme a is due to disequilibrium between free Cytc and Cu_A rather than redox anticooperativity.

Keywords: Cytochrome oxidase; Enzyme kinetics; Enzyme modeling; Midpoint potential; Redox anti-cooperativity and proton pumping.