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

Abstract

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 <i>in vitro</i> kinetic analysis and fluorescence spectroscopy analysis. Among three flavonols tested, myricetin possessed the strongest inhibition effects on α-glucosidase with the lowest dissociation constant (<i>K</i> _i = 15.56 μmol L^-1) of myricetin-α-glucosidase, largest fluorescence quenching constant (<i>K</i> _sv) of (14.26 ± 0.03) × 10⁴ L mol^-1 and highest binding constant (<i>K</i> _a) 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.