Coordination dynamics of heme-copper oxidases. The ligand shuttle and the control and coupling of electron transfer and proton translocation.

Results are presented which, taken with evidence developed by others, suggest a general mechanism for the entry and binding of exogenous ligands (including O2) at the "binuclear site" (CuBFea3) of the heme-copper oxidases. The mechanism includes a "ligand shuttle" wherein the obligatory way station for incoming ligands is CuB and the binding of exogenous ligands at this site triggers the exchange and displacement of endogenous ligands at Fea3. It is suggested that these ligand shuttle reactions might be functionally important in providing a coupling mechanism for electron transfer and proton translocation. Scenarios as to how this might happen are delineated.