ABSTRACT
Heme peroxidases catalyze the oxidation of organic substrates by H2O2 or organic hydroperoxides [1, 2]. The catalytic cycle consists of several reactions including multiple electron/proton transfer steps, which can be summarized by the following three reactions:
(1)
(2)
(3)
The resting state of the peroxidase enzyme is presented by E(FeIII), i.e., the enzyme with Fe-heme redox center in the ferric oxidation state. Upon exposure of peroxidase to a solution containing peroxide, the catalytic cycle starts (reaction 1) with a rapid oxygen transfer from the peroxide to the resting state of the enzyme to yield compound I, EI(FeIV = O, P +). Formally, the process is a two-electron oxidation of the resting enzyme. One oxidation equivalent is conserved in the form of the oxy-ferryl state (FeIV=O) and the other is stored as an organic radical, P+. The last is located at an oxidizable amino acid in case of cytochrome c peroxidase or on the porphyrin as a porphyrin π cation radical in case of plant peroxidases [2]. Reduction of EI to E proceeds by two one-electron transfer steps (reactions 2 and 3). The first electron reduces the organic P+ radical, resulting in an intermediate peroxidase compound II, EII(FeIV=O), and the second electron converts EII to E, thus completing the peroxidase cycle. One-electron donors, AH, e.g., phenolic compounds, are released as phenoxy radicals, A. Reactions (2) and (3) also involve proton transfer, meaning that the way these reactions are presented above reflects the usual simplification used to describe the peroxidase catalytic cycle. The reduction of EI to E can also proceed in a single two-electron step, e.g., in the case of iodide acting as a two-electron donor [2].
