Acetylcholine receptor toxin-binding site

Hydrophilicity analysis of lapemis toxin showed that the central loop is the most hydrophilic region. Since the ligand-binding region of the acetylcholine receptor is in the outside of the cell membranes, it is also hydrophilic. Therefore, it is also logical that the most hydrophilic portion of neurotoxin binds to the acetylcholine receptor-ligand-binding site, which is also hydrophilic. 

Figure 7.23 Diagrammatic representation of cross section of nicotinic acetylcholine receptor showing various sites that bind acetylcholine, insecticides, toxins, and drugs. ACh, acetylcholine NTX, nereistoxin a-BGT, a-bungarotoxin H12-HTX, pH]perhydrohistrionicotoxin PCP, phencyclidine. (From Eldefrawi, M.E. and Eldefrawi, A.T., in Safe Insecticides Development and Use, Hodgson, E. and Ruhr, R.J., Eds., Marcel Dekker, New York, 1990, p. 155. With permission.)...

Most AChR studies were done using skeletal muscle or torpedo tissues. The acetylcholine receptor concentration in the brain is very small, but it is present. Recently, the AChR in the brain has been actively studied using snake postsynaptic neurotoxins. Some of these are rather typical neurotoxins that bind to both skeletal muscles and the brain, and some of them are specific to the brain AChR. Since a brain a-subunit of AChR binds to a-btx, there must be a similarity between the toxin-binding site for the brain AChR and the muscle AChR (McLane et al., 1990 Scheidler et al., 1990). 

Another possible target for toxins are the receptors for neurotransmitters since such receptors are vital, especially for locomotion. In vertebrates the most strategic receptor is that for acetylcholine, the nicotinic receptor. In view of the breadth of action of the various conotoxins it is perhaps not surprising that alpha-conotoxin binds selectively to the nicotinic receptor. It is entirely possible that similar blockers exist for the receptors which are vital to locomotion in lower species. As mentioned previously, lophotoxin effects vertebrate neuromuscular junctions. It appears to act on the end plate region of skeletal muscle (79,59), to block the nicotinic receptor at a site different from the binding sites for other blockers (81). 

When a nerve-muscle preparation is stimulated in the presence of a sea snake neurotoxin, there is no twitch. However, when the muscle itself is stimulated directly in the presence of a neurotoxin, the muscle contracts. This means that neurotoxin does not inhibit the muscle itself. Moreover, postsynaptic neurotoxin does not inhibit the release of acetylcholine from the nerve ending. Therefore, the site of snake toxin inhibition must be in the postsynaptic site 20). Later it was shown that a neurotoxin strongly binds to the acetylcholine receptor (AChR). 

This type of response may be caused by several mechanisms. For instance, the muscle relaxation induced by succinylcholine, discussed in more detail in chapter 7, is due to blockade of neuromuscular transmission. Alternatively, acetylcholine antagonists such as tubocurarine may compete for the receptor site at the skeletal muscle end plate, leading to paralysis of the skeletal muscle. Botulinum toxin binds to nerve terminals and prevents the release of acetylcholine the muscle behaves as if denervated, and there is paralysis. This will be discussed in more detail in chapter 7. 

Nicotiana tabacum contains 0.5-9% nicotine, which is the primary toxin. Nicotine binds to select acetylcholine receptors throughout the body known as nicotine receptors. This produces initial stimulation, but inhibition later, at the receptor sites throughout the nervous system. Low doses enhance the release of catecholamines and sympathetic stimulation. Higher doses produce parasympathetic stimulation. 

B. Arrangement of subunits of the nicotinic acetylcholine receptor, viewed perpendicular to the membrane suiface. The binding sites for acetylcholine and a-toxin are situated on the a-subunits. 

S. Fuchs, R. Kasher, M. Balass, T. Scherf, M. Hard, M. Fridkin, J.L. Sussman and E. Katchalski-Katzir, The Binding Site for a-Bungaro-toxin in the Acetylcholine Receptor From Synthetic Peptides to Solution and Crystal Structure , p. 19... 

A practical detail, useful in recognizing the receptor, is that the Formosan snake toxin, a-bungarotoxin (acetyl- W) binds only to the acetylcholine (ACh) receptor site (in mouse diaphragm muscle), whereas diwopropyl-phosphorofluoridate ( H) 12,15) binds specifically to the active site of acetylcholinesterase. An equal number (3 x 10 per end-plate) of the receptor and the enzyme sites is found, i.e. one active molecule per 5000 A of the membrane, which is consequently densely occupied by these two proteins. Blockade becomes marked only when 70 per cent of the ACh receptor sites are occupied, and hence there are not many spare receptors at this site (Barnard, Wieckowski, and Chiu, 1971). 

Myers, R. A. 1989. Toxins of Conus marine snails bind the nicotinic acetylcholine receptor at sites not identical to those of traditional antagonists. J. Neurosci. 15 678. 

---