Cobratoxin
 Ribbon diagram
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 Space filling diagram
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| Molar mass | 7842.12 g/mol |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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α-Cobratoxin is a substance of the venom of certain Naja cobras. It is a nicotinic acetylcholine receptor (nAChR) antagonist which causes paralysis by preventing the binding of acetylcholine to the nAChR.
α-Cobratoxin is a neurotoxin from the venom of certain Naja genus, including the Thailand cobra, the Indo-Chinese spitting cobra (Naja siamensis) and the Chinese cobra (Naja atra). The cobras that produce the toxin live in tropical and subtropical regions of The Americas, Africa, Asia, and Australia. The venom, produced by these snakes, is a mixture of proteins, carbohydrates, and other substances. The venom is only used when the snake needs it for survival, because it costs a lot of effort to produce. If poisoning a subject is not necessary, it can bite without excreting the venom. When the snake does use it, it mostly tries to immobilize or kill its prey.
α-Cobratoxin forms three hairpin type loops with its polypeptide chain. The two minor loops are loop I (amino acids 1-17) and loop III (amino acids 43-57). Loop II (amino acids 18-42) is the major one. Following these loops, α-cobratoxin has a tail (amino acids 58-71). The loops are knotted together by four disulfide bonds (Cys3-Cys20, Cys14-Cys41, Cys45-Cys56, and Cys57-Cys62). Loop II contains another disulfide bridge at the lower tip (Cys26-Cys30).
Stabilization of the major loop occurs through β-sheet formation. The β-sheet structure extends to amino acids 53-57 of loop III. Here it forms a triple-stranded, antiparallel β-sheet. This ß-sheet has an overall right-handed twist6. This β-sheet consists of eight hydrogen bonds. The folded tip is held stable by two α-helical and two β-turn hydrogen bonds.
The first loop is stabilized because of one β-turn and two β-sheet hydrogen bonds. Loop III stays intact because of a β-turn and hydrophobic interactions.
The tail of the α-cobratoxin structure is attached to the rest of the structure by disulfide bridge Cys57-Cys62. It is also stabilized by the tightly hydrogen bound side chain of Asn63.
In conclusion, the whole is held together by disulfide bonds and the loops are kept stable by ß-turns and ß-sheets.
α-Cobratoxin can occur in both a monomeric form and a disulfide-bound dimeric form. α-Cobratoxin dimers can be homodimeric as well as heterodimeric with cytotoxin 1, cytotoxin 2 and cytotoxin 3. As a homodimer it is still able to bind to muscle type and α7 nAChRs, but with a lower affinity than in its monomeric form. In addition the homodimer acquires the capacity to block α-3/β-2 nAChRs.
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