Catalysis and inactivation of tyrosinase in its action on o-diphenols, o-aminophenols and o-phenylendiamines: Potential use in industrial applications

Abstract

Abstract A study of the diphenolase, o-aminophenol oxidase, and the aromatic o-diamine oxidase activities of tyrosinase carried out by measuring the catalysis and suicide inactivation kinetics provides the following information: catalytic constant, k cat S , Michaelis constant, K M S , the maximum apparent inactivation constant, λ max S , and the partition ratio “r” between the suicide inactivation pathway and catalytic pathway or the number of turnovers made by one mol of enzyme before its inactivation. Analysis of these data, taking into account chemical shifts of the carbon atom supporting the hydroxyl or amino group, (δ) and σ p + , enables a mechanism to be proposed for the transformation of o-diphenols, o-aminophenols and o-phenylendiamines into their products (o-quinones, o-quinoneimine and o-diimine), and, at the same time, for the suicide inactivation. The reaction constants in the representations of log k cat X / k cat H vs. σ p + according to Hammett's equation for the three types of substrate (o-diphenols, o-aminophenols and o-phenylendiamines) confirm that the catalysis mechanism is similar (simultaneous oxidation/reduction process on the two copper atoms). The dependence of log λ max X / λ max H vs. σ p + for the three substrate types reflects a lower reaction constant (in absolute value), which might indicate a similar reaction mechanism for the different molecules, but different from the previous mechanism, in that the oxidation/reduction only involves one copper atom. We also discuss the proposed mechanism and compare it with those described by other authors. Knowledge and quantification of the catalysis/inactivation process of tyrosinase might be of interest for optimizing applications such as wastewater treatment.