Abstract
Cytochrome c oxidase contains a copper ion electron-transfer site, Cu sub(A), which has previously been found to be unreactive with externally added reagents under conditions in which the protein remains structurally intact. The authors have studied the reaction of cytochrome oxidase with sodium p-(hydroxymercuri)benzoate (pHMB) and found that the reaction proceeds, under appropriate conditions, to give an excellent yield of a particular derivative of the Cu sub(A) center that has electron paramagnetic resonance and near-infrared absorption spectroscopic properties which are distinctly different from those of the unmodified center. Spectroscopic and chemical characterization of the other metal ion sites of the enzyme reveals little or no effect of the pHMB modification on the structures of and reactions at those sites. Of particular interest is the observation that the modified enzyme still displays a substantial fraction of the native steady-state activity of electron transfer from ferrocytochrome c to O sub(2). In this work, the authors demonstrate that the unusual features of this site are not prerequisites for competent catalysis of electron transfer and O sub(2) reduction by the enzyme.