α-Glucosidase inhibitors from Chinese bayberry (Morella rubra Sieb. et Zucc.) fruit: molecular docking and interaction mechanism of flavonols with different B-ring hydroxylations

Inhibition of α-glucosidase alleviates postprandial high glycemic levels in diabetic or prediabetic population. In Chinese bayberry fruit, myricetin, quercetin and kaempferol are main flavonols, which differ only in their hydroxylation on the B-ring. Kaempferol (4'-OH) showed high IC50 (65.36 ± 0.27 μmol L-1) against α-glucosidase, while quercetin (3',4'-OH) exhibited stronger inhibition (46.91 ± 0.54 μmol L-1) and myricetin (3',4',5'-OH) possessed the strongest inhibitory activity (33.20 ± 0.43 μmol L-1). Molecular docking analysis illustrated that these flavonols could insert to the active cavity of α-glucosidase. Adjacent hydroxyl groups at B-ring of myricetin and quercetin positively contributed to form hydrogen bonds that were important to the stability of flavonol-enzyme complex, while kaempferol had no adjacent hydroxyl groups. Such observation was further validated by molecular dynamics simulations, and in good consistency with in vitro kinetic analysis and fluorescence spectroscopy analysis. Among three flavonols tested, myricetin possessed the strongest inhibition effects on α-glucosidase with the lowest dissociation constant (Ki = 15.56 μmol L-1) of myricetin-α-glucosidase, largest fluorescence quenching constant (Ksv) of (14.26 ± 0.03) × 104 L mol-1 and highest binding constant (Ka) of (1.38 ± 0.03) × 105 L mol-1 at 298 K with the enzyme. Bio-Layer Interferometry (BLI) and circular dichroism (CD) analysis further confirmed that myricetin had high affinity to α-glucosidase and induced conformational changes of enzyme. Therefore, myricetin, quercetin and kaempferol are all excellent dietary α-glucosidase inhibitors and their inhibitory activities are enhanced by increasing number of hydroxyl groups on B-ring.