The cholecystokinin (CCK) receptor-1 (CCK1R) is a G protein-coupled receptor, which mediates important central and peripheral cholecystokinin actions. Our aim was to progress in mapping of the CCK1R binding site by identifying residues that interact with the methionine and phenylalanine residues of the C-terminal moiety of CCK because these are crucial for its binding and These new and important insights will serve to better understand the activation process of CCK1R and to design or optimize ligands.

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Delineation of CCK receptor binding sites is a prerequisite for the understanding of the molecular basis for ligand recognition, partial agonism, ligand-induced traffiking of receptor signalling. In the current paper, we illustrate how, in the past 5 years, studies from our laboratory and others have provided new data on the molecular basis of the pharmacology and functioning of CCK1 and CCK2 receptors. Available data on CCK1 and CCK2R binding sites indicate that 1) homologous regions of the two receptors are involved in the binding site of CCK, however, positioning of CCK slightly differs; 2) binding sites of non-peptide agonists/ antagonist are buried in the pocket formed by transmembrane helices and overlap that of CCK and 3) residues of the binding sites as well as of conserved motifs such as E/DRY, NPXXY are crucial for receptor activation.The actions of cholecystokinin (CCK) and gastrin are mediated by specific high affinity membrane receptors on target cells. These receptors have been characterized pharmacologically and biologically and are divided into two subtypes based on their affinities for CCK and gastrin. The CCK1 receptor (previously named CCK-A receptor) has an approximately 500 times higher affinity for CCK than for gastrin and the CCK2 receptor (previously named CCKB/ G receptor) has the same high affinity for both CCK and gastrin. An additional difference between the CCK1 and CCK2 receptors resides in their affinities for sulfated and non-sulfated CCK. Indeed, whereas the CCK1 receptor binds and responds to sulfated CCK with 500 times higher affinity/potency than non-sulfated CCK, the CCK2 receptor weakly discriminates between the two (Silvente-Poirot et al. 1993;Wank 1998).

A rational combination of site-directed mutagenesis studies, structure-activity relationships, and dynamic-based docking of pyridopyrimidine-derived CCK1R antagonists into a refined three-dimensional model of the CCK1R allowed us to identify the receptor residues and the ligand functional groups implicated in the molecular recognition process. Our results provided unambiguous evidence that the binding site of these antagonists is overlapping that of the C-terminal tetrapeptide of CCK. In particular, Asn333 and Arg336 residues of the CCK1R are essential for high-affinity binding of these ligands. Moreover, the 2-aryl group in the pyridopyrimidine derivatives shares the same binding pocket as the C-terminal Phe side chain of CCK. Our [pyridopyrimidine.CCK1R] complex model is consistent with previous suggestions concerning the molecular basis that governs functional activity and provides useful considerations about the high CCK1 versus CCK2 selectivity of our derivatives and could contribute to fine-tune the rational design of new molecules with optimized properties.