RT Journal Article SR Electronic T1 The Second Extracellular Loop of the Adenosine A1 Receptor Mediates Activity of Allosteric Enhancers JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP mol.113.088682 DO 10.1124/mol.113.088682 A1 Dylan P. Kennedy A1 Fiona M. McRobb A1 Susan A. Leonhardt A1 Michael Purdy A1 Heidi Figler A1 Melissa A. Marshall A1 Mahendra Chordia A1 Robert A. Figler A1 Joel Linden A1 Ruben Abagyan A1 Mark Yeager YR 2013 UL http://molpharm.aspetjournals.org/content/early/2013/11/11/mol.113.088682.abstract AB Allosteric enhancers of the adenosine A1 receptor amplify signaling by orthosteric agonists. Allosteric enhancers are appealing drug candidates because their activity requires that the orthosteric site be occupied by an agonist, thereby conferring specificity to stressed or injured tissues that produce adenosine. To explore the mechanism of allosteric enhancer activity, we examined their action on several A1 receptor constructs, including (1) species variants, (2) species chimeras, (3) alanine scanning mutants and (4) site-specific mutants. These findings were combined with homology modeling of the A1 receptor and in silico screening of an allosteric enhancer library. The binding modes of known, docked allosteric enhancers correlated with the known structure-activity relationship, suggesting that these allosteric enhancers bind to a pocket formed by the second extracellular loop, flanked by residues S150 and M162. We propose a model in which this vestibule controls the entry and efflux of agonists from the orthosteric site, and agonist binding elicits a conformational change that enables allosteric enhancer binding. This model provides a mechanism for the observations that allosteric enhancers slow the dissociation of orthosteric agonists but not antagonists.