Peroxidases are haem-containing enzymes that use hydrogen peroxide asthe electron acceptor to catalyse a number of oxidative reactions.Most haem peroxidases follow the reaction scheme:Fe3+ + H2O2 --> [Fe4+=O]R' (Compound I) + H2O[Fe4+=O]R' + substrate --> [Fe4+=O]R (Compound II) + oxidised substrate[Fe4+=O]R + substrate --> Fe3+ + H2O + oxidised substrateIn this mechanism, the enzyme reacts with one equivalent of H2O2 to give [Fe4+=O]R' (compound I). This is a two-electron oxidation/reduction reaction where H2O2 is reduced to water and the enzyme is oxidised. One oxidising equivalent resides on iron, giving the oxyferryl intermediate, while in many peroxidases the porphyrin (R) is oxidised to the porphyrin pi-cation radical (R'). Compound I then oxidises an organic substrate to give a substrate radical.Haem peroxidases include two superfamilies: one found in bacteria, fungi, plants and the second found in animals. The animal peroxidases comprise a group of homologous proteins that differ markedly from the plant/fungal/bacterial peroxidases in primary, secondary and tertiary structure, but which share with them a common function. Animal peroxidases probably arose independently of the plant/fungal/bacterial peroxidase superfamily and most likely belong to a different gene family. The crystal structures of a number of these proteins show that the active sites of animal peroxidase and plant/fungal/bacterial peroxidases are remarkably similar.