PT  - JOURNAL ARTICLE
AU  - Nicola J Smith
AU  - Richard J Ward
AU  - Leigh A Stoddart
AU  - Brian D Hudson
AU  - Evi Kostenis
AU  - Trond Ulven
AU  - Joanne C. Morris
AU  - Christian Trankle
AU  - Irina G Tikhonova
AU  - David R Adams
AU  - Graeme Milligan
TI  - Extracellular Loop 2 Of The Free Fatty Acid Receptor 2 Mediates Allosterism Of A Phenylacetamide Ago-Allosteric Modulator
AID  - 10.1124/mol.110.070789
DP  - 2011 Jan 01
TA  - Molecular Pharmacology
PG  - mol.110.070789
4099  - http://molpharm.aspetjournals.org/content/early/2011/04/15/mol.110.070789.short
4100  - http://molpharm.aspetjournals.org/content/early/2011/04/15/mol.110.070789.full
AB  - Allosteric agonists are powerful tools for exploring the pharmacology of closely related G protein-coupled receptors that have non-selective endogenous ligands, such as the short chain fatty acids at Free Fatty Acid receptors 2 and 3 (FFA2/GPR43 and FFA3/GPR41, respectively). We explored the molecular mechanisms mediating the activity of 4-CMTB, a recently described phenylacetamide allosteric agonist and allosteric modulator of endogenous ligand function at human FFA2, by combining our previous knowledge of the orthosteric binding site with targeted examination of 4-CMTB structure-activity relationships and mutagenesis and chimeric receptor generation. Here we show that 4-CMTB is a selective agonist for FFA2 that binds to a site distinct from the orthosteric site of the receptor. Ligand structure-activity relationship studies indicated that the N-thiazolyl amide likely provides hydrogen bond donor/acceptor interactions with the receptor. Substitution at Leu173 and the exchange of the entire extracellular loop 2 of FFA2 with that of FFA3 was sufficient to respectively reduce, or ablate, allosteric communication between the endogenous and allosteric agonists. Thus, we conclude that extracellular loop 2 of human FFA2 is required for transduction of co-operative signaling between the orthosteric and an as yet undefined allosteric binding site of the FFA2 receptor that is occupied by 4-CMTB.