The steady state behaviour of cytochrome c oxidase in proteoliposomes

Electron transfer to oxygen catalysed by cytochrome c oxidase is accompanied by spectral changes at the binuclear a3CuB centre, both in the soluble enzyme and in membranous systems, indicating spin or ligand state transitions of an iron that remains ferric. The other haem group, cytochrome a, does not change its spectral characteristics significantly during the steady state, but remains partially reduced until anaerobiosis. Cytochrome a3, is fully oxidized in each of its major steady state forms, and reduced upon anaerobiosis to a single ferrous species. Although cytochrome a is normally the immediate electron donor to the binuclear centre, its redox state does not alter under conditions in which the flux through the enzyme is changing significantly. A second electron transfer pathway to the binuclear centre may therefore exist, possibly one in which direct reduction of the binuclear a3CuB centre by CuA occurs. Both cytochrome a and CuA behave as simple electron transfer centres. The energy-conserving chemistry takes place at the binuclear centre in concert with the four-electron reduction of molecular oxygen.