Non-Essential Activation of Co 2+ and Zn 2+ on Mushroom Tyrosinase: Kinetic and Structural Stability

Tyrosinase is a widespread enzyme with great promising capabilities. The Lineweaver-Burk plots of the catecholase reactions showed that the kinetics of mushroom tyrosinase (MT), activated by $Co^{2+}$ and $Zn^{2+}$ at different pHs (6, 7, 8 and 9) obeyed the non-essential activation mode. The binding of metal ions to the enzyme increases the maximum velocity of the enzyme due to an increase in the enzyme catalytic constant ($k_{cat}$). From the kinetic analysis, dissociation constants of the activator from the enzyme-metal ion complex ($K_a$) were obtained as $5{\times}10^4M^{-1}$ and $8.33{\times}10^3M^{-1}$ for $Co^{2+}$ and $Zn^{2+}$ at pH 9 and 6 respectively. The structural analysis of MT through circular dichroism (CD) and intensive fluorescence spectra revealed that the conformational stability of the enzyme in these pHs reaches its maximum value in the presence of each of the two metal ions.