The rate of internal heme-heme electron transfer in cytochrome C oxidase

Cytochrome c oxidase catalyzes the one-electron oxidation of four molecules of cytochrome c and the four-electron reduction of dioxygen to water. The process involves a number of intramolecular electron-transfer reactions, one of which takes place between the two hemes of the enzyme, hemes a and a3, with a rate of approximately 3 x 10(5) s(-1) (tau approximately 3 micros). In a recent report [Verkhovsky et al. (2001) Biochim. Biophys. Acta 1506, 143-146], it was suggested that the 3 x 10(5) s(-1) electron transfer may be controlled by structural rearrangements and that there is an additional electron transfer that is several orders of magnitude faster. In the present study, we have reinvestigated the spectral changes occurring in the nanosecond and microsecond time frames after photolysis of CO from the fully reduced and mixed-valence enzymes. On the basis of the differences between them, we conclude that in the bovine enzyme the microscopic forward and reverse rate constants for the electron-transfer reactions from heme a to heme a3 are not faster than approximately 2 x 10(5) and approximately 1 x 10(5) s(-1), respectively.