Neuromuscular acetylcholine receptors have long been a model system for understanding the mechanisms of operation of ligand-gated ion channels and fast chemical synapses. These five subunit membrane proteins have two allosteric (transmitter) binding sites and a distant ion channel domain. Occupation of the binding sites by agonist molecules transiently increases the probability that the channel is ion-permeable. Recent experiments show that the Monod, Wyman and Changeux formalism for allosteric proteins, originally developed for haemoglobin, is an excellent model for acetylcholine receptors. By using mutations and single-channel electrophysiology, the gating equilibrium constants for receptors with zero, one or two bound agonist molecules, and the agonist association and dissociation rate constants from both the closed- and open-channel conformations, have been estimated experimentally. The change in affinity for each transmitter molecule between closed and open conformations provides ~-5.1 kcal mol(-1) towards the global gating isomerization of the protein.
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