A Ligand-Exchange Mechanism of Proton Pumpimg Involving Tyrosine-422 of Subunit I of Cytochrome Oxidase is Ruled Out.

David M. Mitchell, Pia Adelroth, Jonathan P.Hosler, John R. Fetter, Peter Brzezinski, Michelle A. Pressler, Roland Aasa, Bo G. Malmstrom, James O. Alben, Gerald T. Babcock, Robert B. Gennis, and Shelagh Ferguson-Miller Biochemistry , 35, 824-828 (1996).

Summary: The molecular mechanism by which proton pumping is coupled to electron transfer in cytochrome c oxidase has not yet been determined. However, several models of the process have been proposed which are based on changes occurring in the vicinity of the redox centers of the enzyme. Recently, a model was described in which a well conserved tyrosine residue in subunit I (Y422) was proposed to undergo ligand exchange with the histidine ligand (H419) of the high spin cytochrome a3 during the catalytic cycle, allowing both residues to serve as part of the proton transporting system. Site-directed mutants of Y422 have been constructed in the aa3 type cytochrome c oxidase of R. sphaeroides to test his hypothesis (Y422A, Y422F)> The results demonstrate that Y422 is not an essential residue in the electron transfer and proton pumping mechanisms of cytochrome oxidase. However the results support the predicted proximity of Y422 to cytochrome a3. In addition it is shown that the pH dependent reversed electron transfer between cytochrome a and cytochrome a3 is normal in the Y422F mutant. Hence, these data also show that Y422 is not the residue previously postulated to interact electrostatically with cytochrome a3.