Abstract
The redistribution of two electrons in the four redox centers of cytochrome c oxidase following photodissociation of CO from the CO-bound mixed valence species has been examined by resonance Raman spectroscopy. To account for both the kinetic data, obtained from 5 micros to 2 ms, and the equilibrium results, a model is proposed in which the electron redistribution is modulated by a protein conformation transition from a nascent P(1) state to a relaxed P(2) state in a time window longer than 2 ms. In this model, all six possible two-electron reduced species are considered. The high population of species with a one-electron reduced binuclear center, in which the spectrum of heme a(3) is perturbed by the redox state of Cu(B), accounts for the significant residuals in the fitting of the kinetic data with four standard spectra derived from redox species with either zero or two electrons in the binuclear center. Under equilibrium conditions, the conformational change to the P(2) state destabilizes the redox states with only one electron in the binuclear center with respect to those with either zero or two electrons. As a result, the redox equilibrium is perturbed, and the electrons are redistributed. A simulation based on the new kinetics scheme, in which the electron redistribution is modulated by the protein conformation, gives reasonable agreement with both the equilibrium and the kinetic data, demonstrating the validity of this model.
Highlights
Cytochrome c oxidase, (CcO1; ferrocytochrome c: O2 oxidoreductase, EC 1.9.3.1), the terminal enzyme in the electron transport chain, catalyzes the four-electron reduction of oxygen to water
The high population of species with a one-electron reduced binuclear center, in which the spectrum of heme a3 is perturbed by the redox state of CuB, accounts for the significant residuals in the fitting of the kinetic data with four standard spectra derived from redox species with either zero or two electrons in the binuclear center
The electrons in CuA are subsequently transferred to heme a and to the binuclear center consisting of heme a3 and CuB, where oxygen reduction occurs [1,2,3,4]
Summary
Cytochrome c oxidase, (CcO1; ferrocytochrome c: O2 oxidoreductase, EC 1.9.3.1), the terminal enzyme in the electron transport chain, catalyzes the four-electron reduction of oxygen to water. The electrons in CuA are subsequently transferred to heme a and to the binuclear center consisting of heme a3 and CuB, where oxygen reduction occurs [1,2,3,4]. It is very important to understand the electron transfer events to fully determine the molecular mechanism underlying this important enzyme It is a non-trivial task to study the forward electron transfer reactions because they are tightly coupled with the oxygen reduction chemistry. After reinvestigation of this reaction, Namslauer et al [17] concluded that no evidence was found for the presence of this early phase Instead, they reported that the microscopic forward and reverse rate constants for the electron-transfer reactions from heme a to heme a3 are not faster than ϳ2 ϫ 105 and ϳ1 ϫ 105 sϪ1, respectively. Based on optical absorption spectroscopy, Einarsdottir and co-workers [5, 6] pointed out that in addition to the four species included in Scheme 1, other intermediates have to be considered to account for the changes observed in the optical spectra
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