Proton translocation in cytochromec oxidase: Redox linkage through proximal ligand exchange on cytochromea 3

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Abstract
An analysis of resonance Raman scattering data from CO-bound cytochromec oxidase and from the photodissociated enzyme indicates that histidine may not be coordinated to the iron atom of cytochromea 3 in the CO-bound form of the enzyme. Instead, the data suggest that either a water molecule or a different amino acid residue occupies the proximal ligand position. From these data, it is postulated that ligand exchange on cytochromea 3 can occur under physiological conditions. Studies of mutant hemoglobins have demonstrated that tyrosinate binds preferentially to histidine in the ferric forms of the proteins. In cytochromec oxidase tyrosine residues are located near the histidine residues recently implicated in coordination to cytochromea 3 (Shapleighet al., 1992; Hosleret al., this volume). Expanding on these concepts, we propose a model for proton translocation at the O2-binding site based on proximal ligand exchange between tyrosine and histidine on cytochromea 3. The pumping steps take place at the level of the peroxy intermediate and at the level of the ferryl intermediate in the catalytic cycle and are thereby consistent with the recent results of Wilkstrom (1989) who found that proton pumping occurs only at these two steps. It is shown that the model may be readily extended to account for the pumping of two protons at each of the steps.