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

Abstract

Biological function of proteins is frequently associated with the formation of complexes with small-molecule ligands. Experimental structure determination of such complexes at atomic resolution, however, can be time-consuming and costly. Computational methods for structure prediction of protein/ligand complexes, particularly docking, are as yet restricted by their limited consideration of receptor flexibility, rendering them not applicable for predicting protein/ligand complexes if large conformational changes of the receptor upon ligand binding are involved. Accurate receptor models in the ligand-bound state (holo structures), however, are a prerequisite for successful structure-based drug design. Hence, if only an unbound (apo) structure is available distinct from the ligandbound conformation, structure-based drug design is severely limited. We present a method to predict the structure of protein/ligand complexes based solely on the apo structure, the ligand and the radius of gyration of the holo structure. The method is applied to ten cases in which proteins undergo structural rearrangements of up to 7.1 A backbone RMSD upon ligand binding. In all cases, receptor models within 1.5 A backbone RMSD to the target were predicted and close-to-native ligand binding poses were obtained for eight of ten cases in the top-ranked complex models. The developed protocol is expected to enable structure modeling of protein/ligand complexes and structure-based drug design for cases where crystal structures of ligand-bound conformations are not available.

Molecular determinants for selective recognition of antidepressants in the human serotonin and norepinephrine transporters

“…It has been demonstrated that S-citalopram has a competitive mode of inhibition in SERT (22,23), which is consistent with overlapping binding sites for 5-HT and Scitalopram (13,16). To elucidate the mode of inhibition by Rtalopram, we performed uptake-saturation experiments with SERT and NET in the absence or presence of inhibitors (SI Experimental Procedures), which revealed a competitive mode of inhibition for S-citalopram and R-talopram in both SERT and NET (Table S3).…”

“…Citalopram and talopram are racemic mixtures, and it is wellestablished that the inhibitory potency of citalopram toward SERT resides in the S-enantiomer (16,20,21), as exemplified by Lexapro. Accordingly, we found that the affinity of S-citalopram was 35-fold higher for SERT compared with R-citalopram (4 nM versus 136 nM), whereas both enantiomers displayed low affinity binding to NET (3,025 nM versus 1,516 nM) ( Fig.…”

“…4 and Table S4). To further probe the role of the S1 site in NET for talopram binding, we introduced three additional mutations (N153S, F323Y, and S419T), corresponding to three SERT mutants (N177S, F341Y, and S438T) that perturb binding of S-citalopram in SERT with up to two orders of magnitude (13,16). These mutations induced <threefold changes in the inhibitory potency of R-talopram (Table S5), further supporting that talopram does not bind to the S1 site in NET.…”

“…The structures revealed a topology of 12 transmembrane (TM) spanning regions connected by short intra-and extracellular loops with a high-affinity substrate binding site (denoted the S1 site) centrally located in the core of the transporter protein (12). LeuT has proved to be an excellent structural template for construction of homology models of SERT and NET, facilitating identification of the location and molecular structure of binding pockets for substrates, ions, and inhibitors (13)(14)(15)(16)(17)(18).…”

Synthesis and serotonin transporter activity of 1,3-bis(aryl)-2-nitro-1-propenes as a new class of anticancer agents

“…3A), while the tricyclic antidepressant imipramine is also thought to interact strongly with the Asp98 residue 13,32,37 . A study by Koldsø et al (2010) discovered that the two enantiomers of the SSRI citalopram bind to a central binding site of the hSERT homology model with reversed orientations 38 . It has also been previously shown that the protonated amine of a series of sulfur-substituted -alkyl phenethylamines is essential for protein-ligand binding.…”