Elucidation of alpha-amylase inhibition by natural shikimic acid derivates regarding the infrequent uncompetitive inhibition mode and structure-activity relationship

The a-amylase inhibition and binding behaviors of shikimic acid and its structural analogues were studied. Uncompetitive inhibition characteristic of shikimic acid was identified through several linear transformation equations of enzymatic data considering the good linearity. Interestingly, the change in concentrations of starch would not decrease the inhibitory effects of shikimic acid with various concentrations used in present study. The inhibitory effect reduced greatly after the ethyl esterification of shikimic acid. Gallic acid was distinguished from shikimic acid by a benzene ring and exhibited lower inhibition than shikimic acid. Shikimic acid affected the a-amylase structure by binding to the non-active site but did not quench the a-amylase fluorescent intensity. However, gallic acid anchored with a subsite containing some fluorescent residues at the entrance of active pocket, quenching the fluorescence intensity of a-amylase strongly. The difference in shikimic acid and gallic acid made the changes in their binding sites as well as the binding behavior. Conclusively, the carboxyl and hexatomic ring with one double-bond played an important role in binding of shikimic acid with the non-active sites of a-amylase. Shikimic acid has potentials as a functional-factor in a-amylase inhibition for controlling the postprandial blood sugar level.