Cytochrome c oxidase is the terminal enzyme of the electron transfer chain in aerobic bacteria as well as in the mitochondria of plants and animals. Bacterial cytochrome c oxidases are composed of three different subunits and include two hemes a and two copper atoms as prosthetic groups. The enzyme from eukaryotes is more complex and includes three subunits encoded on mitochondrial DNA, which are the homologues of the subunits of the bacterial enzyme, and in addition contains a number of subunits encoded in the nucleus. It is generally agreed that the mitochondrially coded subunits with their associated prosthetic groups are the functional core of the enzyme. The role of the nuclear coded subunits in cytochrome c oxidase function remains a matter of conjecture. cytochrome c oxidase subunit VIb polypeptide 2 Connects the two COX monomers into the physiological dimeric form.