A Further Step in the Kinetic Characterisation of the Tyrosinase Enzymatic System

Tyrosinase is a cuproprotein that hydroxylates monophenols to o-diphenols, which it then oxidises to o-quinones, using molecular oxygen. Based on kinetic studies of the steady state and measuring product formation during the action of the enzyme on o-diphenols, we determine the Michaelis constant and the maximum velocity, respectively. Similarly, we determine these kinetic constants for the enzyme acting on monophenols. From these constants obtained for a monophenol/o-diphenol pair, it is possible to calculate a new constant, the Michaelis constant of the enzyme for an o-diphenol acting on the corresponding monophenol, by means of an equation that relates the above-mentioned kinetic constants. Furthermore, it is also possible to establish the relation between the Michaelis constants for the oxygen in the presence of monophenol and in the presence of o-diphenol from the relation between the maximum velocities of the monophenol and o-diphenol experimentally determined by measuring aminochrome. From applying the equations described above to the kinetic data of the many tyrosinases described in the literature, we find that the Michaelis constant for the o-diphenol in the presence of monophenol is much lower than that obtained when the enzyme acts on o-diphenol alone. The Michaelis constant for oxygen in the presence of monophenol is also much lower than that obtained in the presence of its o-diphenol.