Conotoxin

Having looked at the general structure of PDB files, let us now examine a sample PDB file. The file represents the structure of r conotoxin PNll polypeptide (PDB ID Ipcn) and was retrieved from the Protein Data Bank [53]. Figure 2-109 shows the 3D structure of the molecule. 

Conotoxins are the venoms of the marine cone snails. The >500 Conus species produce >10,000 different toxins. All are cysteine-rich peptides of 10-30 amino... 

Acetylcholine nicotinic (LGIC) Na+, K+ and Ca2 + conductance Nicotine, suxamethonium Tubocurarine, a-conotoxins, a-bungarotoxin... 

Certain occluders also discriminate among Na channels from neuronal and skeletal muscle. But in this case the blocking ligands are small peptides, the x-conotoxins from the mollusc Conus geo aphus. This molecule binds tightly to muscle Na channels, effectively reducing Na current (55 see Figure 6A), and also can displace bound... 

Conotoxins Targeted Peptide Ligands from Snail... 

OLIVERA ET AL, Conotoxins Targeted Peptide Ligands from Snail Venoms 2<>3... 

When a cone snail envenomates its prey, the latter is invariably paralyzed. In all cases, the paralytic toxins in the venom ("conotoxins") appear to be small peptides, most commonly with 3 disulfide bonds (although conotoxins with 2 or 4 S-S bonds... 

In the venom of C. geographus and other fish-hunting species, the conotoxins isolated so far can be divided into three major classes (1-4) o -conotoxms which block neuronal calcium channels at the presynaptic terminus of the neuromuscular junction, a-conotoxins which inhibit the acetylcholine receptor at the postsynaptic terminus, and x-conotoxins which block Na channels on the muscle membrane. 

Between 6 and 10 homologous peptides have been extensively characterized for each toxin class. Although uj- and a-conotoxins have been isolated from several fish-hunting Conus species, x-conotoxins have so far been isolated only from C. geographus venom. 

The biological targets of paralytic conotoxins are presumably ion channels and receptors in the fish neuromuscular system. Amphibian, avian, and mammalian systems may also be affected by a -, a-, and x-conotoxins. However, specific conotoxins vary in their phylogenetic spectrum and detailed physiological effects in vivo. 

In vertebrates, the spectrum of w-conotoxin targets is dependent not only on the species of animal being studied, but on the w-conotoxin being used. In chicks and frogs all synapses tested are almost completely inhibited by w-conotoxin GVIA, while in rodents, w-conotoxin GVIA does not inhibit the neuromuscular synapse, and only a fraction of CNS synapses tested are blocked. In contrast, w-conotoxin MVIIA acts much more reversibly and with a much reduced affinity for many amphibian, and some mammalian, synapses. 

In the periphery, there are no obvious biological effects from w-conotoxin when injected into an adult rodent. However, when injected into a neonatal rodent, a slow depression in respiration takes place suggesting that inhibition of some o -conotoxin binding site in the periphery may result in a dysfunction in respiratory control (J. M. McIntosh, unpublished results). 

Peptides in the a-conotoxin family are inhibitors of nicotinic acetylcholine receptors. They were first isolated from C. geographus venom as components which cause paralysis in mice and fish when injected intraperitoneally (27). Early physiological experiments (28) indicated that a-conotoxins GI, GII, and GIA (see Table III) all act at the muscle end plate region. Mini end-plate potentials and end plate potentials evoked in response to nerve stimulation are inhibited in the presence of a-conotoxins in the nM to pM range. a-Conotoxin GI was subsequently shown to compete with rf-tubocurarine and a-bungarotoxin for the acetylcholine receptor (29). 

Like the other paralytic toxins from Conus venom, a-conotoxins are small and very tightly folded, structural features which may be advantageous for rapid paralysis of prey (1). a-Conotoxins are typically 13 to 15 amino acids long with two disulfide bridges (see Table III). In addition to the five a-conotoxins shown, two new a-conotoxins (SIA and SIB) from C. striatus have recently been isolated, sequenced, and chemically synthesized. SIA is very unusual because it is 19 amino acid residues long and it contains 6 cysteine residues, three of which are contiguous near the amino terminus (C. Ramilo et al., unpublished results). 

Radioiodinated derivatives have been prepared to define more closely the target site of a-conotoxins on the acetylcholine receptor (R. Myers, unpublished data). In membrane preparations from Torpedo electroplax, photoactivatable azidosalicylate derivatives of a-conotoxin GIA preferentially label the p and 7 subunits of the acetylcholine receptor. However, when the photoactivatable derivative is cross-linked to detergent solubilized acetylcholine receptor (AChR), only the 7 subunit is labeled. Since snake a-neurotoxins mainly bind to the a subunits of AChR and a-conotoxins compete directly with a-bungarotoxin, the cross-linking results above are both intriguing and problematic. 

The a -, /z-, and a-conotoxins are the best characterized of the peptides isolated from Conus venoms so far. However, a large number of other peptides are found in these venoms. These comprise both paralytic toxins to immobilize the prey of the cone snail, and other biologically active peptides which are not themselves directly paralytic. Only the briefest overview of these peptide components will be presented here. 

In the fish-hunting cone snail venoms, a- and w-conotoxins are ubiquitously distributed. As noted above, z-conotoxins have only been found in one species. Conus geographus. In addition to these three well-characterized classes, however, a fourth class of paralytic conotoxins has been found. In contrast to the a-, z-, and... 

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