Molecular Docking Assessment of Environmental Pesticides as Potential Endocrine Disruptors of Human Estrogen Receptor Alpha

Technical Summary: Molecular Docking Assessment of Environmental Pesticides as Potential Endocrine Disruptors of Human Estrogen Receptor Alpha

Problem Statement
Environmental pesticides are increasingly recognized as Endocrine-Disrupting Chemicals (EDCs) capable of mimicking or blocking hormonal actions, posing significant risks to human and animal health. While extensive literature documents the acute and chronic toxicities of pesticides—including neurotoxicity, hepatotoxicity, and reproductive harm—there remains a gap in the comprehensive understanding of the specific molecular mechanisms by which these compounds impair reproductive health, particularly through interference with the Estrogen Receptor Alpha (ERα). This study addresses the need to evaluate the binding affinities and interaction profiles of five widely used pesticides (DDT, Chlorpyrifos, Endosulfan, Atrazine, and Glyphosate) against the human ERα to assess their potential as estrogenic disruptors.

Methodology
The research employed an in silico computational approach utilizing molecular docking and interaction profiling:

• Protein Preparation: The 3D structure of the human ERα ligand-binding domain (PDB ID: 3ERT) was retrieved from the Protein Data Bank. The structure was selected for its resolution and co-crystallization with a native ligand to ensure docking accuracy.
• Ligand Preparation: Five pesticides (Atrazine, DDT, Chlorpyrifos, Endosulfan, Glyphosate) and the natural ligand Estradiol (used as a control) were selected. 3D conformers were downloaded from PubChem in .sdf format and validated.
• Docking Protocol: Simulations were conducted using the CB-Dock web server, which utilizes Auto Dock Vina for pose prediction and binding energy calculation. The server automatically detected potential binding cavities, focusing on the top five predicted pockets. Estradiol was docked first to validate the natural binding site.
• Interaction Analysis: The Protein–Ligand Interaction Profiler (PLIP) server was used to analyze specific molecular interactions, including hydrogen bonds, hydrophobic contacts, salt bridges, and π–cation interactions.
• Limitations: The authors note that full-scale Molecular Dynamics (MD) simulations and comprehensive ADMET (Absorption, Distribution, Metabolism, Excretion, and Toxicity) predictions were not performed due to server limitations. Instead, the study relied on existing literature data for ADMET parameters.

Key Results
All tested ligands were found to dock preferentially into "Pocket 1," which corresponds to the natural hormone-binding site of ERα. The binding affinities and interaction profiles varied significantly among the pesticides:

• DDT: Exhibited the strongest binding affinity among the pesticides at -8.5 kcal/mol, closely approaching the control Estradiol (-9.3 kcal/mol). DDT formed extensive hydrophobic contacts (6 contacts) with nonpolar residues (LEU346, ALA350, TRP383, LEU387, PHE404, LEU525) but no hydrogen bonds.
• Chlorpyrifos: Showed moderate binding (-6.3 kcal/mol) driven primarily by hydrophobic contacts (3 contacts) with residues LEU346, LEU391, and ILE424.
• Endosulfan: Demonstrated moderate binding (-6.0 kcal/mol) via a mixed mode involving 3 hydrogen bonds, 2 hydrophobic contacts, and 1 π–cation interaction with ARG394.
• Atrazine: Displayed moderate binding (-5.9 kcal/mol) characterized by a mixed binding mode with 4 hydrogen bonds and 2 hydrophobic contacts.
• Glyphosate: Exhibited the weakest binding (-4.7 kcal/mol) with no hydrophobic interactions. Stabilization occurred mainly through polar interactions, including 3 hydrogen bonds and 1 salt bridge with LYS449.

Key Contributions

• Comparative Binding Assessment: The study provides a direct computational comparison of five distinct environmental pesticides against the ERα binding pocket, establishing a hierarchy of potential endocrine disruption based on binding energy.
• Interaction Profiling: By utilizing PLIP, the research details the specific molecular forces (hydrophobic vs. polar) driving the binding of each pesticide, offering mechanistic insights into how DDT might competitively inhibit natural estrogen signaling compared to weaker binders like Glyphosate.
• Screening Utility: The work demonstrates the utility of molecular docking (specifically CB-Dock) as a predictive screening tool for identifying potential EDCs among environmental chemicals.

Significance and Claims
The paper claims that the computational results provide molecular-level evidence supporting the hypothesis that certain pesticides, particularly DDT, Chlorpyrifos, and Endosulfan, can bind competitively within the natural hormone pocket of ERα. The authors suggest that pesticides with binding energies stronger than -6.0 kcal/mol may act as potential endocrine disruptors by mimicking or interfering with estrogen signaling.

The study emphasizes that while these findings support existing toxicological data regarding the persistence and bioaccumulation of compounds like DDT and Endosulfan, the results are preliminary. The authors explicitly state that the study highlights the need for further experimental validation, including long-term MD simulations to assess complex stability and wet-lab experiments (such as receptor binding assays and gene expression studies) to confirm the in silico predictions and assess real-world impacts on hormonal pathways.