Variations in Binding Among Several Agonists at Two Stoichiometries of the Neuronal, α4β2 Nicotinic Receptor

Drug-receptor binding interactions of four agonists, ACh, nicotine, and the smoking cessation compounds varenicline (Chantix) and cytisine (Tabex), have been evaluated at both the 2:3 and 3:2 stoichiometries of the alpha 4 beta 2 nicotinic acetylcholine receptor (nAChR). Previous studies have established that unnatural amino acid mutagenesis can probe three key binding interactions at the nAChR: a cation-pi interaction, and two hydrogen-bonding interactions to the protein backbone of the receptor. We find that all drugs make a cation-pi interaction to TrpB of the receptor. All drugs except ACh, which lacks an N+B group, make a hydrogen bond to a backbone carbonyl, and ACh and nicotine behave similarly in acting as a hydrogen-bond acceptor. However, varenicline is not a hydrogen-bond acceptor to the backbone NH that interacts strongly with the other three compounds considered. In addition, we see interesting variations in hydrogen bonding interactions with cytisine that provide a rationalization for the stoichiometry selectivity seen with this compound.