Kinetics of competitive inhibition of jack bean urease by boric acid

Inhibitory kinetics of jack bean urease by boric acid has been studied in this paper. According to the kinetic parameters for the enzyme obtained from Lineweaver–Burk plot, it is shown that the Km is equal to 5.11 ± 0.22 mM and Vm is equal to 1.04 ± 0.04 mM min−1. Besides, the inhibition of urease by boric acid at lower than 0.25 mM is a reversible reaction with residual activity and the inhibition belongs to be competitive. Additionally, it is found that the inhibition is maximal on the condition that pH value is between 7.0 and 9.0 at 30 °C. Furthermore, the microscopic rate constants were determined. The values of k+0 equal 0.026 × 10−3, 0.026 × 10−3, 0.027 × 10−3 μM−1 min−1, while the k−0 equal 0.031 × 10−3, 0.032 × 10−3 S−1, which indicates that the established model of inhibition kinetics is suitable. It is turned out that one molecule of boric acid binds to the active unit of the enzyme, which makes the enzyme lose its activity. In accordance with experimental results of the fluorescence spectrum at different boric acid concentrations, it is proved that the microenvironments where Trp and Tyr residues of the enzyme exist have obviously changed after treatment by boric acid.

Boric acid concentrations were respectively 0 (□), 0.10 (■), 0.15 (
• ), 0.20 (▴) and 0.25 (▾) mM. The activity of the enzyme decreases with the increase of concentration of boric acid. The inhibition of urease by boric acid at lower concentration is reversible.Figure optionsDownload as PowerPoint slideHighlights
► The inhibition of urease by lower concentration boric acid is a reversible reaction.
► The inhibition is maximal between pH 7.0 and 9.0 at 30 °C.
► The kinetic parameters and the microscopic rate constants were determined.
► One molecule of boric acid binds to the active unit of the enzyme to make the enzyme lose its activity.
► The microenvironments of both Trp and Tyr residues of urease have obviously changed after inactivation.