Effects of B-ring structures on binding behavior of flavonols with proteins: Experimental and molecular docking approaches

Flavonols are studied extensively because of their interesting biological activities. Galangin, kaempferol, quercetin, and myricetin are structurally related flavonols that differ only in the number of B-ring hydroxyl substituents. In this work, we have carried out a detailed study on the interaction of these structurally related flavonols and γ-globulin/fibrinogen/trypsin/pepsin by using experimental as well as molecular docking approaches. Studies show that the flavonol-protein complex is formed after the interaction of flavonols with the four proteins, which has moderate quenching and binding ability. The effects of the number and position of the hydroxyl groups in flavonols B-ring on the quenching constant, binding constant, number of binding sites, thermodynamic parameters as well as the changes of the secondary structure of proteins are elucidated. The interaction between flavonols and γ-globulin/fibrinogen/trypsin/pepsin is a spontaneous exothermic process driven by enthalpy and entropy, and the main force types are hydrophobic and hydrogen bonds. For some systems, electrostatic forces are also involved. The interaction of flavonols with the four proteins results in loosening and unfolding of backbone of proteins. Moreover, galangin and quercetin also reduce the hydrophobicity of the tryptophan (Trp) microenvironment. The secondary structure of γ-globulin/fibrinogen/trypsin is affected by the number of hydroxyl groups on the B-ring. Except that myricetin does not bind to the catalytic active center of pepsin and seems to have no inhibitory effect on the activity of pepsin, for other flavonol-fibrinogen/trypsin/pepsin systems, flavonols are bound to the active center of the three proteins, and have certain influence on the activity of them.