Snake neurotoxin

Neurotoxins present in sea snake venoms are summarized. All sea snake venoms are extremely toxic, with low LD5Q values. Most sea snake neurotoxins consist of only 60-62 amino acid residues with 4 disulOde bonds, while some consist of 70 amino acids with 5 disulfide bonds. The origin of toxicity is due to the attachment of 2 neurotoxin molecules to 2 a subunits of an acetylcholine receptor that is composed of a2 6 subunits. The complete structure of several of the sea snake neurotoxins have been worked out. Through chemical modification studies the invariant tryptophan and tyrosine residues of post-synaptic neurotoxins were shown to be of a critical nature to the toxicity function of the molecule. Lysine and arginine are also believed to be important. Other marine vertebrate venoms are not well known. 

It is a supposition that the )9-sheet structure of neurotoxin is an essential structural element for binding to the receptor. The presence of -sheet structure was found by Raman spectroscopic analysis of a sea snake neurotoxin (2). The amide I band and III band for Enhydrina schistosa toxin were at 1672 cm and 1242 cm" respectively. These wave numbers are characteristic for anti-parallel -sheet structure. The presence of -sheet structure found by Raman spectroscopic study was later confirmed by X-ray diffraction study on Laticauda semifasciata toxin b. 

The one residue most extensively studied is tryptophan. It is very easily modified, indicating that tryptophan residue is exposed 5-8). Raman spectroscopic analysis of a sea snake neurotoxin indicated that a single tryptophan residue is indeed exposed (2). The tryptophan residue lies in the important loop consisting of segment 4. Modification of the tryptophan residue induces the loss of AChR binding ability as well as the loss of toxicity 5-8). 

There is only one tyrosine residue in some sea snake neurotoxins. This residue is usually quite difficult to modify, but once it is modified, the toxicity is lost (9). Histidine seems not to be essential as the chemical modification of this residue does not affect the toxicity 10). 

Argine and lysine are believed to be important, but results are not clear because sea snake neurotoxins contain several residues of these amino acids (7). 

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). 

Table II. Amino Acid Sequence of Sea Snake Neurotoxins (Type I or Short Chain)...

Table III. Amino Acid Sequences of Sea Snake Neurotoxin (T>pe II, Long Chain)...

While most investigations show that sea snake neurotoxins are postsynaptic type, Gawade and Gaitonde (23) stated that Enhydrina schistosa major toxin has dual actions or postsynaptic as well as presynaptic toxicity. E, schistosa venom phospholipase A is both neurotoxic and myotoxic. Neurotoxic action of the enzyme is weak so that there is sufficient time for myonecrotic action to take place (24), Sea snake, L. semifasciata toxin also inhibits transmission in autonomic ganglia, but has no effect on transmission in choroid neurons. 

Elapidae Cobras Kraits Coral snakes Neurotoxin (some very potent) Fixed fangs, usually low dose Nervous system effects, paralysis, numbness, respiratory failure... 

Another example of a quantal repeat—but with considerable variation in sequence—is seen in the keratin-associated proteins (KAPs). In sheep, these display pentapeptide and decapeptide consensus repeats of the form G—G—Q—P—S/T and C-C-Q/R—P—S/T—C/S/T—C—Q—P/T—S, respectively (Parry et al., 1979). Some of the positions, as indicated by the presence of a consensus sequence, contain residues that occur much more frequently than others, but the absolute conservation of a residue in any position is not observed. The decapeptide consists of a pair of five-residue repeats closely related, but different to that displayed by the pentapeptide. Although the repeats have an undetermined structure, the similarity of the repeat to a sequence in snake neurotoxin suggests that the pentapeptides will adopt a closed loop conformation stabilized by a disulphide bond between cysteine residues four apart (Fig. 5 Fraser et al., 1988 Parry et al, 1979). Relative freedom of rotation about the single bond connecting disulphide-bonded knots would give rise to the concept of a linear array of knots that can fold up to form a variety of tertiary structures. The KAPS display imperfect disulphide stabilization of knots and have interacting... 

