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:<reaction>Fe<sup>3+</sup> + H<sub>2</sub>O<sub>2</sub> --> [Fe<sup>4+</sup>=O]R' (Compound I) + H<sub>2</sub>O</reaction><reaction>[Fe<sup>4+</sup>=O]R' + substrate --> [Fe<sup>4+</sup>=O]R (Compound II) + oxidised substrate</reaction><reaction>[Fe<sup>4+</sup>=O]R + substrate --> Fe<sup>3+</sup> + H<sub>2</sub>O + oxidised substrate</reaction><p>In this mechanism, the enzyme reacts with one equivalent of H<sub>2</sub>O<sub>2</sub> to give [Fe<sup>4+</sup>=O]R' (compound I). This is a two-electron oxidation/reduction reaction where H<sub>2</sub>O<sub>2</sub> is reduced to water and the enzyme is oxidised. One oxidising equivalent resides on iron, giving the oxyferryl [<cite idref="PUB00001259"/>] 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 [<cite idref="PUB00005246"/>].</p><p>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 [<cite idref="PUB00013944"/>].</p>