Binding site of AChR

Introduction

There are two kinds of acetylcholine receptor in nature: nicotinic acetylcholine receptors and muscarinic acetylcholine receptors. The nicotinic acetylcholine receptor(nAChR) is a pentameric ligand-gated ion channel activated by binding of acetylcholine in nature. In this page we will show the binding site of nAChR.

Pentameric ligand-gated ion channel

Pentameric ligand gated ion channels(), or Cys-loop receptors,are a group of transmembrane ion channel proteins which open to allow ions such as Na+, K+, Ca2+, or Cl- to pass through the membrane in response to the binding of a chemical messenger, such as a neurotransmitter^[1]. Nicotinic acetylcholine receptor is a kind of pentameric ligand gated ion channels. So at first of this page, we introduce some facts of the pentameric ligand gated ion channels, which will help us to understand the structure and function of AChR.

In overall organization, the pLGIC have five subunits. The five subunits are arranged in a barrel-like manner around a central symmetry axis that coincides with the ion permeation pathway.^[2] In each subunit, the extracellular domin(ECD) of pLGIC encompasses 10β-strands that are organized as a sandwich of two tightly interacting β-sheets, while the transmembrane domain(TMD) folds into a bundle of four α-helices.

Structure of Acetylcholine binding site

The X-ray structure of AChR has not yet been solved since its hydrophobic character hampers its successful crystallization.^[3] But the X-ray structure of an acetylcholine binding protein() has been solved(Brejc et al., 2001). AChBP is most closely related to the α-subunits of the nAChR. Nearly all residues that are conserved within the nAChR family are present in AChBP, including those that are relevant for lignad binding.^[4] We already know that the ligand binding site of AChR is mainly located at the α-subunits. And AChBP can bind with α-Neurotoxins, so AChBP can be use as an example to study the structure of AChR.

The high affinity and specific interaction of α-bungarotoxin () with AChR has been of considerable importance in the study of the binding site of AChR.^[5] There is a 13-mer high affinity peptides() which corresponding to residues 187-199 of the AChR that can inhibits the binding of α-BTX eo AChR. And through the crystal structure we can study the structure binding site of AChR.

The 13-mer HAP assumes an antiparallel β hairpin structure, and is held snugly between of α-BTX. The shortest and most numerous interactions are formed with finger 2 of α-BTX. The intermolecular interaction between finger2 and two arms of the HAP hairpin make the complex stable, like .

Affinity labeling experiments which indentified position 10 and 33 of -neurotoxin to be with in 11.5-15.5 Å from AChR residues Cys192-Cys193(Michalet et al ., 2000) agree with the α-BTX-HAP structure where the corresponding Cα distance are 11.05 Å(Pro10-Ser193), 12.72 Å(Cys33-Ser193), 14.45 Å（Pro10-Ser 192）,9.76 Å(Cys33-Ser192),respectively.^[6]So through the complex of α-BTX-HAP, we can see the structure of Acetylcholine binding site.

Superimpose HAP on AChBP

The crystal shows it's a pentamer like the AChR molecule,which is obviously an ideal candidate for testing the relevance of the conformation of the HAP(The small brown loop binding to blue subunit is the HAP.) when bound to α-BTX, to that of the corresponding binding region in AChR.And the Ach binding site in AChBP is assigned to the 187-199 loop of the AChR αsubunit.

α-BTX binds perpendicular to the 5-fold axis of the AChBP molecule and therefore, there are no steric hindrance limitations even when five toxin molecules bind to AChBP.The X-ray confirm that the major interaction between α-BTX and the HAP occur residues 187-192 of AchR α subunit.The overlay of the first 12 residues of the short 13-mer binding HAP assumes a structure similar to the corresponding region of AChR upon binding to α-BTX.

The superimposed model of AchBP and α-BTX shows residues 34–36 (corresponding to residues 36–38 of AChR σ subunit) and 162–165 (181–184) of the neighboring AChBP subunit (subunit B) as abutting the α-BTX molecule.

Image:Combined model of α-BTX-HAP and AchBP.png

This figure(Michal Harel,Joel Sussman,2001) shows the stereo view of the combined model of α-BTX-HAP(Red) and AChBP structure with subunit A in green and subunit B in yellow showing the insertion of loop 2 of the toxin into the interface of the to subunits.

Function of Acetylcholine receptor

The α-Neurotoxins such as α-bungarotoxin (α-BTX)can compete antagonists of acetylcholine for its site. So studying the binding site of AChR is very important for the development of antidotesagainstα-BTX poisoning as well as drugs against, like Alzheimer's disease and nicotine addiction.

Nicotinic receptors, with a molecular mass of 290 kDa,^[7] are made up of five subunits, arranged symmetrically around a central pore.Each subunit comprises four transmembrane domains with both the N- and C-terminus located extracellularly.So as with all ligand-gated ion channels, opening of the nAChR channel pore requires the binding of a chemical messenger.Several different terms are used to refer to the molecules that bind receptors, such as ligand. As well as the endogenous agonist acetylcholine, agonists of the nAChR are nicotine, epibatidine, and choline.

In muscle-type nAChRs, the acetylcholine binding sites are located at the α and either ε or δ subunits interface (or between two α subunits in the case of homomeric receptors) in the extracellular domain near the N terminus.When an agonist binds to the site, all present subunits undergo a conformational change and the channel is opened^[8] and a pore with a diameter of about 0.65 nm opens.

Nicotinic AChRs may exist in different interconvertible conformational states. Binding of an agonist stabilises the open and desensitised states. Opening of the channel allows positively charged ions to move across it; in particular, sodium enters the cell and potassium exits. The net flow of positively-charged ions is inward.The nAChR is a non-selective cation channel, meaning that several different positively charged ions can cross through.It is permeable to Na+ and K+, with some subunit combinations that are also permeable to Ca2+^[9][10] The amount of sodium and potassium the channels allow through their pores (their conductance) varies from 50–110 pS, with the conductance depending on the specific subunit composition as well as the permeant ion.^[11]

The nAChR is unable to bind ACh when bound to any of the snake venom α-neurotoxins. These α-neurotoxins antagonistically bind tightly and noncovalently to nAChRs of skeletal muscles, thereby blocking the action of ACh at the postsynaptic membrane, inhibiting ion flow and leading to paralysis and death. The nAChR contains two binding sites for snake venom neurotoxins. Some studies have shown that a twist-like motion caused by ACh binding is likely responsible for pore opening, and that one or two molecules of α-bungarotoxin (or other long-chain α-neurotoxin) suffice to halt this motion. The toxins seem to lock together neighboring receptor subunits, inhibiting the twist and therefore, the opening motion.^[12]

The activation of receptors by nicotine modifies the state of neurons through two main mechanisms. On one hand, the movement of cations causes a depolarizationof the plasma membrane (which results in an excitatory postsynaptic potential in neurons), but also by the activation of voltage-gated ion channels. On the other hand, the entry of calcium acts, either directly or indirectly, on different intracellular cascades leading, for example, to the regulation of the activity of some genes or the release of neurotransmitters.