title: Docking studies of agonists and antagonists suggest an activation pathway of the A3 adenosine receptor.
abstract: Structural determinants of ligand efficacy in the human A(3) adenosine receptor (AR) were studied using pharmacophore and docking analyses of various categories of A(3) selective ligands: inverse agonist, neutral antagonist (nonnucleoside and nucleoside), and agonist (partial and full). The homology modeling of GPCRs was adapted to provide two templates: the rhodopsin-based resting state for antagonist binding and a putative Meta I state, conformationally altered at a key residue (W6.48), for agonist binding. The preferential binding domains and/or local conformational changes associated with docking of three high affinity A(3)AR ligands were compared: inverse agonist PSB-11 1 ((R)-8-ethyl-4-methyl-2-phenyl-imidazo[2,1-i]purin-5-one); neutral antagonist MRE-3008F20 7 (5-[[(4-methoxyphenyl)amino]carbonyl]amino-8-methyl-2-(2-furyl)pyra-zolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine), and full agonist Cl-IB-MECA 21 (2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarboxamidoadenosine) to define a distinct recognition mode for each. Ribose-containing agonists were more hydrophilic than nonnucleoside antagonists, and H-bonding ability at the ribose 3'- and 5'-positions was required for agonism. From the receptor perspective, common requirements for activation included the destabilization of H-bond networks at W6.48 and H7.43, the specific interactions of the ribose moiety in its putative hydrophilic pocket at T3.36, S7.42, and H7.43, the stabilization of the complex by inward movement of F5.43, and the characteristic rotation of W6.48. By analogy, outward rotation of the W6.48 side-chain upon activation of an internally-crosslinking mutant M(3) muscarinic receptor was indicated by constrained molecular dynamics (MD). Our results are consistent with an anti-clockwise rotation (from the extracellular view) of transmembrane domains 3, 5, 6, and 7, as proposed for other Family A GPCRs. Thus, the putative conformational changes associated with A(3)AR activation indicate a shared mechanism of GPCR activation similar to rhodopsin.
authors.affiliation: Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), National Institutes of Health (NIH), Bethesda, MD 20892, USA.
authors.forename: Soo-Kyung
authors.initials: SK
authors.lastname: Kim
authors.forename: Zhan-Guo
authors.initials: ZG
authors.lastname: Gao
authors.forename: Lak Shin
authors.initials: LS
authors.lastname: Jeong
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Jacobson
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives / Q000737:chemistry / Q000494:pharmacology; D058909:Adenosine A3 Receptor Agonists; D058918:Adenosine A3 Receptor Antagonists; D000818:Animals; D001665:Binding Sites; D003198:Computer Simulation; D006801:Humans; D006860:Hydrogen Bonding; D007093:Imidazoles / Q000737:chemistry / Q000494:pharmacology; D066298:In Vitro Techniques; D008024:Ligands; D008958:Models, Molecular; D010671:Phenylurea Compounds / Q000737:chemistry / Q000494:pharmacology; D011487:Protein Conformation; D011687:Purines / Q000737:chemistry / Q000494:pharmacology; D051381:Rats; D043684:Receptor, Adenosine A3 / Q000737:chemistry / Q000378:metabolism; D043587:Receptor, Muscarinic M3 / Q000737:chemistry; D013329:Structure-Activity Relationship; D014230:Triazoles / Q000737:chemistry / Q000494:pharmacology
chemical_list: C438613:8-ethyl-4-methyl-2-phenyl-4,5,7,8-tetrahydro-1H-imidazo(2,1-i)purin-5-one; D058909:Adenosine A3 Receptor Agonists; D058918:Adenosine A3 Receptor Antagonists; D007093:Imidazoles; D008024:Ligands; C405605:MRE 3008-F20; D010671:Phenylurea Compounds; D011687:Purines; D043684:Receptor, Adenosine A3; D043587:Receptor, Muscarinic M3; D014230:Triazoles; D000241:Adenosine; C090034:2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide