4-Ethyl phenyl sulfate binding to human serum albumin

Protein-bound uremic toxins (PBUTs) such as 4-ethylphenyl sulfate (4-EPS) challenge kidney failure management due to strong binding to human serum albumin (HSA), thus limiting dialysis clearance. This study examined 4-EPS-HSA interactions using saturation transfer difference nuclear magnetic resonance spectroscopy (STD-NMR), isothermal titration calorimetry (ITC), in silico analysis via molecular docking, molecular dynamics (MD), and molecular mechanics Generalized Born surface area (MM/GBSA) simulations. STD-NMR qualitatively showed competitive binding with the site-specific ligands, warfarin and ibuprofen, indicating interaction with both Sudlow Sites I and II; competitive ITC results also showed competition between these molecules. ITC analysis using a two-component model indicated two distinct binding interactions with association constants K_a1 = 6.62 × 10⁵ M^-1 (entropically unfavorable) and K_a2 = 2.71 × 10⁴ M^-1 (enthalpically favorable), respectively. Docking models were used to analyze representative binding poses at Sudlow Sites I and II, while MD revealed hydrophobic stabilization at Site I and polar interactions at Site II. MM/GBSA per-residue decomposition identified key stabilizing residues. Circular dichroism measurements showed that the observed binding effects are not accompanied by gross protein unfolding. Comparisons with other PBUTs, including indoxyl sulfate and p-cresyl sulfate, indicated the role of electrostatic and hydrophobic forces involved in binding. These findings advance understanding of PBUT-HSA interactions and inform strategies for improved toxin removal in kidney failure treatments.