Haem peroxidase, plant/fungal/bacterial 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>Fe³⁺ + H₂O₂ --&gt; [Fe⁴⁺=O]R' (Compound I) + H₂O</reaction><reaction>[Fe⁴⁺=O]R' + substrate --&gt; [Fe⁴⁺=O]R (Compound II) + oxidised substrate</reaction><reaction>[Fe⁴⁺=O]R + substrate --&gt; Fe³⁺ + H₂O + oxidised substrate</reaction><p>In 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 [<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 first one can be viewed as consisting of 3 major classes [<cite idref="PUB00001075"/>]. Class I, the intracellular peroxidases, includes: yeast cytochrome c peroxidase (CCP), a soluble protein found in the mitochondrial electron transport chain, where it probably protects against toxic peroxides; ascorbate peroxidase (AP), the main enzyme responsible for hydrogen peroxide removal in chloroplasts and cytosol of higher plants; and bacterial catalase- peroxidases, exhibiting both peroxidase and catalase activities. It is thought that catalase-peroxidase provides protection to cells under oxidative stress [<cite idref="PUB00000617"/>].</p> <p> Class II consists of secretory fungal peroxidases: ligninases, or lignin peroxidases (LiPs), and manganese-dependent peroxidases (MnPs). These are monomeric glycoproteins involved in the degradation of lignin. In MnP, Mn²⁺ serves as the reducing substrate [<cite idref="PUB00001743"/>]. Class II proteins contain four conserved disulphide bridges and two conserved calcium-binding sites. </p> <p> Class III consists of the secretory plant peroxidases, which have multiple tissue-specific functions: e.g., removal of hydrogen peroxide from chloroplasts and cytosol; oxidation of toxic compounds; biosynthesis of the cell wall; defence responses towards wounding; indole-3-acetic acid (IAA) catabolism; ethylene biosynthesis; and so on. Class III proteins are also monomeric glycoproteins, containing four conserved disulphide bridges and two calcium ions, although the placement of the disulphides differs from class II enzymes. </p><p> The crystal structures of a number of these proteins show that they share the same architecture - two all-alpha domains between which the haem group is embedded. </p>