Cytochrome c oxidase subunit III. Cytochrome c oxidase (CcO), the terminal oxidase in the respiratory chains of eukaryotes and most bacteria, is a multi-chain transmembrane protein located in the inner membrane of mitochondria and the cell membrane of prokaryotes. CcO catalyzes the reduction of O2 and simultaneously pumps protons across the membrane. The number of subunits varies from three to five in bacteria and up to 13 in mammalian mitochondria. Only subunits I and II are essential for function, but subunit III, which is also conserved, is believed to play a role in assembly of the multimer complex. Rhodobacter CcO subunit III stabilizes the integrity of the binuclear center in subunit I. Subunits I, II, and III of mammalian CcO are encoded within the mitochondrial genome and the remaining 10 subunits are encoded within the nuclear genome. Subunit III contains bound phospholipids in several crystal structures and is proposed to contain a "lipid pool." These phospholipids are believed to intrinsic constituents similar to cofactors of the enzyme.
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