The Two Enantiomers of Citalopram Bind to the Human Serotonin Transporter in Reversed Orientations

The two enantiomers of the antidepressant citalopram inhibit the human serotonin transporter substantially differently. Previous studies revealed Tyr95 and lle172 as important for citalopram binding, however, the overall orientation of the ligands in the binding site and the protein-ligand interaction points remain unknown. The binding of S- and R-citalopram to a human serotonin transporter homology model are herein examined via docking simulations including induced fit effects. For a better description of the formal charges of the ligand when bound inside the protein, polarization effects of the protein were included by additional quantum-polarized ligand docking calculations, where ligand charges are evaluated using QM/MM calculations. By this approach a much clearer picture emerged of the positions of the functional groups of citalopram. The two enantiomers are predicted to bind in the substrate binding pocket with opposite orientations of their aromatic groups. The predicted binding modes are experimentally validated using human wild type and 15 serotonin transporter mutants and 13 optically pure citalopram analogues. Important protein-ligand interaction points were identified validating one binding model for each enantiomer. In the validated model of the high affinity enantiomer, S-citalopram, the fluorine atom is located near Ala173 and Thr439 and the cyano group is in close proximity of Phe341; these contacts are found to be reversed for the R-enantiomer.