Catalase (CAT) catalyzes the conversion of hydrogen peroxide to water and oxygen. CAT contains porphyrin-heme Fe group which reacts with the hydrogen peroxide. Catalase is a tetramer of four polypeptide chains, each over 500 amino acids long. It contains four porphyrin groups that allow the enzyme to react with the hydrogen peroxide. It provides defense against oxidative stress by reducing hydrogen peroxide.
To get to the active site, hydrogen peroxide must first travel through a hydrophilic channel to reach the active site at the heme. When hydrogen peroxide enters the active site, it interacts with the amino acids , causing a proton to transfer from one of the oxygen atoms to the other. The recipient oxygen is released as water, while the other oxygen atom coordinates with the iron to form Fe(IV)=O. Fe(IV)=O reacts with a to reform Fe(III)-E and produce water and oxygen. The heme reactivity is enhanced by the phenolate ligand of a in the 5th iron ligand position which can assist in the oxidation of the Fe(III) to Fe(IV). The efficiency of the reaction may also be improved by the interactions of His74 and Asn147 with reaction intermediates.
