Muscarine Conformation

The debut of the selective AChR agonist (+)-anatoxin-a has provided a new tool for AChR physiology and pharmacology. (+)-Anatoxin not only has high affinity for the nicotinic AChR but it also has high selectivity for nicotinic over muscarinic receptors in the mammalian CNS. Recently, the use of (+)-anatoxin-a was essential to the identification of nicotinic receptors on cultured neurons (4), We are studying the features which allow it to bind with high affinity to the peripheral and central nicotinic receptors and the kinetic effects on receptor conformational... 

Henis, Y. I. and Sokolovsky, M. (1983) Muscarinic antagonists induce different receptor conformations in rat adenohypophysis. Mol. Pharmacol. 24, 357-365. 

In derivatives structurally related to pirenzepine (144), a muscarine antagonist, biological activity is likely to differ (84MI7) as a function of conformer composition. In pirenzepine (144), NMR reveals two conformers (84M17) at 10°C, with the relative amount changing with temperature. 

S ATP -I- muscarinic cholinergic receptor <3> (<3> from chick heart [7-9] <3> in vitro as good as -adrenergic receptor, phosphorylation depends on the presence of a muscarinic agonist ligand, not merely receptor occupancy, the agonist induces a conformational change, which allows phosphorylation, phosphorylation sites 70% Ser- and 30% Thr-residues, incorporation of 3-4 mol phosphate/mol receptor [7]) (Reversibility <3> [7-9]) [7-9]... 

Ward, S. D., Hamdan, F. F., Bloodworth, L. M., and Wess, J. (2002). Conformational changes that occur during M3 muscarinic acetylcholine receptor activation probed by the use of an in situ disulfide cross-linking strategy. J. Biol. Chem. 277, 2247-2257. 

Acetylcholine (9.12) binds both types of cholinergic receptors muscarinic and nicotinic (Figure 9.9). The names of these receptor types were based on muscarine (9.13) and nicotine (9.14), selective agonists for each receptor. A muscarinic response is characterized by nausea, salivation, and tearing. Nicotinic responses are noted by an acceleration of the heart rate. Acetylcholine s ability to elicit a response from both subtypes of receptors may imply that different conformations of acetylcholine may be responsible for binding to each receptor. 

A ring system that is able to restrict conformational flexibility while minimizing additional bulk is cyclopropane. As a strained system, its angles differ from the standard 60° and 180° possibilities with cyclohexane. The (+)-trans cyclopropane analogue (9.26) does show muscarinic activity comparable to acetylcholine (Figure 9.13). The dihedral angle between the ester and ammonium group is approximately 145°. The (—)-trans enantiomer and both... 

Figure 1. Predicted conformation of acetylcholine in muscarinic variant...

In the same study EHT predictions were reported for the potent muscarinic agents muscarine, Figure 2, and muscarone, Figure 3(8). The experimental evidence available for comparison are x-ray analyses of the muscarine (10) and muscarone (80) crystals, with which the predicted conformations agree. 

Superpositioning these three potent muscarinic agents, in their preferred conformations resulted in the observation of a common pattern of similarly charged structural features. The pattern, Figure 4, was proposed as the muscarinic pharmacophore (8). This pattern, predicted from theoretical considerations, bears a striking similarity to the muscarinic pharmacophore proposed by Beckett based on extensive structure—activity studies... 

Calculations on oxotremorine (15), presumed to be a CNS muscarinic agent, revealed that this molecule, in its predicted conformation (Figure 5) mimicks this predicted muscarinic pharmacophore. The assumption is made that the electronic character of the triple bond is the receptor equivalent of the ether oxygen atom in acetylcholine as proposed by Bebbington (16). These calculations support his proposal and reveal how oxotremorine may meet the structural requirements of a muscarinic agent. 

An alternative approach uses the distance geometry paradigm, in which all the constraints are combined to form the distance matrix from which energetically feasible conformations of the set of molecules are sought mathematically. Sheridan et al. (439) demonstrated this approach on acetylcholine analogs that are muscarinic agonists. Both of these approaches ask the same question and suffer from the same limitations, and differ only in computational technique. Each suffers from the local minima problem, in that each uses a... 

Spiro-DAMP (27) was slightly more potent at Mg muscarinic receptors than at Mg receptors. It was proposed that the geometry of the spiro-molecule might reflect the receptor-bound conformation of 4-DAMP (26) this conformation differs from that observed in the crystal structure of 4-DAMP. 

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