Kinetics and mechanism of jack bean urease inhibition by Hg(2+)

BACKGROUND: Jack bean urease (EC 3.5.1.5) is a metalloenzyme, which catalyzes the hydrolysis of urea to produce ammonia and carbon dioxide. The heavy metal ions are common inhibitors to control the rate of the enzymatic urea hydrolysis, which take the Hg(2+) as the representative. Hg(2+) affects the enzyme activity causing loss of the biological function of the enzyme, which threatens the survival of many microorganism and plants. However, inhibitory kinetics of urease by the low concentration Hg(2+) has not been explored fully. In this study, the inhibitory effect of the low concentration Hg(2+) on jack bean urease was investigated in order to elucidate the mechanism of Hg(2+) inhibition. RESULTS: According to the kinetic parameters for the enzyme obtained from Lineweaver–Burk plot, it is shown that the K(m) is equal to 4.6±0.3 mM and V(m) is equal to 29.8±1.7 μmol NH(3)/min mg. The results show that the inhibition of jack bean urease by Hg(2+) at low concentration is a reversible reaction. Equilibrium constants have been determined for Hg(2+) binding with the enzyme or the enzyme-substrate complexes (K(i) =0.012 μM). The results show that the Hg(2+) is a noncompetitive inhibitor. In addition, the kinetics of enzyme inhibition by the low concentration Hg(2+) has been studied using the kinetic method of the substrate reaction. The results suggest that the enzyme first reversibly and quickly binds Hg(2+) and then undergoes a slow reversible course to inactivation. Furthermore, the rate constant of the forward reactions (k(+0)) is much larger than the rate constant of the reverse reactions (k(-0)). By combining with the fact that the enzyme activity is almost completely lost at high concentration, the enzyme is completely inactivated when the Hg(2+) concentration is high enough. CONCLUSIONS: These results suggest that Hg(2+) has great impacts on the urease activity and the established inhibition kinetics model is suitable.