Intramolecular Electron Transfer in CO-Bound Mixed-Valence Cytochrome c Oxidase Following CO Photolysis

摘要

Internal electron transfer in bovine cytochrome c oxidase was intiated by CO photolysis of the CO-bound mixed-valence form of the enzyme. Transient absorption spectroscopy was used to monitor changes in the redox states of the 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_3 was reoxidized with biphasic rate constants of k_1=1.0*10~6 s~(-1) and k_2=7.8*10~4 s~(-1). Concomitant reduction of cytochrome a was also observed with biphasic rate constants of k_1=1.6*10~6 s(-1) and k_2=9*10~4 s~(-1). The stoichiometry of cytochrome a_3 oxidized to cytochrome a reduced was found to be close to 1:1. Contrary to similar studies in the literature, no reduction of Cu_A was observed. As a control, no transient absorption changes corresponding to electron transfer was observed in the CO-inhibited fully reduced form of the enzyme. These results indicate that there is significant electron reequilibration only between cytochrome a_3 and cytochrome a upon photolysis of the CO-bound mixed-valence enzyme.