TY - JOUR T1 - The Second Extracellular Loop of the Adenosine A<sub>1</sub> Receptor Mediates Activity of Allosteric Enhancers JF - Molecular Pharmacology JO - Mol Pharmacol SP - 301 LP - 309 DO - 10.1124/mol.113.088682 VL - 85 IS - 2 AU - Dylan P. Kennedy AU - Fiona M. McRobb AU - Susan A. Leonhardt AU - Michael Purdy AU - Heidi Figler AU - Melissa A. Marshall AU - Mahendra Chordia AU - Robert Figler AU - Joel Linden AU - Ruben Abagyan AU - Mark Yeager Y1 - 2014/02/01 UR - http://molpharm.aspetjournals.org/content/85/2/301.abstract N2 - 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. ER -