Cytochrome c oxidase CcO is a metalloenzyme that plays a crucial role in the respiratory process of living organisms. The active site of CcO contains two metal ions: a copper ion CuB and a heme iron Fea3 .The coordination geometry of the heme iron Fea3 is typically square pyramidal. The iron is coordinated to a histidine residue, a porphyrin ring, and a water molecule or hydroxide ion. The copper ion CuB has a distorted tetrahedral coordination geometry, being coordinated to three histidine residues and a water molecule or hydroxide ion.The oxidation state of the heme iron Fea3 in the active site of cytochrome c oxidase can vary between +2 ferrous and +3 ferric during the catalytic cycle. The copper ion CuB also undergoes redox changes, with its oxidation state varying between +1 cuprous and +2 cupric .