The active site of cytochrome c oxidase, a metalloenzyme involved in cellular respiration, consists of a binuclear center formed by a high-spin heme a3 and a copper ion CuB . The heme a3 and CuB are bridged by a hydroxide ion, and both metal centers are coordinated to nearby amino acid residues. The heme a3 is coordinated to a histidine residue, while the CuB is coordinated to three histidine residues. Additionally, there is a low-spin heme a and a copper ion CuA that are involved in electron transfer but not directly in the active site.The active site of cytochrome c oxidase facilitates the reduction of oxygen to water through a series of redox reactions. The process begins with the transfer of an electron from cytochrome c to the CuA center, followed by the transfer of the electron to the low-spin heme a. The electron is then transferred to the high-spin heme a3, which is part of the binuclear center.Upon receiving the electron, the high-spin heme a3 and CuB work together to bind and reduce molecular oxygen. The oxygen molecule binds to the heme a3 iron center, and the electron is used to reduce the oxygen molecule to a peroxide species. The CuB center donates an electron to further reduce the peroxide species to a hydroxide ion, which is then bridged between the heme a3 and CuB.The hydroxide ion is then protonated, forming a water molecule that is released from the active site. This process occurs twice, resulting in the reduction of one oxygen molecule to two water molecules. The enzyme then returns to its initial state, ready to accept another electron from cytochrome c and continue the process of cellular respiration.