Oxidation of 4-methoxymandelic acid by lignin peroxidase: Mediation by veratryl alcohol

The mechanism of veratryl alcohol-mediated oxidation of 4- methoxymandelic acid by lignin peroxidase was studied by kinetic methods. For monomethoxylated substrates not directly oxidized by lignin peroxidase, veratryl alcohol has been proposed to act as a redox mediator. Our previous study showed that stimulation of anisyl alcohol oxidation by veratryl alcohol was not due to mediation but rather due to the requirement of veratryl alcohol to complete the catalytic cycle. Anisyl alcohol can react with compound I but not with compound II. In contrast, veratryl alcohol readily reduces compound II. We demonstrate in the present report that the oxidation of 4-methoxy mandelic acid is mediated by veratryl alcohol. Increasing veratryl alcohol concentration in the presence of 2 mM 4-methoxymandelic acid resulted in increased oxidation of 4-methoxymandelic acid yielding anisaldehyde. This is in contrast to results obtained with anisyl alcohol where increased concentrations of veratryl alcohol caused a decrease in product formation. ESR spectroscopy demonstrated that 4-methoxymandelic acid caused a decrease in the enzyme-bound veratryl alcohol cation radical signal, which is consistent with its reaction at the active site of the enzyme.