PROTON TRANSLOCATION IN CYTOCHROME-C-OXIDASE - REDOX LINKAGE THROUGH PROXIMAL LIGAND-EXCHANGE ON CYTOCHROME-A3

An analysis of resonance Raman scattering data from CO-bound cytochrom e c oxidase and from the photodissociated enzyme indicates that histid ine may not be coordinated to the iron atom of cytochrome a3 in the CO -bound form of the enzyme. Instead, the data suggest that either a wat er molecule or a different amino acid residue occupies the proximal li gand position. From these data, it is postulated that ligand exchange on cytochrome a3 can occur under physiological conditions. Studies of mutant hemoglobins have demonstrated that tyrosinate binds preferentia lly to histidine in the ferric forms of the proteins. In cytochrome c oxidase tyrosine residues are located near the histidine residues rece ntly implicated in coordination to cytochrome a3 (Shapleigh et al., 19 92; Hosler et al., this volume). Expanding on these concepts, we propo se a model for proton translocation at the 02-binding site based on pr oximal ligand exchange between tyrosine and histidine on cytochrome a3 . The pumping steps take place at the level of the peroxy intermediate and at the level of the ferryl intermediate in the catalytic cycle an d are thereby consistent with the recent results of Wilkstrom (1989) w ho found that proton pumping occurs only at these two steps. It is sho wn that the model may be readily extended to account for the pumping o f two protons at each of the steps.