The enzyme cytochrome c oxidase (PDB 2OCC, EC 22.214.171.124 ) is a large transmembrane protein found in the mitochondrion and is the terminal electron acceptor in the electron transfer chain, taking 4 reducing equivalents from cytochrome c and converting molecular oxygen to water. In the process, it translocates protons, helping to establish a chemiosmotic potential that the ATP synthase then uses to synthesize ATP.
4 Fe+2-cytochrome c + 4H+ + O2 → 4 Fe+3-cytochrome c + H2O.
The complex is a large lipoprotein composed of several metal prosthetic sites and 13 protein subunits. In mammals, 10 subunits are nuclear in origin and 3 are synthesized mitochondrially. The complex contains 2 cytochromes, the a and a3 cytochromes, and two copper centers, the CuA and CuB centers. In fact, the cytochrome a3 and CuB are a binuclear center that is the site of oxygen reduction. The mechanism of action of this large complex is still an active research topic.