The neuromuscular acetylcholine receptor (AChR) is an allosteric protein that alternatively adopts inactive versus active conformations (R<-->R). The R shape has a higher agonist affinity and ionic conductance than R. To understand how agonists trigger this gating isomerization, we examined single-channel currents from adult mouse muscle AChRs that isomerize normally without agonists but have only a single site able to use agonist binding energy to motivate gating. We estimated the monoliganded gating equilibrium constant E(1) and the energy change associated with the R versus R change in affinity for agonists. AChRs with only one operational binding site gave rise to a single population of currents, indicating that the two transmitter binding sites have approximately the same affinity for the transmitter ACh. The results indicated that E(1) approximately 4.3 x 10(-3) with ACh, and approximately 1.7 x 10(-4) with the partial-agonist choline. From these values and the diliganded gating equilibrium constants, we estimate that the unliganded AChR gating constant is E(0) approximately 6.5 x 10(-7). Gating changes the stability of the ligand-protein complex by approximately 5.2 kcal/mol for ACh and approximately 3.3 kcal/mol for choline.
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