From ab initio quantum mechanics to molecular neurobiology: A cation–π binding site in the nicotinic receptor

Abstract

The nicotinic acetylcholine receptor is the prototype ligand-gated ion channel. A number of aromatic amino acids have been identified as contributing to the agonist binding site, suggesting that cation-interactions may be involved in binding the quaternary ammonium group of the agonist, acetylcholine. Here we show a compelling correlation between: (i) ab initio quantum mechanical predictions of cation-binding abilities and (ii) EC 50 values for acetylcholine at the receptor for a series of tryptophan derivatives that were incorporated into the receptor by using the in vivo nonsense-suppression method for unnatural amino acid incorporation. Such a correlation is seen at one, and only one, of the aromatic residues-tryptophan-149 of the ␣ subunit. This finding indicates that, on binding, the cationic, quaternary ammonium group of acetylcholine makes van der Waals contact with the indole side chain of ␣ tryptophan-149, providing the most precise structural information to date on this receptor. Consistent with this model, a tethered quaternary ammonium group emanating from position ␣149 produces a constitutively active receptor.

Insight in nAChR subtype selectivity from AChBP crystal structures

“…A c c e p t e d M a n u s c r i p t 8 interactions in the stabilization of the nicotinic ligand cation [36,37]. Structures of AChBP complexes with these small agonists indeed reveal that the tertiary nitrogen of nicotine and epibatidine or the quaternary nitrogen of carbamylcholine interact with the negative charge generated by the π electrons from the side chains of the residues making up the aromatic cage [23,38].…”

Nicotinic Acetylcholine Receptors

“…All were identified as being near the agonist binding site by labelling with a small photo-activatable ligand that covalently reacts with the receptor upon UVirradiation and acts as a competitive antagonist (Dennis et al, 1988), and the first three are highly conserved in aligned positions of all muscle and neuronal α subunits. Experiments in which a series of unnatural tryptophan derivatives were substituted in place of the natural residues, have suggested that the side-chain of αW149 is in van der Waals contact with the quaternary ammonium group of ACh in the bound state of the receptor (Zhong et al, 1998). Chemical labelling has also shown that the pair of adjacent cysteines (αC192 and αC193) is likely to be close to the binding site (Kao & Karlin, 1986).…”

“…Although the actual ligand-binding site has not yet been identified definitively within the three-dimensional structure of the receptor, ACh is expected to bind through cation-π interactions, where the positive charge of its quaternary ammonium moiety interacts with electron-rich aromatic sidechains (Zhong et al, 1998). The recently solved structure of AChBP (Brejc et al, 2001), discussed above, shows that the 'signature' aromatic residues lie in a pocket next to the interface with the anticlockwise-positioned protomer, as seen from the 'synaptic cleft'.…”

“…An ingenious study by Zhong et al, (1998) used unnatural amino acids to conclude that αW149 was "the primary cation-π binding site" in the nicotinic receptor. This sort of study cannot, however, distinguish between effects on binding and gating, because it relies on macroscopic EC 50 data only.…”

Role of the Cation-π Interaction in Therapeutic Proteins: A Comparative Study with Conventional Stabilizing Forces

“…In proteins, cation-π interactions occur between the cationic side chain of lysine (K) or arginine (R) and the aromatic side chains of phenylalanine (F), tyrosine (Y) and tryptophan (W) (Chakravarty and Varadarajan, 2000). Previous studies on cation-π interactions have focused on various aspects such as their role in ligand recognition (Zacharias and Dougherty, 2002;Zhong et al, 1998;Scrutton and Raine, 1996) and protein drug interactions (Liu et al, 2002). There are several instances where cation-π interactions have shown to play a significant role.…”