Study on the Binding of Methylphenanthrene Isomers with Different Methylated Positions to Human Serum Albumin Employing Spectroscopic Techniques Combined with Molecular Docking

Abstract

The interactions of methylphenanthrene (MP) isomers (1-MP, 3-MP, 4-MP, and 9-MP) with human serum albumin (HSA) were studied in simulated physiological conditions. A fluorescence analysis revealed that each MP formed a 1:1 ground-state complex with HSA. The binding constants at 308 K decreased in the order of 3-MP (9.13 × 104 L·mol−1) > 9-MP (3.57 × 104 L·mol−1) > 4-MP (2.37 × 104 L·mol−1) > 1-MP (1.46 × 104 L·mol−1), which was consistent with the docking results. The K b values at 298 K, 308 K, and 318 K revealed that the β-isomer has a stronger binding affinity with HSA than the α-isomer. Thermodynamic and docking parameters showed that MPs combined with HSA through van der Waals forces. The results of the binding site competition experiment and molecular docking indicated that 4-MP bound at subdomain IIA in HSA molecule, while the other MPs bound at a site between subdomains IB and IIA. The circular dichroism (CD) spectra showed that the α-helical content of HSA reduced in the presence of 1-MP, 4-MP and 9-MP, while no significant change was observed in the presence of 3-MP. Further, the fluorescence spectroscopic studies revealed that the Vitamin B2 (VB2)-transporting function of HSA was inhibited by 3-MP and 9-MP, and enhanced by 1-MP and 4-MP. Moreover, the impact degrees of MPs on the VB2-transporting function had a positive correlation with the binding affinities of the MPs to HSA. Highlights Interactions of MP isomers methylated at varying sites with HSA were studied. The α-substituted MP had a weaker affinity to HSA than β-substituted MP. Binding site of 4-MP in HSA was different from that of 1-MP/3-MP/9-MP. MP isomers unfolded the α-helical structure of HSA to varying degrees. Transporting function of HSA was inhibited by 3-MP/9-MP but enhanced by 1-MP/4-MP.