Tyrosinase: kinetic analysis of the transient phase and the steady state

The transient phase of tyrosinase activity acting on monophenols has been investigated. Although an analytical solution for the lag period (tau) cannot be obtained, its dependence on reagent concentration and pH is studied. It is established that decreases as the quantity of enzyme increases, although it increases when monophenol or pH are increased. The computer simulation shows those rate constants whose variations affect the transient phase most significantly. In addition, the steady state of the pathway is studied using tyrosinases from several sources such as mushroom, frog epidermis and grape. The kinetic analysis, which is based on not imposing restrictions on the values of the rate constants involved in the mechanism, allows us to obtain analytical expressions for both monophenolase and diphenolase activities and explains the experimental results obtained with the different enzymes. The values determined for the kinetic parameter, R, point to the monophenol hydroxylation step as being the limiting step of the turnover, while the values obtained for n suggest the absence of fast equilibrium in the oxidation of diphenol by Emet.