Acetylcholine binding

Muscarinic acetylcholine receptors (mAChRs) form a class of cell surface receptors that are activated upon binding of the neurotransmitter, acetylcholine. Structurally and functionally, mAChRs are prototypical members of the superfamily of G protein-coupled receptors. Following acetylcholine binding, the activated mAChRs interact with distinct classes of heterotrimeric G proteins resulting in the activation or inhibition of distinct downstream signaling cascades. 

Sussman, J.L., Harel, M., and Frolow, F. et al. (1991). Atomic structure of acetylcholinesterase from Torpedo califomica a prototypic acetylcholine-binding protein. Science 253, 872-879. 

Figure 6.1 Synthesis and metabolism of acetylcholine. Choline is acetylated by reacting with acetyl-CoA in the presence of choline acetyltransferase to form acetylcholine (1). The acetylcholine binds to the anionic site of cholinesterase and reacts with the hydroxy group of serine on the esteratic site of the enzyme (2). The cholinesterase thus becomes acetylated and choline splits off to be taken back into the nerve terminal for further ACh synthesis (3). The acetylated enzyme is then rapidly hydrolised back to its active state with the formation of acetic acid (4)...

Each muscle fiber is innervated by a branch of an alpha motor neuron. The synapse between the somatic motor neuron and the muscle fiber is referred to as the neuromuscular junction. Action potentials in the motor neuron cause release of the neurotransmitter acetylcholine. Binding of acetylcholine to its receptors on the muscle fiber causes an increase in the permeability to Na+ and K+ ions. The ensuing depolarization generates an action potential that travels along the surface of the muscle fiber in either direction that is referred to as a propagated action potential. This action potential elicits the intracellular events that lead to muscle contraction. 

Stauffer, D.A., and Karlin, A. (1994) Electrostatic potential of the acetylcholine binding sites in the nicotinic receptor probed by reaction of binding-site cysteines with charged methanethiosulfonates. Biochemistry 33, 6840-6849. 

AChBP acetylcholine binding protein BOAA P-N-oxylylamino-1 -alanine... 

Nakanishi studied philanthotoxin (polyamine-amide) interaction with nicotinic acetylcholine ion-channel [58]. Philanthotoxin-133 (PhTX-133) is a noncompetitive channel-blocker found in venom of the wasp Philanthus. Nicotinic acetylcholine ion-channel is composed of five transmembrane subunits (a, o, P, y, and S), which forms a 270-kDa glycoprotein. The major acetylcholine binding sites are in the a,ex subunits. A 43-kDa cytoplasmic protein is associated non-covalently with the receptor, but interaction with the receptor is not essential for the channel opening (Fig. 6). 

J. L. Sussman, M. Harel, F. Frolow, C. Oefner, A. Goldman, L. Toker, I. Silman, Atomic Structure of Acetylcholinesterase from Torpedo califomica A Prototypic Acetylcholine-Binding Protein , Science 1991, 253, 872 - 879. 

Acetylcholine Binds to cholinergic receptor, which are of two types— muscarinic and nicotinic. 

Representative data for [ H]acetylcholine binding to the membrane-bound Torpedo nAChR. Bindng was measured either by equilibrium dialysis (closed circles) as described in Protocol 4.1 or by centrifugation (open squares, see Protocol 4.2). Estimated Kd values from nonlinear regression curve fitting were 12 nM and 10 nM, respectively with corresponding Rq values of 0.14 ulM and 0.135 ulM... 

Middleton RE, Cohen JB. 1991. Mapping of the acetylcholine binding site of the nicotinic acetylcholine receptor [ H] nicotine as an agonist photoaffinity label. Biochemistry 30 6987-6997. 

In contrast to muscarine, atropine blocks these actions of acetylcholine and muscarine. Atropine is, therefore, an acetylcholine antagonist. It binds where acetylcholine binds and therefore prevents the binding of the latter but does not activate it. Two molecules cannot occupy the same binding site at the same time. Atropine is isolated from the plant Atropa belladonna. Extracts of this plant have been used for millennia for a variety of purposes. Although large doses are poisonous (Atropos is the name of the Fate who cuts the thread of life), small doses causes dilation of the pupils, a consequence of its action as an acetylcholine antagonist, and has been used for cosmetic purposes by women, hi Itahan, belladonna means beautiful woman. 

Dougherty, D.A. and Staueeer, D.A. Acetylcholine binding by a synthetic receptor implications for biological recognition. Science. 1990, 250, 1558-1560. 

The site of inhibition is in case of hexamethonium in the sodium channel, whereas trimethaphan blocks the acetylcholine binding site of the receptor. 

Whitehouse PJ, Price DL, Clark AW, et al Alzheimer disease evidence for selective loss of cholinergic neurons in the nucleus basahs. Ann Neurol 10 122-126, 1981 Whitehouse PJ, Price DL, Struble RG, et al Alzheimer s disease and senile dementia—loss of neurons in the basal forebrain. Science 215 1237-1239, 1982 Whitehouse PJ, Hedreen JC, White CL, et al Basal forebrain neurons in dementia of Parkinson s disease. Ann Neurol 13 243-248, 1983 Whitehouse P, Martino A, Antuono P, et al Nicotinic acetylcholine binding sites in Alzheimer s disease. Brain Res 371 146-151, 1986 Whitehouse PJ, Martino AM, Marcus KA, et al Reductions in acetylcholine and nicotine binding in several degenerative diseases. Arch Neurol 45 722-724, 1988 Whitton PS, Sama GS, O Connell MT The effect of the novel antidepressant tianeptine on the concentration of 5-hydroxytryptamine in rat hippocampal diasylates in vivo. Neuropharmacology 39 1-4, 1991 Whitworth P, Kendall DA Lithium selectively inhibits muscarinic receptor-stimulated inositol tetrakisphosphate accumulation in mouse cerebral cortex slices. J Neurochem 51 258-265, 1988... 

The AChR is one of the best characterized of all cell-surface receptors for hormones or neurotransmitters (Figure 2-9). One form of this receptor is a pentamer made up of four different polypeptide subunits (eg, two chains plus one B, one 7, and one 5 chain, all with molecular weights ranging from 43,000 to 50,000). These polypeptides, each of which crosses the lipid bilayer four times, form a cylindrical structure that is 8 nm in diameter. When acetylcholine binds to sites on the subunits, a conformational change occurs that results in the transient opening of a central aqueous channel through which sodium ions penetrate from the extracellular fluid into the cell. 

Acetylcholinesterase is the primary target of these drugs, but butyrylcholinesterase is also inhibited. Acetylcholinesterase is an extremely active enzyme. In the initial catalytic step, acetylcholine binds to the enzyme s active site and is hydrolyzed, yielding free choline and the acetylated enzyme. In the second step, the covalent acetyl-enzyme bond is split, with the addition of water (hydration). The entire process occurs in approximately 150 microseconds. 

Pathophysiology can influence muscarinic activity in other ways as well. Circulating autoantibodies against the second extracellular loop of cardiac M2 muscarinic receptors have been detected in some patients with idiopathic dilated cardiomyopathy and those afflicted with Chagas1 disease caused by the protozoan Trypanosoma cruzi. These antibodies exert parasympathomimetic actions on the heart that are prevented by atropine. In animals immunized with a peptide from the second extracellular loop of the M2 receptor, the antibody is an allosteric modulator of the receptor. Although their role in the pathology of heart failure is unknown, these antibodies should provide clues to the molecular basis of receptor activation because their site of action differs from the orthosteric site where acetylcholine binds (see Chapter 2). 

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