α-amylase inhibitory activity of chitooligosaccharide from shrimp shell chitosan and its epigallocatechin gallate conjugate: kinetics, fluorescence quenching and structure–activity relationship

The inhibitory effects of chitooligosaccharide-epigallocatechin gallate (COS-EGCG) conjugate in comparison with COS, EGCG, and acarbose on α-amylase were studied. The half-maximum inhibitory concentration (IC50) of COS-EGCG conjugate against α-amylase was 4.62 mg/mL, which was lower than other samples expect acarbose (IC50 = 0.48 mg/mL) (p < 0.05). COS-EGCG conjugate showed the mixed-type inhibition kinetic and had Kic of 1.516 mg/mL, which was less than those of COS (Kic = 4.572 mg/mL) and EGCG (Kic = 2.248 mg/mL) (p < 0.05). COS-EGCG conjugate linked with amino acids of α-amylase via hydrogen bonding, van der Waals and hydrophobic interactions as elucidated by molecular docking. Interaction between COS-EGCG conjugate and α-amylase was achieved with the binding affinity of -14.30 kcal/mol (p < 0.05). COS-EGCG conjugate spontaneously alterated the micro-environment around the active site as shown by fluorescence quenching. Therefore, mechanism for retarding digestibility of starch by COS-EGCG conjugate was elucidated. COS-EGCG conjugate could be incorporated as functional ingredient in various foods to possess low-glycaemic index.