Molecular Dynamics Simulation and Binding Studies of β-Sitosterol with Human Serum Albumin and Its Biological Relevance

beta-Sitosterol is a naturally occurring phytosterol that is widely used to cure atherosclerosis, diabetes, cancer, and inflammation and is also an antioxidant. Here, we studied the interaction of beta-sitosterol, isolated from the aerial roots of Ficus bengalensis, with human serum albumin (HSA) at physiological pH 7.2 by using fluorescence, circular dichroism (CD), molecular docking, and molecular dynamics simulation methods. The experimental results show that the intrinsic fluorescence of HSA is quenched by addition of A-sitosterol through a static quenching mechanism. The binding constant of the compound to HSA, calculated from fluorescence data, was found to be K beta-sitosterol = 4.6 +/- 0.01 x 10(3) M-1, which corresponds to -5.0 kcal M-1 of free energy. Upon binding of P-sitosterol to HSA, the protein secondary structure was partially unfolded. Specifically, the molecular dynamics study makes an important contribution to understanding the effect of the binding of beta-sitosterol on conformational changes of HSA and the stability of a protein drug complex system in aqueous solution. Molecular docking studies revealed that the P-sitosterol can bind in the large hydrophobic cavity of subdomain IIA, mainly by the hydrophobic interaction but also by hydrogen bond interactions between the hydroxyl (OH) group of carbon-3 of beta-sitosterol to Arg(257), Ser(287), and Ala(261) of HSA, with hydrogen bond distances of 1.9, 2.4, and 2.2 angstrom, respectively.