Caffeoyl substitution changes the inhibition mode of tartaric acid against α-amylase: Analysis of the enzyme inhibition by four caffeic and tartaric acid derivates

alpha-Amylase inhibition by four dietary organic acids was characterized by inhibition assay, kinetics, fluorescence quenching and molecular docking. Tartaric acid was found as an uncompetitive inhibitor, with 2,3-OH playing an important role in binding with the enzymic non-active site. Although caffeic acid had a low inhibitory activity, caffeoyl substitution at 2,3-OH of tartaric acid gradually increased its competitive inhibition character. As a result, caftaric acid (one caffeoyl-substituted) and chicoric acid (two caffeoyl-substituted) were suggested as mixed-type and competitive inhibitors, respectively, despite that the substitution decreased the inhibitory activity of tartaric acid. Fluorescence quenching was only observed for compounds with caffeoyl(s), and the effect increased with the moiety number increasing, consistent with the changing trend in number of active fluorescent residues involved in ligand-enzyme interactions. These results suggest that caffeoyl moiety entered into alpha-amylase active pocket. Therefore, caffeoyl has potentials as a functional-factor in alpha-amylase inhibition for alleviating type-II diabetes symptoms.