The binding interactions of imidacloprid with earthworm fibrinolytic enzyme

• Binding of imidacloprid to earthworm fibrinolytic enzyme was discussed.
• Noncovalent binding interactions stabilized the imidacloprid–protein complex.
• There was one major binding site of imidacloprid in earthworm fibrinolytic enzyme.
• Molecular docking study supported the experimental results.

In this paper, several studies were conducted to elucidate the binding mechanism of earthworm fibrinolytic enzyme (EFE) with imidocloprid (IMI) by using theoretical calculation, fluorescence, UV–vis, circular dichroism spectroscopy and an enzymatic inhibition assay. The spectral data showed that the binding interactions existed between IMI and EFE. The binding constants, binding site, thermodynamic parameters and binding forces were analyzed in detail. The results indicate a single class of binding sites for IMI in EFE and that this binding interaction is a spontaneous process with the estimated enthalpy and entropy changes being 2.195 kJ mol−1 and 94.480 J mol−1 K−1, respectively. A single class of binding site existed for IMI in EFE. The tertiary or secondary structure of EFE was partly destroyed by IMI. The visualized binding details were also exhibited by the theoretical calculation and the results indicated that the interaction between IMI and Phe (Tyr, or Trp) or EFE occurred. Combining the experimental data with the theoretical calculation data, we showed that the binding forces between IMI and EFE were mainly hydrophobic force accompanied by hydrogen binding, and π–π stacking. In addition, IMI did not obviously influence the activity of EFE. In a word, the above analysis offered insights into the binding mechanism of IMI with EFE and could provide some important information for the molecular toxicity of IMI for earthworms.