Allosteric binding pockets in peptide-binding GPCRs create opportunities for development of small molecule drugs with substantial benefits over orthosteric ligands. To gain insights into molecular determinants for this pocket within type 1 and 2 cholecystokinin receptors (CCK1R and CCK2R), we prepared a series of receptor constructs in which six distinct residues in TM2, 3, 6, and 7 were reversed. Two novel iodinated CCK1R- and CCK2R-selective 1,4-benzodiazepine antagonists differing only in stereochemistry at C3 were used. When all six residues within CCK1R were mutated to corresponding CCK2R residues, benzodiazepine selectivity was reversed, yet peptide-binding selectivity was unaffected. Detailed analysis, including observations of gain-of-function, demonstrated that residues 6.51, 6.52, and 7.39 were most important for binding CCK1R-selective ligand, while 2.61 and 7.39 were most important for binding CCK2R-selective ligand, although the effect of substitution of 2.61 was likely indirect. Ligand-guided homology modeling was applied to wild type receptors and those reversing benzodiazepine-binding selectivity. Models had high predictive power in enriching known receptor-selective ligands from related decoys, indicating a high degree of precision in pocket definition. The benzodiazepines docked in similar poses in both receptors, with C3 urea substituents pointing upward, while different stereochemistry at C3 directed the C5 phenyl rings and N1 methyl groups into opposite orientations. The geometry of the binding pockets and specific interactions predicted for ligand docking in these models provide a molecular framework for understanding ligand selectivity at these receptor subtypes. Furthermore, the strong predictive power of these models suggests their usefulness in lead discovery and development programs.
Cawston, E. E., Lam, P. C. H., Harikumar, K. G., Dong, M., Ball, A. M., Augustine, M. L., ... Miller, L. (2012). Molecular basis for binding and subtype selectivity of 1,4-benzodiazepine antagonist ligands of the cholecystokinin receptor
. Journal of Biological Chemistry
(22), 18618 - 18635. https://doi.org/10.1074/jbc.M111.335646