My watch list
my.bionity.com  
Login  

Acetylcholine receptor



  An acetylcholine receptor (abbreviated AChR) is an integral membrane protein that responds to the binding of the neurotransmitter acetylcholine.

Additional recommended knowledge

Contents

Classification

Like other transmembrane receptors, acetylcholine receptors are classified according to their "pharmacology", or according to their relative affinities and sensitivities to different molecules. Although all acetylcholine receptors, by definition, respond to acetylcholine, they respond to other molecules as well.

Receptor types

Molecular biology has shown that the nicotinic and muscarinic receptors belong to distinct protein superfamilies.

nAChR

The nAChRs are ligand-gated ion channels, and, like other members of the "cys-loop" ligand-gated ion channel superfamily, are composed of five protein subunits symmetrically arranged like staves around a barrel. The subunit composition is highly variable across different tissues. Each subunit contains four regions named M1, M2, M3, and M4, which span the membrane and consist of approximately 20 amino acids. The M2 region, which sits closest to the pore lumen, forms the pore lining.

Binding of acetylcholine to the N termini of each of the two alpha subunits results in the 15° rotation of all M2 helices.[1] The cytoplasm side of the nAChR receptor has rings of high negative charge that determine the specific cation specificity of the receptor and remove the hydration shell often formed by ions in aqueous solution. In the intermediate region of the receptor, within the pore lumen, valine and leucine residues (Val 255 and Leu 251) define a hydrophobic region which the dehydrated ion must pass through.[2]

nAChR is found at the edges of junctional folds at the neuromuscular junction on the postsynaptic side, and is activated by acetylcholine release across the synapse. The diffusion of Na+ and K+ across the receptor causes depolarization, the end-plate potential, that opens voltage-gated sodium channels, which allows for firing of the action potential and potentially muscular contraction.

mAChR

In contrast, the mAChRs are not ion channels, but belong instead to the superfamily of G-protein-coupled receptors that activate other ionic channels via a second messenger cascade.

Role in health and disease

Nicotinic acetylcholine receptors can be blocked by curare and toxins present in the venoms of snakes and shellfishes, like α-bungarotoxin. Drugs such as the neuromuscular blocking agents bind reversibly to the nicotinic receptors in the neuromuscular junction and are used routinely in anaesthesia.

Nicotinic receptors are the primary mediator of the effects of nicotine. In myasthenia gravis, the receptor is targeted by antibodies, leading to muscle weakness. Muscarinic acetylcholine receptors can be blocked by the drugs atropine and scopolamine.

See also

  • Muscarinic acetylcholine receptor M5

The muscarine cholinergic receptor activates a G protein when bound to ex.c. ach. The alpha subunit of the Gprotein deactivates adenylate cyclase while the betagamma subunit activates the Kchannels and therefore hyperpolarise the cell. This causes a decrease in cardiac activity.

References

  1. ^ Doyle DA (2004). "Structural changes during ion channel gating". Trends Neurosci. 27 (6): 298–302. doi:10.1016/j.tins.2004.04.004. PMID 15165732.
  2. ^ Miyazawa A, Fujiyoshi Y, Unwin N (2003). "Structure and gating mechanism of the acetylcholine receptor pore". Nature 423 (6943): 949–55. doi:10.1038/nature01748. PMID 12827192.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Acetylcholine_receptor". A list of authors is available in Wikipedia.
Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE