Homology Modeling of the Serotonin Transporter: Insights into the Primary Escitalopram‐binding Site

Abstract

The serotonin transporter (SERT) is one of the neurotransmitter transporters that plays a critical role in the regulation of endogenous amine concentrations and therefore is an important target for therapeutic agents affecting the central nervous system. The recently published, high resolution X-ray structure of the closely related amino acid transporter, Aquifex aeolicus leucine transporter (LeuT), provides an opportunity to develop a three-dimensional model of the structure of SERT. We present herein a homology model of SERT using LeuT as the template and containing escitalopram as a bound ligand. Our model explains selectivities known from mutational studies and varying ligand data, which are discussed and illustrated in the paper.

Transmembrane Domain 6 of the Human Serotonin Transporter Contributes to an Aqueously Accessible Binding Pocket for Serotonin and the Psychostimulant 3,4-Methylene Dioxymethamphetamine

“…In LeuT Aa , TMs 1, 3, 6, and 8 form the leucine binding pocket. Biochemical analyses and homology modeling of SERT proteins predict a similar binding pocket for 5-HT (32)(33)(34)(35)(36). Consistent with these models, pre-structure studies identified residues in hSERT TMs 1 and 3 that confer high affinity interactions and ligand selectivity to substrates and antagonists.…”

“…Because hSERT TM6 is predicted to lie adjacent to and share topology with TM1 (27,(32)(33)(34)(35)(36), we explored potential contributions of TM6 to the interactions of 5-HT, MDMA, and cocaine via an assessment of their ability to protect hSERT C109A Cys mutants from inactivation by MTSET and biotinylation with MTSEA-biotin. Before initiation of protection experiments, we examined the sensitivity of each Cys mutant to MTSET at 0.1, 1, and 10 mM to ensure we were using a concentration of MTSET appropriate for detecting either protection or exposure.…”

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

“…Currently, there is no existing crystal structure of SERT; however, a number of homology models of hSERT are available which were constructed using the bacterial leucine transporter (LeuT) as a template 32,33 . For this study the homology model of hSERT constructed by Jørgensen et al (2007) was used 33 . A number of previous studies have identified key binding residues involved in the binding of SERT ligands [33][34][35][36] .…”

“…For this study the homology model of hSERT constructed by Jørgensen et al (2007) was used 33 . A number of previous studies have identified key binding residues involved in the binding of SERT ligands [33][34][35][36] . 5-HT, the natural substrate of SERT, and amphetamines such as 4-MTA interact with residues Ala96, Asp98 and Phe335 via binding of the protonated amine (Fig.…”

LeuT: A prokaryotic stepping stone on the way to a eukaryotic neurotransmitter transporter structure

“…For some nontransportable, competitive drugs like the SSRI citalopram, convincing biochemical evidence for a primary binding site that overlaps with that of the substrate 5-HT 148 was acquired before the LeuT structure was published but is now also supported by LeuT-based homology models of SERT. 149 For other drugs, like cocaine, a combination of LeuT-based homology models and mutagenesis on DAT suggests that the binding sites of dopamine and cocaine also overlap. 150 Although these studies are reassuring and probably identify the correct antagonist binding sites, they still do not detail the conformational changes predicted to be associated with competitive inhibition and an extracellular inhibitor like cocaine [151][152][153] or the GAT1 inhibitor SKF100330, 154 i.e., stabilization of an open-to-out state.…”