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The kinetics of inhibition of rice bran lipase (RBL) by phenylboronic acid (PBA) was studied to elucidate the nature of inhibition and the effect of the inhibitor on the structure-function of RBL. The effectiveness of an inhibitor is normally expressed by the constant Ki, which is calculated from the Lineweaver-
Burk plot and found to be 1.7 mM at pH 7.4. The kinetics of inhibition by PBA was competitive,indicating the presence of serine in the active site of the enzyme. The loss of activity of RBL was concentration dependent on the inhibitor (PBA), and the inactivation followed a pseudo-first-order kinetics. Fluorescence emission measurements indicated a decrease in the fluorescence emission
intensity and a red shift in the emission maximum as the inhibitor concentration was increased. The inhibition of the enzyme by PBA was also confirmed by thermal denaturation measurements, which indicated a shift in the thermal denaturation temperature of the enzyme toward lower temperatures. The far-UV-CD data suggest that there were no significant changes in the conformation of the enzyme
as a result of binding of PBA. These results indicate that PBA is a potential inhibitor of RBL and binds to the enzyme in bringing about inhibition without any structural alterations.