Occurs in almost all aerobically respiring organisms and serves to protect cells from the toxic effects of hydrogen peroxide. Its levels are highest in the light period and are lowest in the dark period, hence it may be important for scavenging hydrogen peroxide at night, rather than during the day. 2 H2O2 = 2 H2O + O2 Homotetramer. Leaf mesophyll cells, pericarp, seedling roots and the coleoptile. Belongs to the catalase family.