Figure 1

Dinesh Indurthi

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Background and Purpose: Agonists turn on receptors because they have higher affinity for active versus resting conformations of their target sites. Agonist efficiency (distinct from efficacy) is the fraction of binding energy applied to the receptor’s activating conformational change and depends only on the resting/active equilibrium dissociation constant ratio. Our goal was to estimate agonist efficiency from concentration-response curves (CRCs). Experimental Approach: Adult skeletal muscle acetylcholine receptors (AChRs) were expressed in HEK cells and CRCs were compiled from single-channel currents. The efficiencies of 13 agonists were estimated from the midpoints and maxima of the CRCs by using equations that pertain to a cyclic activation scheme. Key Results: Agonist efficiency was greater for small- compared to large-volume agonists, 52±2% (n=9) for 70 A3 versus 40±5% (n=4) for 101 A3. The smoking-cessation drugs varenicline and cytisine belong to the lower efficiency group. An examination of AChRs having a binding site mutation showed that Y190A was the only one of 22 that affected the efficiency of ACh, switching it from the more- to the less-efficient group. IF agonist efficiency is known, EC50 can be estimated from the maximum response and the unliganded gating equilibrium constant can be calculated from the CRC parameters. Conclusion and Implications: We hypothesize that larger agonists have lower efficiencies because they limit the extent of binding-pocket contraction upon activation, and that efficiency values may be modal. Knowledge of agonist efficiency simplifies and extends CRC analysis.