Cytochrome c oxidase subunit VIa. 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 VIa is expressed in two tissue-specific isoforms in mammals but not fish. VIa-H is the heart and skeletal muscle isoform; VIa-L is the liver or non-muscle isoform. Mammalian VIa-H induces a slip in CcO (decrease in proton/electron stoichiometry) at high intramitochondrial ATP/ADP ratios, while VIa-L induces a permanent slip in CcO, depending on the presence of cardiolipin and palmitate.