ABSTRACT
As mentioned before, a partial, but mutually exclusive, overlap of agonists and antagonists can also be a molecular explanation for competitive antagonism. From site-directed mutagenesis studies on the !32-adrenoceptor it became evident that substitution of Asp-113 (somewhere halfway in helix III if we think of GPCRs as seven-helix bundles) with asparagine led to dramatic decreases in both agonist and antagonist affinity. Since other Gprotein-coupled receptors that bind biogenic amines such as serotonin and acetylcholine also have this aspartate present, it was concluded that this residue was the counterion for the protonated amino function in the ligands (16). It has also been postulated that two serine residues in helix V of the f3radrenoceptor (Ser-204 and Ser-207, respectively) are involved in binding the meta and para positions on the agonist phenyl ring (e.g., the catecholtwo hydroxyl groups-moiety in isoproterenol) without significant effects on antagonist binding (17). This quite hydrophilic interaction via hydrogen bonds could form an explanation for the relatively modest contribution of lipophilicity to agonist affinity apparent from the QSAR analysis. Consequently, the receptor region around the aromatic nucleus of antagonists should contain amino acid residues more lipophilic than the two serines. For antagonists another anchor site was found in an asparagine residue in helix VII (Asn-312). It was proposed that this residue forms a hydrogen bond with the oxygen atom in the oxymethylene bridge. All these findings are incorporated in Figure 7, from which it is suggested that Asp-113 is the crucial element in the competition between isoproterenol (agonist) and propranolol (antagonist), whereas the other residues are relatively specific for either agonists or antagonists.
