CYTOCHROME-OXIDASE - A PERSPECTIVE

Cytochrome oxidase from beef heart mitochondria is a 204 kDa large protein consisting of 13 subunits of which the numbers I-III are encoded by the mitochondrial DNA, while the remaining subunits (IV-VIII) are encoded by the nuclear genome. The protein contains 2 iron heme groups (cytochrome a and cytochrome a//3), 2 Cu centers (Cu//A and Cu//B) and 1 Zn atom. The heme groups and the Cu//B center are located in subunit I, the Cu//A center is located on subunit II, whereas subunit Vb contains the Zn site. The spectroscopic features of the oxidase are reviewed in relation to the structure of the metal centers. Cytochrome a occurs in the form of a low-spin bis-imidazole center; Cu//A exhibits a high degree of covalency through which a large part of the unpaired electron is delocalized over 1 (or possibly even 2) sulfur ligands. Cytochrome a//3 and the Cu//B site form an antiferromagnetically, tightly coupled dinuclear center, located at the dioxygen reducing side of the protein. The mechanism of electron transfer and the possible proton pumping activity of the enzyme are reviewed in the light of the available structural evidence.