Alpha-Bungarotoxin
Alpha-bungarotoxin is a 74-amino acid three-finger neurotoxin from the banded krait snake that irreversibly blocks nicotinic acetylcholine receptors at the neuromuscular junction.
Chemical Identity
| Property | Value |
|---|---|
| Chemical Formula | C339H526N94O99S8 |
| Molecular Weight | 7984.45 g/mol |
| CAS Number | 11032-79-4 |
| IUPAC Name | Alpha-bungarotoxin (Bungarus multicinctus) |
| Peptide Class | Three-Finger Neurotoxin |
| Origin | Bungarus multicinctus (banded krait) |
| Disulfide Bonds | 4 |
Structure
Alpha-bungarotoxin is a 74-amino acid protein of the three-finger toxin (3FTx) family from the venom of the banded krait (Bungarus multicinctus). It contains four intramolecular disulfide bonds that stabilize three beta-strand loops extending from a central core, resembling three fingers of a hand. The three loops (I, II, III) are functionally distinct, with loops II and III primarily responsible for receptor binding.
Mechanism of Action
Alpha-bungarotoxin irreversibly blocks nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction:
- Binding: The toxin binds with extremely high affinity (Kd ~ 10^-10 M) to the alpha-subunit of muscle-type nAChRs
- Competition: Occupies the acetylcholine binding site, preventing ACh from activating the receptor
- Irreversible block: Unlike ACh, the toxin does not dissociate from the receptor, producing a permanent block
- Neuromuscular paralysis: Failure of neuromuscular transmission leads to flaccid paralysis and respiratory failure
The toxin also binds with high affinity to alpha7 nAChRs in the brain, making it a valuable research tool.
Biological Functions
Alpha-bungarotoxin is a major lethal component of krait venom:
- Predation: Rapid immobilization of prey through respiratory paralysis
- Defense: Deterrence of predators through envenomation
- Research tool: Gold standard for labeling and quantifying nAChRs since the 1970s
Research and Clinical Applications
Alpha-bungarotoxin is primarily used as a research tool:
- nAChR localization: Fluorescent and radiolabeled alpha-bungarotoxin maps receptor distribution in tissues
- Receptor quantification: Binding assays measure nAChR density and affinity
- Neuromuscular junction studies: Visualization of motor endplates in muscle
- Neuroscience: Characterization of alpha7 nAChRs in brain function and disease
No direct therapeutic use due to irreversible receptor blockade and paralytic toxicity.
Safety and Side Effects
Alpha-bungarotoxin is highly lethal. Banded krait envenomation causes progressive neuromuscular paralysis: ptosis, ophthalmoplegia, dysphagia, limb weakness, and respiratory failure. Death results from respiratory paralysis within 1-6 hours if untreated. Antivenom is the primary treatment. The toxin is not absorbed through intact skin.
References
- Tsetlin, V., & Hucho, F. (2009). Snake and snail toxins acting on nicotinic acetylcholine receptors. Nature Reviews Neuroscience, 10, 469-479.
- Changeux, J.P. (2012). The nicotinic acetylcholine receptor: the founding father of the pentameric ligand-gated ion channel superfamily. Journal of Biological Chemistry, 287, 40207-40215.
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