κ-Bungarotoxin
The three-dimensional structure of κ-bungarotoxin, highlighting disulfide bonds, from PDB: 1KBA​.[1]
Identifiers
Organism	Bungarus multicinctus
Symbol	N/A
PDB	1KBA
UniProt	P01398
Search for Structures Swiss-model Domains InterPro

κ-Bungarotoxin (often written κ-Bgt; historically also called toxin F^[2]) is a protein neurotoxin of the bungarotoxin family that is found in the venom of the many-banded krait, a snake found in Taiwan. κ-Bungarotoxin is a high affinity antagonist of nicotinic acetylcholine receptors (nAChRs), particularly of CHRNA3; it causes a post-synaptic blockade of neurotransmission. Although there is significant variability in the clinical effects of snake bites, neuromuscular paralysis and respiratory failure are associated with krait bites.^[3]

Discovery

κ-Bungarotoxin was first reported in 1983 as a component of the venom of Bungarus multicinctus that differed in biological effect from the previously known α-bungarotoxin: κ-bungarotoxin, but not α-bungarotoxin, was capable of impeding nicotinic signaling in the chick ciliary ganglion.^[4] Bungarotoxin toxin was designated "kappa" as an allusion to the Latin word kiliaris ("from the eye"), and to the root of "ciliary".^[4] Separately identified toxins designated "toxin F" and "bungarotoxin 3.1" were identified by protein sequencing as identical to κ-bungarotoxin.^[2]

Mechanism and biological effects

κ-Bungarotoxin binds to the nicotinic acetylcholine receptors of the autonomic ganglia, predominantly to the nicotinic receptor subunit alpha 3 (CHRNA3) and to a lesser extent alpha 4. Two distinct binding surfaces, both on the N-terminal extracellular face of the receptor subunit, have been identified.^[5]

κ-Bungarotoxin is a receptor antagonist, meaning it blocks the normal response of the receptor to acetylcholine, which inhibits neurotransmission and therefore causes neuromuscular paralysis. Like the α-bungarotoxins, κ-bungarotoxin causes a post-synaptic blockade of signaling; this is in contrast to the β-bungarotoxins which induce a pre-synaptic block.^[3] The distinction between the effects of α-bungarotoxin and κ-bungarotoxin was first identified functionally, as differences in effects on specific neural structures.^[4][6] The basis of this functional difference has been molecularly characterized as differences in receptor subtype specificity; the pentameric receptors are assembled from different distributions of subunits in neurons and in muscles.^[5]

Structure

The κ-bungarotoxin polypeptide is 66 amino acids long and folds into an antiparallel beta sheet structure stabilized by five conserved disulfide bonds, a structural feature shared by many peptide toxins. Unlike other members of the bungarotoxin family, κ-bungarotoxin is a dimer.^[1]