Cytochrome c oxidase subunit Vb. 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 Vb is one of three mammalian subunits that lacks a transmembrane region. Subunit Vb is located on the matrix side of the membrane and binds the regulatory subunit of protein kinase A. The abnormally extended conformation is stable only in the CcO assembly.
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