Structures of the Ligand-Binding Core of iGluR2 in Complex with the Agonists (R)- and (S)-2-Amino-3-(4-hydroxy-1,2,5-thiadiazol-3-yl)propionic Acid Explain Their Unusual Equipotency

Chalcogen Bonding "2S-2N Squares" versus Competing Interactions: Exploring the Recognition Properties of Sulfur

Chalcogen bonding (CB) is the focus of increased attention for its applications in medicinal chemistry, materials science, and crystal engineering. However, the origin of sulfur's recognition properties remains controversial, and experimental evidence for supporting theories is still emerging. Here, a comprehensive evaluation of sulfur CB interactions is presented by investigating 2,1,3-benzothiadiazole X-ray crystallographic structures gathered from the Cambridge Structure Database (CSD), Protein Data Bank (PDB), and own laboratory findings. Through the systematic analysis of substituent effects on a subset library of over thirty benzothiadiazole derivatives, the competing interactions have been categorized into four main classes, namely 2S-2N CB square, halogen bonding (XB), S⋅⋅⋅S, and hydrogen-bonding (HB). A geometric model is employed to characterize the 2S-2N CB square motifs and discuss the role of electrostatic, dipole, and orbital contributions toward the interaction.

Studies on Aryl-Substituted Phenylalanines: Synthesis, Activity, and Different Binding Modes at AMPA Receptors

A series of racemic aryl-substituted phenylalanines was synthesized and evaluated in vitro at recombinant rat GluA1-3, at GluK1-3, and at native AMPA receptors. The individual enantiomers of two target compounds, (RS)-2-amino-3-(3,4-dichloro-5-(5-hydroxypyridin-3-yl)phenyl)propanoic acid 37 and (RS)-2-amino-3-(3'-hydroxybiphenyl-3-yl)propanoic acid 38, were characterized. (S)-37 and (R)-38 were identified as the only biologically active isomers, both being antagonists at GluA2 receptors with Kb of 1.80 and 3.90 μM, respectively. To address this difference in enantiopharmacology, not previously seen for amino acid-based AMPA receptor antagonists, X-ray crystal structures of both eutomers in complex with the GluA2 ligand binding domain were solved. The cocrystal structures of (S)-37 and (R)-38 showed similar interactions of the amino acid parts but unexpected and different orientations and interactions of the biaromatic parts of the ligands inside the binding site, with (R)-38 having a binding mode not previously identified for amino acid-based antagonists.

Size matters in activation/inhibition of ligand-gated ion channels

Cys loop, glutamate, and P2X receptors are ligand-gated ion channels (LGICs) with 5, 4, and 3 protomers, respectively. There is now growing atomic level understanding of their gating mechanisms. Although each family is unique in the architecture of the ligand-binding pocket, the pathway for motions to propagate from ligand-binding domain to transmembrane domain, and the gating motions of the transmembrane domain, there are common features among the LGICs, which are the focus of the present review. In particular, agonists and competitive antagonists apparently induce opposite motions of the binding pocket. A simple way to control the motional direction is ligand size. Agonists, usually small, induce closure of the binding pocket, leading to opening of the channel pore, whereas antagonists, usually large, induce opening of the binding pocket, thereby stabilizing the closed pore. A cross-family comparison of the gating mechanisms of the LGICs, focusing in particular on the role played by ligand size, provides new insight on channel activation/inhibition and design of pharmacological compounds.

Lessons from more than 80 structures of the GluA2 ligand-binding domain in complex with agonists, antagonists and allosteric modulators

Ionotropic glutamate receptors (iGluRs) constitute a family of ligand-gated ion channels that are essential for mediating fast synaptic transmission in the central nervous system. These receptors play an important role for the development and function of the nervous system, and are essential in learning and memory. However, iGluRs are also implicated in or have causal roles for several brain disorders, e.g. epilepsy, Alzheimer's disease, Parkinson's disease and schizophrenia. Their involvement in neurological diseases has stimulated widespread interest in their structure and function. Since the first publication in 1998 of the structure of a recombinant soluble protein comprising the ligand-binding domain of GluA2 extensive studies have afforded numerous crystal structures of wildtype and mutant proteins including different ligands. The structural information obtained combined with functional data have led to models for receptor activation and desensitization by agonists, inhibition by antagonists and block of desensitization by positive allosteric modulators. Furthermore, the structural and functional studies have formed a powerful platform for the design of new selective compounds.