Deciphering conformational changes in human serum albumin induced by bile salts using spectroscopic and molecular modeling approaches

Abstract

Bile salts are physiologically-important natural amphiphilic biosurfactants synthesized in the liver and play a vital role in the solubilization and digestion of dietary lipids, cholesterol, and other fat-soluble compounds in the body. Bile salts also exhibit pharmacological and biological uses as carriers for transporting scarce water-soluble drugs and other chemicals owing to their unique emulsifying, solubilizing capacities and micelle forming ability. In this context, we present an endeavor towards the possibilities underlying the interaction and binding between the most abundant plasma protein namely, human serum albumin (HSA) and bile salts to demonstrate an intriguing interplay of hydrophobic, electrostatic and hydrogen bonding effects using steady state absorption, fluorescence emission, anisotropy, time-resolved emission, and molecular modelling approaches. The outcome illuminates how amphiphilic interfaces of bile salts under various physico-chemical conditions trigger the conformational changes and binding affinities of native and molten-globule forms of HSA. To elucidate non-covalent interactions during HSA-bile salt supramolecular host–guest complex formation, changes in intrinsic (tryptophan) and extrinsic (ANS) fluorescence have been investigated. The results reveal upon binding of bile salts in subdomain IIA of HSA, the protein undergoes conformational changes mediated primarily by hydrophobic interactions. Furthermore, time-resolved fluorescence measurements provide important structural and dynamical insights into the protein-bile salt supramolecular complexes. Additionally, molecular docking studies on these complexes clearly reveal spontaneous binding of bile salts into subdomain IIA of HSA while suggesting that the binding affinity decreases with the decreasing order of hydrophobicity of the bile salts (NaDC > NaC > NaTC) (ΔGdock = −29.64 kJ mol−1, −26.15 kJ mol−1, −14.35 kJ mol−1), respectively. This study exclusively highlights the molecular mechanism of conformational perturbation in the native (pH 7) and molten-globule (pH 3) forms of HSA, induced by bile salts. We believe that the results reported herein will be helpful in the design and formulation of protein-bile salt-based pharmacological carriers suitable for drug delivery.