<p>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:</p><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>Peroxidases are found in bacteria, fungi, plants and animals and can be viewed as members of a superfamily consisting of 3 major classes [<cite idref="PUB00001075"/>]. Class III comprises 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. The wide spectrum of peroxidase activity, coupled with the participation in various physiological processes, is in keeping with its relative lack of specificity for substrates and the occurrence of a variety of isozymes. Plant peroxidases are monomeric glycoproteins containing 4 conserved disulphide bridges and 2 calcium ions. The 3D structure of peanut peroxidase has been shown to possess the same helical fold as class I and II peroxidases [<cite idref="PUB00005278"/>].</p> Plant peroxidase