The inhibition kinetics and thermodynamic changes of tyrosinase via the zinc ion

We found that Zn2+ conspicuously inactivated tyrosinase in a mixed-type inhibition manner: the final level of residual activity was abolished at the equilibrium state with concentration of 0.25 mM Zn2+. Changes of both K-m and V-max by various concentrations of Zn2+ in Lineweaver-Burk plot were observed. To see whether Zn2+ also induced conformational change of tyrosinase and how thermodynamical changes by ligand binding were occurred, the intrinsic fluorescence studies as well as calorimetric measurements were conducted. The results showed that the Zn2+ binding to tyrosinase directly induced conformational change of tyrosinase, and the changes of thermodynamic parameters such as enthalpy (Delta H), Gibbs free-energy (Delta G), and entropy (Delta S) were obtained as 60 +/- 7.0 kJ/mol, -14.54 kJ/mol and 248.53 J/(K mol), respectively. The inactivating effect of Zn2+ on tyrosinase was completely prevented by incubation with bovine serum albumin, which has a Zn2+ binding motif in its structure. We suggested that Zn2+ ligand-binding affected the substrate's accessibility due to the conformational changes and thus, the complex type of inhibition has occurred with the calorimetric changes. (c) 2007 Elsevier B.V. All rights reserved.