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

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, kcatS, Michaelis constant, KMS, the maximum apparent inactivation constant, λmaxS, 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 σp+, 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 kcatX/kcatH vs.  σp+ 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 λmaxX/λmaxH vs.  σp+ 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.

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► Different types of substrates induce suicide inactivation of tyrosinase.
► The rate of this process is: o-diphenols > o-aminophenols > aromatic o-diamines.
► The catalytic and inactivation constants fit to Hammett's equation.
► The catalysis and suicide inactivation mechanisms are similar in all substrates.