Understanding the molecular basis of drug action can facilitate development of more potent and selective drugs. Here, we explore the molecular basis for action of a unique small molecule ligand that is a type 1 cholecystokinin (CCK) receptor agonist and type 2 CCK receptor antagonist, GI181771X. We characterize its binding utilizing structurally related radioiodinated ligands selective for CCK receptor subtypes that utilize the same allosteric ligand-binding pocket, using wild-type receptors and chimeric constructs exchanging the distinct residues lining this pocket. Intracellular calcium assays were performed to determine biological activity. Molecular models for docking small molecule agonists to the type 1 CCK receptor were developed using a ligand-guided refinement approach. The optimal model was distinct from the previous antagonist model for the same receptor and was mechanistically consistent with the current mutagenesis data. This study revealed a key role for Leu(7.39) that was predicted to interact with the isopropyl group in the N1 position of the benzodiazepine that acts as a "trigger" for biological activity. The molecular model was predictive of binding of other small molecule agonists, effectively distinguishing these from 1065 approved drug decoys with an area under curve value of 99%. The model also selectively enriched for agonist compounds, with 130 agonists identified by ROC analysis when seeded in 2175 non-agonist ligands of the type 1 CCK receptor (area under curve 78%). Benzodiazepine agonists in this series docked in consistent pose within this pocket, with a key role played by Leu(7.39), whereas the role of this residue was less clear for chemically distinct agonists.
Keywords: Allosteric Regulation; Benzodiazepine Agonist; Binding; Biological Activity; Calcium Signaling; Cholecystokinin Receptor; G Protein-coupled Receptors (GPCR); Receptor Modification; Signaling.