Cytochrome c oxidase is a metalloenzyme that plays a crucial role in cellular respiration. The active site of this enzyme contains two metal ions: a copper ion Cu and a heme-bound iron ion Fe . The coordination geometry of the copper ion CuB in the active site is approximately tetrahedral. It is coordinated by three histidine nitrogen atoms and a nearby oxygen atom from a water molecule or hydroxide ion.The heme-bound iron ion Fe in the active site has a distorted octahedral coordination geometry. It is coordinated by four nitrogen atoms from the porphyrin ring, one histidine nitrogen atom, and an oxygen atom from a water molecule or hydroxide ion.The oxidation states of the metal ions in the active site of cytochrome c oxidase can vary during the catalytic cycle. The copper ion CuB can switch between Cu I and Cu II oxidation states, while the heme-bound iron ion Fe can switch between Fe II and Fe III oxidation states. These changes in oxidation states are essential for the enzyme's function in transferring electrons and reducing molecular oxygen to water.