A model for the acetylcholine binding site of the nicotinic acetylcholine receptor

Abstract

A detailed model for the acetylcholine binding site on the nicotinic acetylcholine receptor is proposed. It is derived from assumptions based on existing biochemical, structural, and pharmacological data, combined with molecular modeling and principles of protein evolution and architecture. Acetylcholine is proposed to fit into a pocket on one face of an antiparallel beta-pleated sheet formed by residues 128-142 on the alpha-subunit. This sheet is flexible yet stable, in part because of a double cystine bridge at its end. Asp138, Thr133, and Gln140 provide a ring of negative charges around the quaternary ammonium group of acetylcholine, Ile131 and alkane segments of the other residues in the binding site provide hydrophobic interactions, and Gln140 provides a hydrogen bond for acetylcholine's carbonyl group; Glu129 would form part of the second anionic subsite for the bis-quaternary ammonium compounds and curares. The model is compatible with the available evidence pertaining to the binding site and with structure-activity relationship studies. It is precise and detailed, thereby making clear predictions, which are directly testable by affinity labeling and site-directed mutagenesis. It should prove useful in the design of such experiments.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

“…[4,209,210] Extending the sequence comparison to receptor subunits from different species, [211] it was possible to identify conserved residues critical for ligand binding or receptor function which, together with hydropathy analysis, guided the first attempts to derive secondary structure predictions and topographical models of the receptor channel and of the ligand binding site. [212][213][214][215][216] Site-directed mutagenesis experiments on Torpedo receptor confirmed the importance of several residues for protein-ligand interactions and delineated two components of the binding site: a "principal component" located on the a subunit and a "complementary component" on the g or d subunit.…”

Dissecting the Chemistry of Nicotinic Receptor-Ligand Interactions with Infrared Difference Spectroscopy

“…mide, and the nAChR. 2 The difference between spectra of the nAChR recorded in the presence and absence of TMA is thus referred to as a TMA R3 D difference spectrum.…”

“…Each of the two acetylcholine (ACh) binding sites on the nAChR consists of two subsites, an esterophilic subsite that binds the ester functional group of ACh and an anionic subsite that binds the quaternary ammonium cation (1,2). Sequence analysis, affinity labeling, and site-directed mutagenesis identify both aromatic and negatively charged residues in the anionic subsite that likely interact with the charged nitrogen of ACh via cation-electron interactions and/or hydrogen bonding (3)(4)(5)(6)(7)(8)(9).…”

Cholinergic nicotinic receptors: Competitive ligands, allosteric modulators, and their potential applications

“…[145][146][147][148] Putting together all the evidence coming from X-ray diffraction, photoaffinity labeling, and mutagenesis studies, researchers have tried to build structural models of the neurotransmitter binding site. [149][150][151] Most recently, homology modeling techniques have been used, choosing as a template the enzyme AChE, a protein with known three-dimensional structure which recognizes the same neurotransmitter ACh, 152 or the blue copper binding protein plastocyanine which shows structural similarity with the extracellular domain of the nAChR a subunits. 153 The greatest improvement in the modelling of the neurotransmitter binding site has come after the discovery of a glia-derived AChBP protein, 144 whose X-ray structure reveals a striking resemblance to the nAChR.…”