Abstract
Lignin peroxidase (LiP), manganese-dependent peroxidase (MnP) and laccase are extracellular enzymes of white-rot fungi able to oxidise phenolic compounds to phenoxy radicals that can polymerise. LiP, however, is inhibited in the course of oxidation of phenolics, which may constitute a form of feedback control to regulate the rate of lignin oxidation and minimise the extent of lignin repolymerisation in vivo. By contrast, LiP is not inhibited to the same extent during the oxidation of veratryl alcohol. Data are presented which show that veratryl alcohol is oxidised to radical cations able to participate in intermolecular charge transfer reactions. This property of radical cations to charge transfer has led to the proposal that radical cations of veratryl alcohol, the products of LiP catalysis, may mediate in the oxidation of lignin. In addition, radical cations may assist in the reaction of LiP Compound II with reductant and thereby maintain the active peroxidase cycle. A model for net lignin breakdown is presented.
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