Serotonin transporter (S2 allosteric site) (SERT S2 site)

The Serotonin Transporter (SERT) S2 site is a high-affinity allosteric binding pocket located within the extracellular vestibule of the SLC6A4 protein, distinct from the primary orthosteric (S1) site where serotonin and most SSRIs bind (Chen et al., 2005, J Biol Chem). This site plays a crucial role in the pharmacodynamics of certain antidepressants, most notably escitalopram, which binds to both the S1 and S2 sites. Occupancy of the S2 site triggers a conformational change that effectively traps the ligand in the S1 site, slowing its dissociation rate and resulting in a more sustained inhibition of serotonin reuptake compared to drugs that only target the S1 site (Coleman et al., 2016, Nature). This allosteric mechanism is hypothesized to underlie the superior clinical efficacy and faster onset of action observed with escitalopram in the treatment of major depressive disorder and anxiety (Plenge et al., 2007, Eur Neuropsychopharmacol). Structurally, the S2 site is situated above the S1 site and is formed by residues in transmembrane helices 1, 3, 6, 10, and 11. Targeting this allosteric site represents a sophisticated approach to modulating serotonergic signaling, offering a pathway to fine-tune transporter activity beyond simple competitive inhibition (Zhong et al., 2012, J Biol Chem).

Other names

S2 binding siteAllosteric site of SLC6A4Secondary binding site of the serotonin transporterHigh-affinity allosteric site of SERT

Allosteric modulation of the serotonin transporter; binding of a ligand to the S2 site in the extracellular vestibule induces a conformational change that sterically hinders the dissociation of a ligand bound at the primary orthosteric (S1) site, thereby prolonging the inhibition of serotonin reuptake (Coleman et al., 2016, Nature; Chen et al., 2005, J Biol Chem).