Cytochrome c oxidase CcO is a metalloenzyme that plays a crucial role in the electron transport chain during cellular respiration. The active site of CcO contains two metal ions: a copper ion Cu and a heme-bound iron ion Fe . These metal ions are referred to as CuB and heme a3, respectively.The coordination geometry of the CuB ion is distorted tetrahedral, with three histidine residues and a hydroxide ion as ligands. The heme a3 iron ion has a square pyramidal geometry, with four nitrogen atoms from the porphyrin ring and a histidine residue as ligands.The oxidation state of the metal ions in the active site varies during the catalytic cycle of CcO. The resting state of the enzyme has CuB in the +2 oxidation state Cu II and heme a3 iron in the +3 oxidation state Fe III . During the electron transfer process, both metal ions can be reduced to lower oxidation states: Cu I for CuB and Fe II for heme a3 iron.The metal ions in the active site of CcO facilitate the transfer of electrons during respiration by acting as redox centers. When an electron is transferred from cytochrome c to the CuB ion, it reduces CuB from Cu II to Cu I . This electron transfer also induces the reduction of the heme a3 iron from Fe III to Fe II , which allows the enzyme to bind molecular oxygen. The bound oxygen is then reduced to water through a series of electron transfers and proton-coupled steps, ultimately regenerating the resting state of the enzyme.In summary, the coordination geometry of the metal ions in the active site of cytochrome c oxidase is distorted tetrahedral for CuB and square pyramidal for heme a3 iron. The oxidation states of these metal ions vary during the catalytic cycle, and they play a crucial role in facilitating electron transfer and oxygen reduction during cellular respiration.