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

Inhibition of alpha-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 mu mol L-1) against alpha-glucosidase, while quercetin (3 ',4 '-OH) exhibited stronger inhibition (46.91 +/- 0.54 mu mol L-1) and myricetin (3 ',4 ',5 '-OH) possessed the strongest inhibitory activity (33.20 +/- 0.43 mu mol L-1). Molecular docking analysis illustrated that these flavonols could insert to the active cavity of alpha-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 within vitrokinetic analysis and fluorescence spectroscopy analysis. Among three flavonols tested, myricetin possessed the strongest inhibition effects on alpha-glucosidase with the lowest dissociation constant (K-i= 15.56 mu mol L-1) of myricetin-alpha-glucosidase, largest fluorescence quenching constant (K-sv) of (14.26 +/- 0.03) x 10(4)L mol(-1)and highest binding constant (K-a) of (1.38 +/- 0.03) x 10(5)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 alpha-glucosidase and induced conformational changes of enzyme. Therefore, myricetin, quercetin and kaempferol are all excellent dietary alpha-glucosidase inhibitors and their inhibitory activities are enhanced by increasing number of hydroxyl groups on B-ring.