<p>Peroxidases are haem-containing enzymes that use hydrogen peroxide as the electron acceptor to catalyse a number of oxidative reactions. They are found in bacteria, fungi, plants and animals. On the basis of sequence similarity, fungal, plant and bacterial peroxidases can be viewed as members of a superfamily consisting of 3 major classes [<cite idref="PUB00001075"/>]. Class I, the intracellular peroxidases, includes yeast cytochrome c peroxidase (CCP), ascorbate peroxidase (AP) and bacterial catalase-peroxidases.</p> <p>In chloroplasts of higher plants, oxygen consumption in the absence of electron acceptors is accompanied by production of H2O2 and activated forms of oxygen. Chloroplasts contain several protective systems (such as superoxide dismutase (SOD), alpha-tocopherol and carotenoids), which are effective against various forms of activated oxygen. However, they lack catalase, and the disposal of H2O2 is accomplished by other means.</p> <p>Ascorbic acid is a strong antioxidant that is effective in scavenging superoxide (O2-'), hydroxyl (OH') radicals and singlet oxygen. It can also remove H2O2 in the following reaction:</p> <reaction> Ascorbate + H2O2 --> dehydroascorbate + 2 H2O</reaction> <p>Ascorbate peroxidase (AP) is the main enzyme responsible for hydrogen peroxide removal in the chloroplasts and cytosol of higher plants.</p> <p>The 3D structure of pea cytosolic ascorbate peroxidase has an overall fold virtually identical to that of CCP [<cite idref="PUB00000418"/>]. The protein consists of 2 all-alpha domains, between which is embedded the haem group. The most pronounced difference between the AP and CCP structures is the absence of an antiparallel beta-hairpin between the G and H helices in the AP molecule.</p>
Plant ascorbate peroxidase