Cupredoxin domain of Heme-copper oxidase subunit II
Heme-copper oxidases are transmembrane protein complexes in the respiratory chains of prokaryotes and mitochondria which catalyze the reduction of O2 and simultaneously pump protons across the membrane. The superfamily is diverse in terms of electron donors, subunit composition, and heme types. The number of subunits varies from two to five in bacteria and up to 13 in mammalian mitochondria. Subunits I, II, and III of mammalian cytochrome c oxidase (CcO) are encoded within the mitochondrial genome and the remaining 10 subunits are encoded within the nuclear genome. It has been proposed that archaea acquired heme-copper oxidases through gene transfer from gram-positive bacteria. Subunit II is found in CcO, ubiquinol oxidase, and the ba3-like oxidases, while the cbb3 oxidases contain alternative additional subunits. Additionally, nitrous oxide reductase contains the globular portion of subunit II as a domain within its structure. In some families, subunit II contains a copper-copper binuclear center that is involved in the transfer of electrons from the substrate to the binuclear center (active site) in subunit I.