A cation-π interaction between the ammonium group of an agonist and a conserved tryptophan termed TrpB is a near universal feature of agonist binding to nicotinic acetylcholine receptors (nAChRs). TrpB is one of five residues that form the aromatic box of the agonist binding site, and for the prototype agonists ACh and nicotine, only TrpB makes a functional cation-π interaction. We report that, in addition to TrpB, a significant cation-π interaction is made to a second aromatic – TyrC2 – by the agonists metanicotine, TC299423, varenicline, and nornicotine. A common structural feature of these agonists – and a distinction from ACh and nicotine – is a protonated secondary amine that provides the cation for the cation-π interaction. These results indicate a distinction in binding modes between agonists with subtly different structures that may provide guidance for the development of subtype-selective agonists of nAChRs.
Significance Statement The α4β2 nicotinic acetylcholine receptor binding site is made of several loops contributing five aromatic residues. Here we show four secondary ammonium agonists – TC299423, metanicotine, varenicline, and nornicotine – make a cation-π interaction with TyrC2 in addition to the canonical cation-π interaction with TrpB. The prototypical agonists acetylcholine (a quaternary ammonium), and nicotine (a tertiary ammonium) only make a cation-π interaction with TrpB. This result indicates a new binding mode for agonists with only subtle structural differences, and suggests that a more compact cation allows for greater interaction with Loop C in the binding site.
Authors report no conflict of interest.
HHS | NIH | National Institute of Neurological Disorders and Stroke (NINDS) (100000065, grant no. NS34407); HHS | NIH | National Institute on Drug Abuse (NIDA)(funding id. 100000026, grand no. DA 019375) Beckman Institute.