Molecular insights into the binding model and response mechanisms of triclosan with lysozyme

Abstract

Triclosan is an emerging pollutant, which belongs to Pharmaceutical and Personal Care Products (PPCPs). The high exposure risk of triclosan is triggered by its high persistence and accumulation levels. Accordingly, we investigated the interaction mechanism of triclosan with lysozyme by biophysical methods, including multiple spectroscopic techniques, enzyme activity measurements, isothermal titration calorimetry (ITC) and molecular docking. The results indicated that the binding of triclosan and lysozyme contributed to the fluorescence sensitization and repositioned the fluorophore, such as Trp108, in a weaker polar microenvironment. Meanwhile, the binding of lysozyme and triclosan broke the secondary structure of lysozyme, as well as induced a small amount of α-Helix structure. The local surrounding of amide bond (CO) in lysozyme was altered and the content of α-Helix structure increased, suggesting that the protein scaffold and polypeptide chain of lysozyme were stretched and a lysozyme agglomerated with a larger particle size. In addition, the thermodynamic results indicated that triclosan docked to lysozyme at relatively moderate affinity, relying on hydrophobic forces, which was an endothermic reaction. The LysozymeTriclosan system became more disordered, indicating that the conformation of lysozyme was broken. Molecular simulations demonstrated that the docking pocket of lysozyme and triclosan contained 8 residues, consisting of Ala 107, Arg 112, Trp 108, Asn 59, Ile 98, Gln 57, Val 109 and Asp 52, which was core residues of lysozyme activity. Triclosan at high concentrations could occupy the native binding site of substrate and disturb Asp 52 residue, showing the antagonism with natural substrate, which inhibited the activity of lysozyme. In summary, this study elucidated the structural and functional responses of lysozyme after triclosan exposure simulated physiological conditions in vitro, as well as the interaction mode between triclosan and lysozyme.