ABSTRACT The Key function of plant peroxidases (EC 1.11.1.7; hydrogen donor: H2O2 oxidoreductase) is to oxidize phenolic substrates at the expense of H2O2. However, and paradoxically, peroxidases may also generate O2·-/H2O2 through two mechanisms: one directly and the other indirectly. The direct mechanism is restricted to the O2·-/H2O2 generating step of plant peroxidases during their catalytic cycle, which is represented by the decay of Compound III (CoIII) into ferriperoxidase. The reaction most likely involves the dissociation of a ferric-O2·- complex to yield the ferric form of the enzyme and O2·-, which may further dismutate to H2O2, either chemically or enzymatically. However, the Kdecay of CoIII into ferriperoxidase is very slow, and it is improbable that it is responsible for net O2·-/H2O2 production. The indirect mechanism involves the oxidation through a peroxidase cycle of a suitable reductor (either NADH or thiol [RSH] to its corresponding radical (NAD· and [RSSR] ·-), which further react with O2 through a reaction of mediation redox, to give O2·-. However, caution should be exercised when considering peroxidase as a possible enzyme responsible for O2·-/H2O2 production since the overall balance of this last oxidase/peroxidase reaction is a non net O2·-/H2O2 production.
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