Agonist efficiency from concentration-response curves: structural implications and applications

Abstract

Agonists are evaluated by a concentration-response curve (CRC), with a midpoint (EC50) that indicates potency, a high-concentration asymptote that indicates efficacy and a low-concentration asymptote that indicates constitutive activity. A third agonist attribute, efficiency ({eta}), is the fraction of binding energy that is applied to the conformational change that activates the receptor. We show that {eta} can be calculated from EC50 and the asymptotes of a CRC derived from either single-channel or whole-cell responses. For 20 agonists of skeletal muscle nicotinic receptors, the distribution of {eta} values is bimodal with population means at 51% (including ACh, nornicotine and DMPP) and 40% (including epibatidine, varenicline and cytisine). The value of {eta} is related inversely to the size of the agonists head-group, with high-versus low-efficiency ligands having an average volume of 70 [A]3 versus 102 [A]3. Most binding site mutations have only a small effect on ACh efficiency except for Y190A (35%), W149A (60%) and those at G153 (42%). If {eta} is known, the midpoint and high-concentration asymptote can be calculated from each other. Hence, an entire CRC can be estimated from the response to a single agonist concentration, and efficacy can be estimated from EC50 of a CRC that has been normalized to 1. Given {eta}, the level of constitutive activity can be estimated from a single CRC. Statement of significanceReceptors are molecular machines that convert chemical energy from agonist binding into mechanical energy of a global conformational change that generates a cell response. Agonists are distinguished by their potency (proportional to affinity) and efficacy, but also by the efficiency at which their binding energy is applied to receptor activation. Here we show that agonist efficiency can be estimated from a single concentrationresponse curve, and estimate efficiencies of 20 nicotinic receptor agonists. These have a bimodal distribution with larger agonists belonging to the lower-efficiency population. We further show that mutations of some binding site residues alter efficiency, and that knowledge of agonist efficiency simplifies and extends dose-response curve analysis.