Cytochrome c oxidase (CcO) is the terminal enzyme of the respiratory chains of mitochondria and aerobic bacteria. 1 Recent high-resolution X-ray diffraction analyses2 have disclosed that the active site of CcO is composed of a five-coordinate myoglobin-type iron center (heme a3) with an imidazole ligand from a histidine residue on the proximal side of the heme and a copper atom (CuB) coordinated to three histidine imidazoles on the ...