Kinetic study of the oxidation of 3-hydroxyanisole catalysed by tyrosinase

Tyrosinase hydroxylates 3-hydroxyanisole in the 4-position. The reaction product accumulates in the reaction medium with a lag time (tau) which diminishes with increasing concentrations of enzyme and lengthens with increasing concentrations of substrate, thus fulfilling all the predictions of the mechanism proposed by us for 4-hydroxyphenols. The kinetic constants obtained, kcatM = (46.87 +/- 2.06) s-1 and KmM = (5.40 +/- 0.60) mM, are different from those obtained with 4-hydroxyanisole, kcatM = (184.20 +/- 6.1) s-1 and KmM = (0.08 +/- 0.004) mM. The catalytic efficiency, kcatM/KmM is, therefore, 265.3 times greater with 4-hydroxyanisole. The possible rate-determining steps for the reaction mechanism of tyrosinase on 3- and 4-hydroxyanisole, based on the NMR spectra of both monophenols, are discussed. These possible rate-determining steps are the nucleophilic attack of hydroxyl's oxygen on the copper and the electrophilic attack of the peroxide on the aromatic ring. Both steps may be of similar magnitude, i.e. take place in the same time scale.