Effect of Kolavenic Acid on the Structure of Human Serum Albumin

The effect of Kolavenic acid (KA), an active component isolated from the genus Polyalthia, on the structure of human serum albumin (HSA) was investigated by fluorescence polarization, synchronous fluorescence, three-dimensional (3D) fluorescence, and absorption spectroscopy in combination with molecular modeling techniques under physiological conditions. The synchronous and absorption fluorescence spectra confirmed that KA has an effect on the microenvironment around HSA in aqueous solution. The two-dimensional (2D) and 3D fluorescence spectra showed that KA could quench the intrinsic fluorescence of HSA and make its conformation change. Fluorescence polarization measurements provided useful information on the relaxation time and aggregation behavior of the complex formed between HSA and KA, and indicated that the presence of KA caused changes in the fluidity and microviscosity of HSA. The binding constants and thermodynamic parameters for KA-HSA systems were obtained under different temperatures (298, 308, and 318 K). Molecular docking showed that the KA moiety bound to the hydrophobic cavity of HSA, and there were three hydrogen-bonding interactions between KA and the Lys195 and Asp451 residues. Fluorescent displacement measurements confirmed that KA bound to HSA at site II. In addition, the binding mechanism of KA and HSA was revealed by the physicochemical parameters at the molecular level. The results showed that the interaction between KA and HSA was strong, indicating that KA may be stored and transferred by serum albumin.