Cytochrome c oxidase subunit Va. 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. It 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. 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. Found only in eukaryotes, subunit Va is one of three mammalian subunits that lacks a transmembrane region. Subunit Va is located on the matrix side of the membrane and binds thyroid hormone T2, releasing allosteric inhibition caused by the binding of ATP to subunit IV and allowing high turnover at elevated intramitochondrial ATP/ADP ratios.
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