Two-electron reduction of cytochrome c oxidase triggers a conformational transition.

The slow increase of a cyanide-induced optical change at 437 nm following rapid cyanide inhibition of cytochrome oxidase has been followed as a function of the number of electrons donated from ferrocytochrome c to cytochrome a and CuA. The initial rate of optical change is a parabolic function of this number. The results have been analyzed in terms of a model where addition of electrons causes a conformational transition allowing rapid cyanide binding. The binding is followed by a slow intramolecular responsible for the optical change. The analysis demonstrates that only molecules with both cytochrome a and CuA reduced can undergo the conformational change, which is suggested to be involved in the proton-pump mechanism of the oxidase.