The cytochrome c oxidase (COX) family of proteins function as the final electron donor in the respiratory chain to drive a proton gradient across the inner mitochondrial membrane, ultimately resulting in the production of water. The mammalian COX apoenzyme is a dimer, with each monomer consisting of 13 subunits, some of which are mitochondrial and some of which are nuclear. Found in the inner mitochondrial membrane, COX5a, one of two subunits of the COX5 protein, is the heme A-containing chain of the oxidase family that converts one molecule of oxygen and four molecules of hydrogen to two molecules of water. When oxygen levels within the cell are high, transcription of COX5a, the aerobic subunit of the COX5 protein, is up-regulated as the rate of cellular respiration increases. Conversely, when oxygen levels are low, COX5a transcription decreases as the cell works to conserve oxygen by slowing the creation of water.