Oxidation of guaiacol by lignin peroxidase: Role of veratryl alcohol

We have investigated the lignin peroxidase-catalyzed oxidation of guaiacol and the role of veratryl alcohol in this reaction by steady-state and pre- steady-state methods. Pre-steady-state kinetic analyses demonstrated that guaiacol is a good substrate for both compounds I and II, the two- and one- electron oxidized enzyme intermediates, respectively, of lignin peroxidase. The rate constant for the reaction with compound I is 1.2 x 10⁶ M^-1 s^{- 1}. The reaction of guaiacol with compound II exhibits a K(d) of 64 μM and a first-order rate constant of 17 s^-1. Oxidation of guaiacol leads to tetraguaiacol formation. This reaction exhibits classical Michaelis-Menten kinetics with a K(m) of 160 μM and a k(cat) of 7.7 s^-1. Veratryl alcohol, a secondary metabolite of ligninolytic fungi, is capable of mediating the oxidation of guaiacol. This was shown by steady-state inhibition studies. Guaiacol completely inhibited the oxidation of veratryl alcohol, whereas veratryl alcohol had no corresponding inhibitory effect on guaiacol oxidation. In fact, at low guaiacol concentrations, veratryl alcohol stimulated the rate of guaiacol oxidation. These results collectively demonstrate that veratryl alcohol can serve as a mediator for phenolic substrates in the lignin peroxidase reaction.