Acetylcholine Receptor Channels Activated by a Single Agonist Molecule

The neuromuscular acetylcholine receptor (AChR) is an allosteric: protein that alternatively adopts inactive versus active conformations (R <-> R-star). The R-star 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-star 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 approximate to 4.3 x 10(-3) with ACh, and approximate to 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 approximate to 6.5 x 10(-7). Gating changes the stability of the ligand-protein complex by similar to 5.2 kcal/mol for ACh and similar to 3.3 kcal/mol for choline.