Binding analysis of glycyrrhetinic acid to human serum albumin: Fluorescence spectroscopy, FTIR, and molecular modeling

Fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR), and molecular modeling methods were employed to analyze the binding of glycyrrhetinic acid (GEA) to human serum albumin (HSA) under physiological conditions with GEA concentrations from 4.0 × 10−6 to 4.5 × 10−5 mol L−1. The binding of GEA to HSA was via two types of sites: the numbers of binding site for the first type was near 0.45 and for the second type it was approximately 0.75. The binding constants of the second type binding site were lower than those of the first type binding site at corresponding temperatures, the results suggesting that the first type of binding site had high affinity and the second binding site involved other sites with lower binding affinity and selectivity. The fluorescence titration results indicated that GEA quenched the fluorescence intensity of HSA through static mechanism. The FTIR spectra evidence showed that the protein secondary structure changed with reduction of α-helices about 26.2% at the drug to protein molar ratio of 3. Thermodynamic analysis showed that hydrogen bonds were the mainly binding force in the first type of binding site, and hydrophobic interactions might play a main role in the second type of binding site. Furthermore, the study of computational modeling indicated that GEA could bind to the site I of HSA and hydrophobic interaction was the major acting force for the second type of binding site, which was in agreement with the thermodynamic analysis.

The interaction model between glycyrrhetinic acid (GEA) and HSA. (a) Only residues around 6.5 Å of GEA are displayed. The residues of HSA are represented using lines and the GEA structure is represented using a stick model. (b) GEA–HSA spacefill conformation. Protein atoms are shown in CPK representation and the GEA structure is represented using a stick model. The model shown GEA can bind to subdomain IIA of HSA, namely, site I. This result indicated that hydrophobic interaction played an important role.Figure optionsDownload as PowerPoint slide