Non-conventional toxins from Elapid venoms.
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Non-conventional toxins constitute a poorly characterized class of three-finger toxins isolated exclusively from Elapidae venoms. These toxins are monomers of 62-68 amino acid residues and contain five disulfide bridges. However, unlike a/?-neurotoxins and ?-neurotoxins which have the fifth disulfide bridge in their middle loop (loop II), the fifth disulfide bridge in non-conventional toxins is located in loop I (N-terminus loop). Overall, non-conventional toxins share ~28-42% identity with other three-finger toxins including a-neurotoxins, a/?-neurotoxins and ?-neurotoxins. Recent structural studies have revealed that non-conventional toxins also display the typical three-finger motif. Non-conventional toxins are typically characterized by a lower order of toxicity (LD50~5-80 mg/kg) in contrast to prototype a-neurotoxins (LD50~0.04-0.3 mg/kg) and hence they are also referred to as 'weak toxins'. Further, it is generally assumed that non-conventional toxins target muscle (a2߿d) receptors with low affinities several orders of magnitude lower than a-neurotoxins and a/?-neurotoxins. However, it is now known that some non-conventional toxins also antagonize neuronal a7 nicotinic acetylcholine receptors. Hence, non-conventional toxins are not a functionally homogeneous group and other, yet unknown, molecular targets for this class of snake venom toxins may exist. Non-conventional toxins may therefore be a useful source of ligands with novel biological activity targeting the plethora of neuronal nicotinic receptors as well as other physiological processes.
Biochemistry and Cell Biology not elsewhere classified
Physiology not elsewhere classified