This domain is found in Class III secretory plant peroxidases.
Peroxidases are haem-containing enzymes that use hydrogen peroxide as the 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 substrate
In this mechanism, the enzyme reacts with one equivalent of H2O2to give [Fe4+=O]R' (compound I). This is a two-electron oxidation/reduction reaction where H2O2is reduced to water and the enzyme is oxidised. One oxidising equivalent resides on iron, giving the oxyferryl
[4] 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
[3].
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. 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
[2].
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
[1]. Class II proteins contain four conserved disulphide bridges and two conserved calcium-binding sites.
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.