Cation-π interactions in ligand recognition by serotonergic (5-HT_3A) and nicotinic acetylcholine receptors: The anomalous binding properties of nicotine

A series of tryptophan analogues has been introduced into the binding site regions of two ion channels, the ligand-gated nicotinic acetylcholine and serotonin 5-HT_3A receptors, using unnatural amino acid mutagenesis and heterologous expression in Xenopus oocytes. A cation-π interaction between serotonin and Trp183 of the serotonin channel 5-HT_3AR is identified for the first time, precisely locating the ligand-binding site of this receptor. The energetic contribution of the observed cation-π interaction between a tryptophan and the primary ammonium ion of serotonin is estimated to be approximately 4 kcal/mol, while the comparable interaction with the quaternary ammonium of acetylcholine is approximately 2 kcal/mol. The binding mode of nicotine to the nicotinic receptor of mouse muscle is examined by the same technique and found to differ significantly from that of the natural agonist, acetylcholine.