Description

Class III peroxidases or the secreted plant peroxidases (EC 1.11.1.7) are found only in plants, where they form large multigenic families. Although their primary sequence differs in some points from the classes I and II, their three-dimensional structures are very similar to those of class II, and they also possess calcium ions, disulfide bonds, and an N-terminal signal for secretion. Class III peroxidases are additionally able to undertake a second cyclic reaction, called hydroxylic, which is distinct from the peroxidative one. During the hydroxylic cycle, peroxidases pass through a Fe(II) state and use mainly the superoxide anion (O₂^.-) to generate hydroxyl radicals (^.OH). Class III peroxidases, by using both these cycles, are known to participate in many different plant processes from germination to senescence, for example, auxin metabolism, cell wall elongation and stiffening, or protection against pathogens.