Further insight into the pH effect on the catalysis of mushroom tyrosinase

Abstract

The pH has a great physiological effect on the catalysis of tyrosinase because protons can act as competitive-type inhibitors. In this work, a reaction mechanism is proposed whereby only the protonation of the free forms Em (met-tyrosinase) and Eox (oxy-tyrosinase) has a significant kinetic effect. The pKa value calculated for tyrosinase is 4.63±0.04. This pKa value could correspond to a residue that acts as a gatekeeper of the active site, controlling the access of the substrate depending on its protonation state. Regarding the location and the nature of the responsible residue, we propose that glutamic acid E322 could be a key residue in the pH effect on the enzyme activity. Moreover, the substrate nature has a large effect on the kinetic behavior as a function of pH, depending on the pKa values of the phenolic hydroxyl groups and, to a lesser extent, on the charge of the R group. The sigmoid behavior of the initial rate vs. pH due to the pKa of the enzyme in the pH range studied could change to a bell-shaped behavior when the pKa values of the hydroxyl groups are low enough, and the R group is negatively charged. These aspects are interesting for controlling food browning and understanding the relationship melanosome pH plays in human skin pigmentation.