Action mechanism of tyrosinase on meta- and para-hydroxylated monophenols

The relationship between the structure and activity of meta- and para-hydroxylated monophenols was studied during their tyrosinase-catalysed hydroxylation and the rate-limiting steps of the reaction mechanism were identified, The para-hydroxylated substrates permit us to study the effect of a substituent (R) in the carbon-1 position (C-l) of the benzene ring on the nucleophilic attack step, while the meta group permits a similar study of the effect on the electrophilic attack step. Substrates with a -OCH3 group on C-1, as p-hydroxyanisol (4HA) and m-hydroxyanisol (3HA), or with a -CH2OH group, as p-hydroxybenzylalcohol (4HBA) and m-hydroxybenzylalcohol (3HBA), were used because the effect of the substituent (R) size was assumed to be similar. However, the electron-donating effect of the -OCH3 group means that the carbon-4 position (C-4) is favoured for nucleophilic attack (para-hydroxylated substrates) or for electrophilic attack (meta-hydroxylated substrates), The electron-attracting effect of the -CH2OH group has the opposite effect, hindering nucleophilic (para) or electrophilic (meta) attack of C-4. The experimental data point to differences between the maximum steady-state rate (V-max(M)) of the different substrates, the value of this parameter depends on the nucleophilic and electrophilic attack, However, differences are greatest in the Michaelis constants (K-m(M)), with the meta-hydroxylated substrates having very large values. The catalytic efficiency k(cat)(M)/K-m(M) is much greater for the para-hydroxylated substrates although it varies greatly between one substrate and the other, However, it varies much less in the meta-hydroxylated substrates since this parameter describes the power of the nucleophilic attack, which is weaker in the meta OH. The large increase in the K-m(M) of the meta-hydroxylated substrates might suggest that the phenolic OH takes part in substrate binding. Since this is a weaker nucleophil than the para-hydroxylated substrates, the binding constant decreases, leading to an increase in K-m(M). The catalytic efficiency of tyrosinase on a monophenol (para or meta) is directly related to the nucleophilic power of the oxygen of the phenolic OH, The oxidation step is not limiting since if this were the case, the para and meta substrates would have the same V-max(M). The small difference between the absolute values of V-max(M) suggests that the rate constants of the nucleophilic and electrophilic attacks are on the same order of magnitude.