Heme-copper oxidase subunit III. Heme-copper oxidases are transmembrane protein complexes in the respiratory chains of prokaryotes and mitochondria which couple the reduction of molecular oxygen to water to, proton pumping across the membrane. The heme-copper oxidase superfamily is diverse in terms of electron donors, subunit composition, and heme types. This superfamily includes cytochrome c and ubiquinol oxidases. Bacterial oxidases typically contain 3 or 4 subunits in contrast to the 13 subunit bovine cytochrome c oxidase (CcO). 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. Subunits I, II and III of ubiquinol oxidase are homologous to the corresponding subunits in CcO. This group additionally contains proteins which are fusions between subunits I and III, such as Sulfolobus acidocaldarius SoxM, a subunit of the SoxM terminal oxidase complex. It also includes NorE which has been speculated to be a subunit of nitric oxide reductase. Some archaebacterial cytochrome oxidases lack subunit III. Although not required for catalytic activity, subunit III 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. It has been proposed that archaea acquired heme-copper oxidases through gene transfer from gram-positive bacteria.
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