Fig. 5. Predicted conformation of the pentapeptide repeat C-X-Y-Z-C in trichocyte keratin-associated proteins. Glutamine and arginine residues are found commonly in the X position, prolines in the Yposition, and serines and threonines in the Z position. The structure is based on the known conformation of a similar repeat in snake neurotoxin. The model shows a disulphide bond-stabilized /5-bend with a potential hydrogen bond (dotted). A string of these /5-bends, linked by bonds about which there is relatively free rotation, has been proposed as a model for this important family of matrix proteins in trichocyte keratin (Fraser et al, 1988). Figure from Fraser et al (1988) with permission from Elsevier.

Presynaptic snake neurotoxins endowed with PLA2 activity (SPANs) are major components of the venom of four families of venomous snakes (Crotalidae, Elapidae, Hydrophiidae, and Viperidae). These neurotoxins play a crucial role in envenomation of the prey (Harris 1997) by causing a persistent blockade of neurotransmitter release from nerve terminals with a peripheral paralysis very similar to that of botulism (Connolly et al. 1995 Gutidrrez et al. 2006 Kularatne 2002 Prasampun et al. 2005 Theakston et al. 1990 Trevett et al. 1995 Warrell et al. 1983). 

Regeneration of the Skeletal Neuromuscular Junction and the Innervated Muscle Fibers after Poisoning by Botulinum or Snake Neurotoxins... 

Rigoni M, Caccin P, Gschmeissner S, Koster G, Postle AD et al. (2005) Equivalent effects of snake pla2 neurotoxins and lysophospholipid-fatty acid mixtures. Science 310 1678-80 Rigoni M, Pizzo P, Schiavo G, Weston AE, Zatti G et al. (2007) Calcium influx and mitochondrial alterations at synapses exposed to snake neurotoxins or their phospholipid hydrolysis products. JBiolChem 282 11238-45... 

It is interesting to note that an endogenous molecule related to snake neurotoxins, lynxl, is present in the rodent CNS, where it is surface anchored and expressed by neurons positive for nicotinic a4)32 and a7 receptors. In recombinant systems, the effects of coexpressing this molecule with a4jS2 receptors are complex increases in EC50 and in the relative fre-... 

The derived solution conformation of short neurotoxins (Figures 8 and 9) is in general agreement with the X-ray crystal structure of erabutoxins(46,48), although the amino acid composition of these sea snake toxins differs by ca 30% from that of the terrestrial snake neurotoxins mostly studied by NMR. The conformational reorientation of the fragment 30-34 (3 turn in the central loop. Figure 9)... 

One major protein, MW 40,000, is labeled labeling is blocked by reversible cholinergic ligands or snake neurotoxins... 

Almost aU publications on the metabolites of marine reptiles are devoted to snake venoms. Like their terrestrial homologs, these venoms are mixtures of proteins. The most studied are netirotoxins, particularly erabutoxins A and B, isolated from species of the genus Laticauda, the amino acid sequences of which have been determined (Gttinea, Tamiya, and Cogger, 1983 Tamiya et al., 1983). The toxic sites of snake neurotoxin molecules from marine and land snakes always include a number of invariant amino adds (Menez et al., 1986). 

There are several types of neurotoxins and their structures the site of action and the mechanism are not identical. Snake neurotoxins are peripheral neurotoxins, rather than centrally neurotoxic apparently they do not pass through the blood-brain barrier. 

Snake neurotoxins are the main toxic proteins of cobra, krait, tiger snake and sea snake venoms which block neuromuscular transmission and cause animals death of respiratory paralysis. Snake neurotoxins are classified into two distinct types, postsynaptic and presynaptic neurotoxins, in relation to the neuromuscular junction. Postsynaptic neurotoxins bind specifically to nicotinic acetylcholine receptor (AChR) at the motor endplate and produce a nondepolarizing block of neuromuscular transmission. Presynaptic neurotoxins block the release of acetylcholine from the presynaptic motor nerve terminals. 

Selective and stepwise chemical modifications of cobrotoxin indicate that at least two cationic groups, e-amino group of Lys-47 and guanidino group of Arg-33, both of which are common to all known postsynaptic snake neurotoxins, held at a certain critical distance in the molecule are functionally important for its neuromuscular blocking activity. 

Tamiya, N., Lamouroux, A., Julien, J.F., Grima, B., Mallet, J., Fromageot, P. and Menez, A. (1985) Cloning and sequence analysis of the cDNA encoding a snake neurotoxin precursor. Biochimie 61 185-189. 

Yang, C.C. (1992) Chemistry of snake neurotoxins and future perspective. J. Chin. Chem. Soc. 39 731-740. 

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