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.

Graphical Abstract

Keywords:

• Methylphenanthrene isomer
• human serum albumin
• intermolecular interaction
• spectroscopic method
• molecular docking

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China (No. 21577110), the National Major Scientific Instruments Development Project of China (No. 21627814), and the Natural Science Foundation of Fujian Province, China (No. 2018J05024). We also appreciate the assistance of senior engineer Fang Wu in using apparatus.