Comparison of lignin peroxidase, horseradish peroxidase and laccase in the oxidation of methoxybenzenes

Lignin peroxidase oxidizes non-phenolic substrates by one electron to give aryl-cation-radical intermediates, which react further to give a variety of products. The present study investigated the possibility that other peroxidative and oxidative enzymes known to catalyse one-electron oxidations may also oxidize non-phenolics to cation-radical intermediates and that this ability is related to the redox potential of the substrate. Lignin peroxidase from the fungus Phanerochaete chrysosporium, horseradish peroxidase (HRP) and laccase from the fungus Trametes versicolor were chosen for investigation with methoxybenzenes as a homologous series of substrates. The twelve methoxybenzene congeners have known half-wave potentials that differ by as much as approximately 1 V. Lignin peroxidase oxidized the ten with the lowest half-wave potentials, whereas HRP oxidized the four lowest and laccase oxidized only 1,2,4,5-tetramethoxybenzene, the lowest. E.s.r. spectroscopy showed that this congener is oxidized to its cation radical by all three enzymes. Oxidation in each case gave the same products: 2,5-dimethoxy-p-benzoquinone and 4,5-dimethoxy-o-benzoquinone, in a 4:1 ratio, plus 2 mol of methanol for each 1 mol of substrate. Using HRP-catalysed oxidation, we showed that the quinone oxygen atoms are derived from water. We conclude that the three enzymes affect their substrates similarly, and that whether an aromatic compound is a substrate depends in large part on its redox potential. Furthermore, oxidized lignin peroxidase is clearly a stronger oxidant than oxidized HRP or laccase. Determination of the enzyme kinetic parameters for the methoxybenzene oxidations demonstrated further differences among the enzymes.