The Roles of veratryl alcohol and oxalate in fungal lignin degradation

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Veratryl alcohol (3,4-dimethoxybenzyl alcohol) and oxalate are secondary metabolites of Phanerochaete chrysosporium and other white-rot fungi. Veratryl alcohol is involved in lignin peroxidase catalysis as a substrate, and oxalate is involved in Mn peroxidase activity in its ability to chelate Mn 2+. The role of veratryl alcohol in lignin degradation has been the subject of numerous studies and considerable debate. Several investigators have proposed that veratryl alcohol can act as a redox mediator of lignin degradation. In this mechanism, veratryl alcohol is oxidized by lignin peroxidase to form a cation radical. This cation radical then acts as a diffusible oxidant, mediating the oxidation of compounds that are putatively inaccessible to the lignin peroxidase active site, such as the bulky lignin polymer. Other investigators suggested that the short lifetime of the veratryl alcohol cation radical would make diffusion from the active site unlikely. Still others have suggested that veratryl alcohol is not a mediator at all and that its primary role is protecting lignin peroxidase from inactivation by H 2O 2. Recent evidence clearly shows that veratryl alcohol does form a cation radical upon oxidation by lignin peroxidase and that it can mediate the oxidation of some substrates. While the possibility of cation radical diffusion exists, it now appears that an enzyme-bound radical would have greater stability. In contrast, diffusible oxidation does play a role in Mn peroxidase activity. Mn peroxidase catalyzes the oxidation of Mn 2+ to Mn 3+. Mn 3+ is a diffusible oxidant and is capable of oxidizing phenolic substrates. Studies have shown that chelating Mn 3+ with oxalate enhances its reactivity with its organic substrates. Other investigations have shown that Mn peroxidase activity is stimulated by oxalate. This article summarizes current understanding of the roles veratryl alcohol and oxalate play in the enzymatic degradation of lignin.