Introduction
Nicotinic AChRs are ligand-gated ion channels that couple an increase in agonist binding energy to an increase in gating equilibrium constant. These receptors switch globally between resting and active conformations (‘gating’) that bind agonists weakly (with low affinity) and have a C(losed) channel, or strongly (with high affinity) and have an O(pen) channel (Fig. 1) (Monod, Wyman & Changeux, 1965). Constitutive AChR activation is uphill energetically, so without a bound agonist(s) the probability of being in the O conformation (PO) is vanishingly small (Jackson, 1986; Nayak, Purohit & Auerbach, 2012; Purohit & Auerbach, 2009). However, when a resting receptor with a bound agonist(s) begins spontaneously to undergo the channel-opening transition, the newfound, favorable binding energy serves to increase PO above the basal level. This increase can be dramatic. At adult human neuromuscular synapses, 2 bound neurotransmitter molecules increase transiently PO from ~0.0000005 to ~0.95 (Auerbach, 2012; Karlin, 1967).
Agonists are differentiated by two features of their CRCs, the concentration that produces a half-maximal response (EC50, related to ‘affinity’) and the maximum response (POmax, related to ‘efficacy’) (Colquhoun, 1998). Recently, a third distinguishing attribute, ‘efficiency’, was proposed (Nayak, Vij, Bruhova, Shandilya & Auerbach, 2020). Agonist efficiency is defined as the fraction of ligand binding energy that is converted into the mechanical energy of the gating conformational change. As described below, agonist affinity, efficacy and efficiency all are functions of the equilibrium dissociation constants for binding to the resting state (KdC) and to the active state (KdO).
Previously, KdC and KdO for agonists related structurally to either ACh or the frog toxin epibatidine were measured at individual human AChR neurotransmitter binding sites (Nayak & Auerbach, 2017). Kinetic analyses of single-channel currents showed that the average efficiency of agonists related to ACh was greater than for agonists related to epibatidine. Investigations of corresponding AChR structures showed that binding energy is correlated linearly and inversely with the distance between a key nitrogen atom in the ligand and the center of a binding cavity (dx) that contracts upon receptor activation (Tripathy, Zheng & Auerbach, 2019). Further, these studies showed that the bound ligand is more centered in the active versus resting pocket, and that agonist efficiency can be estimated from the relative change in dx upon activation.
Below, we calculate agonist efficiency from EC50 and POmax values for agonists of adult-type mouse endplate AChRs. We report efficiencies for 16 agonists, as well as for ACh in receptors having point mutations at the binding sites. The efficiencies fall into 2 groups, with larger-volume ligands being ~15% less efficient than smaller-volume ligands. Knowledge of agonist efficiency simplifies and extends CRC analysis.