The functional role of the binding site aspartate in muscarinic acetylcholine receptors, probed by site-directed mutagenesis
Page, K.M.; Curtis, C.A.; Jones, P.G.; Hulme, E.C.
European Journal of Pharmacology 289(3): 429-437
Mutation of the Asp in transmembrane domain three of the muscarinic receptors to Asn (M-1) or Glu (M-1 and M-2) strongly reduced the high-affinity component of agonist binding, and the M-1 phosphoinositide response. Formation of the acetylcholine-receptor binary complex was also strongly inhibited. In contrast, binary complex formation by other agonists, as well as the antagonist (-)-N-methylscopolamine, was less affected. Ionic bonding between the carboxylate oxyanion and the positively-charged headgroup probably anchors acetylcholine when it is bound in its active conformation, but alternative, non-productive, binding modes, promoted by non-polar forces, may contribute to binary complex formation by other ligands.