Structural Insights into σ₁ Receptor Interactions with Opioid Ligands by Molecular Dynamics Simulations

Despite considerable advances over the past years in understanding the mechanisms of action and the role of the σ₁ receptor, several questions regarding this receptor remain unanswered. This receptor has been identified as a useful target for the treatment of a diverse range of diseases, from various central nervous system disorders to cancer. The recently solved issue of the crystal structure of the σ₁ receptor has made elucidating the structure-activity relationship feasible. The interaction of seven representative opioid ligands with the crystal structure of the σ₁ receptor (PDB ID: 5HK1) was simulated for the first time using molecular dynamics (MD). Analysis of the MD trajectories has provided the receptor-ligand interaction fingerprints, combining information on the crucial receptor residues and frequency of the residue-ligand contacts. The contact frequencies and the contact maps suggest that for all studied ligands, the hydrophilic (hydrogen bonding) interactions with Glu172 are an important factor for the ligands' affinities toward the σ₁ receptor. However, the hydrophobic interactions with Tyr120, Val162, Leu105, and Ile124 also significantly contribute to the ligand-receptor interplay and, in particular, differentiate the action of the agonistic morphine from the antagonistic haloperidol.