Catalase is the classical marker for peroxisomes and is the most abundant protein within peroxisomes. It is found in almost all aerobically respiring organisms and serves to protect cells from the toxic effects of hydrogen peroxide. Likewise, catalase has one of the highest turnover numbers of all enzymes; one catalase molecule can convert millions of hydrogen peroxide molecules to water and oxygen each second. Human catalase forms a tetramer composed of four subunits, each of which can be conceptually divided into four domains and each over 500 amino acids long.