The enzyme cytochrome c oxidase or Complex IV, (was , now reclassified as a translocase [https://www.enzyme-database.org/query.php?name=EC+7.1.1.9&search=search_all&display=show_all&order=ec_num&nr=50 EC 7.1.1.9]) is a large transmembrane protein complex found in bacteria, archaea, and the mitochondria of eukaryotes.
It is the last enzyme in the respiratory electron transport chain of cells located in the membrane. It receives an electron from each of four cytochrome c molecules and transfers them to one oxygen molecule and four protons, producing two molecules of water. In addition to binding the four protons from the inner aqueous phase, it transports another four protons across the membrane, increasing the transmembrane difference of proton electrochemical potential, which the ATP synthase then uses to synthesize ATP.