α-Amylase inhibition of a certain dietary polyphenol is predominantly affected by the concentration of α-1, 4-glucosidic bonds in starchy and artificial substrates

[Display omitted] •Physiochemical property of starchy substrates varied with their molecular structures.•IC50 values were similar for different starchy substrates with the same concentration.•Inhibitory activity of tannic acid predominantly depended on substrate concentration.•Tannic acid bound with α-amylase active site, competing with α-1,4-glucosidic bond.•1/Kic was a fixed constant describing tannic acid-amylase binding in digestion system. To elucidate why the inhibitory activity of one same polyphenol against α-amylase varies in different works. Seven starchy and three artificial substrates, and a polyphenolic competitive inhibitor, tannic acid (TA) were applied to study the enzyme inhibition in different digestion systems. The results showed that the IC50 values of TA were similar for all starches at the same starch concentration, although there existed difference in starch physiochemical properties, like branching degree, amylose chain distribution, viscosity, and digestion rate. However, the IC50 values significantly decreased with the substrate concentration decreasing, regardless of substrate types. Notably, TA had a similar competitive inhibition constant (Kic) for all the starches, despite the difference in substrate concentration, indicating that the fixed constant unconditionally describes the inhibitor-enzyme binding property. In the TA/amylase/starch system, the physical adsorption of TA with starch was much weaker than the specific binding of TA with α-amylase that was driven by hydrogen bondings and π-stackings. Therefore, it was the substrate, i.e., α-1,4-glucosidic bond concentration, rather than the existing matrix of the bonds, that predominantly affected the inhibitory activity of a polyphenol, because of the competitive action between α-1,4-glucosidic bond and the polyphenol regarding binding with the enzyme.