Mitochondrial DNA is multicopy and maternally inherited. mtDNA has 16,500 basepairs and codes for 37 genes; 13 polypeptides, 22 transfer RNA (tRNA), and two ribosomal RNA (rRNA). Cytochrome c oxidase (MT-CO1) is the catalytic subunit of the respiratory complex IV, which catalyzes the reduction of oxygen to water. Subunits 1, 2, and 3 are the functional core of this enzyme complex. Electrons originating in cytochrome c are transferred via the copper A center of subunit 2 and heme A of subunit 1 to the bimetallic center formed by heme A3 and copper B.