Intramolecular electron transfer in CO-bound mixed-valence cytochrome c oxidase following CO photolysis

Internal electron transfer in bovine cytochrome c oxidase was initiated by CO photolysis of the CO-bound mixed-valence form of the enzyme. Transient a bsorption spectroscopy was used to monitor changes in the redox states of t he metal centers in the enzyme brought about by electron re-equilibration, Upon CO photodissociation, reduced high spin cytochrome a(3) was generated in less than 0.1 mu sec, and a portion of the reduced cytochrome a, was reo xidized with biphasic rate constants of k(1) = 1.0 x 10(6) s(-1) and k(2) = 7.8 x 10(4) s(-1). Concomitant reduction of cytochrome a was also observed with biphasic rate constants of k(1) = 1.6 x 10(6) s(-1) and k(2) = 9 x 10 (4) s(-1). The stoichiometry of cytochrome a, oxidized to cytochrome a redu ced was found to be close to 1:1. Contrary to similar studies in the litera ture, no reduction of Cu-A was observed. As a control, no transient absorpt ion changes corresponding to electron transfer was observed in the CO-inhib ited fully reduced form of the enzyme. These results indicate that there is significant electron reequilibration only between cytochrome a(3) and cyto chrome a upon photolysis of the CO-bound mixed-valence enzyme.