Structural Insights into Partial Activation of the Prototypic G Protein-Coupled Adenosine A Receptor.

The adenosine A receptor (AAR) belongs to the rhodopsin-like G protein-coupled receptor (GPCR) family, which constitutes the largest class of GPCRs. Partial agonists show reduced efficacy as compared to physiological agonists and can even act as antagonists in the presence of a full agonist. Here, we determined an X-ray crystal structure of the partial AAR agonist 2-amino-6-[(1-imidazol-2-ylmethyl)sulfanyl]-4--hydroxyphenyl-3,5-pyridinedicarbonitrile (LUF5834) in complex with the AAR construct A-PSB2-bRIL, stabilized in its inactive conformation and being devoid of any mutations in the ligand binding pocket. The determined high-resolution structure (2.43 Å) resolved water networks and crucial binding pocket interactions. A direct hydrogen bond of the -hydroxy group of LUF5834 with T88 was observed, an amino acid that was mutated to alanine in the most frequently used AAR crystallization constructs thus preventing the discovery of its interactions in most of the previous AAR co-crystal structures. G protein dissociation studies confirmed partial agonistic activity of LUF5834 as compared to that of the full agonist -ethylcarboxamidoadenosine (NECA). In contrast to NECA, the partial agonist was still able to bind to the receptor construct locked in its inactive conformation by an S91K mutation, although with an affinity lower than that at the native receptor. This could explain the compound's partial agonistic activity: while full AAR agonists bind exclusively to the active conformation, likely following conformational selection, partial agonists bind to active as well as inactive conformations, showing higher affinity for the active conformation. This might be a general mechanism of partial agonism also applicable to other GPCRs.