Botulinum Toxins
Botulinum neurotoxin is a family of neurotoxins produced by three types of anaerobic spore-forming bacterium, Clostridium botulinum, Clostridium barati and Clostridium butyricum. In 1920, the first botulinum toxin type A was isolated and purified. Type A is the most powerful of the family of botulinum neurotoxins. There are six other varieties of toxin, designated B through G. Botulism is a typical neuroparalytic disease caused by botulinum toxin (BoNT). Human botulism is caused by toxin types A, B, E and rarely, F; botulism associated with toxin type A is most severe. BoNTs are synthesized as single chain polypeptides (approximately 150 kD) that are proteolytically activated to yield a light chain (LC, approximately 50 kD) and a heavy chain (HC, approximately 100 kD). These chains remain linked by a disulfide bridge. BoNT binds to cholinergic nerve terminals by the c-terminal half of heavy chain (HC, approximately 50 kD). Binding is thought to require the presence of gangliosides (in particular GD1b, GT1b, and/or GQ1b), as well as a protein receptor. The n-terminal half of heavy chain (HN, approximately 50 kD) promotes penetration and translocation of the LC across the endosomal membrane. Active LC enters the cytosol, where it is known to act as a metalloendoprotease. The light chains of each of the seven BoNT serotypes possesses the sequence His-Glu-Xaa-Xaa-His, which is a signature sequence for zinc-dependent endoproteases. Unlike many other zinc-dependent endoproteases, BoNTs display a rather remarkable specificity of action. BoNT/B, BoNT/D, BoNT/F and BoNT/G cleave VAMP (vesicle-associated membrane protein); BoNT/A and BoNT/E cleave SNAP-25 (synaptosomal-associated protein of 25kDa); and BoNT-C cleaves both syntoxin and SNAP-25. BoNT cleaves any one of these substrates, the complex is non-functional and transmitter release is blocked. Toxin-induced blockade of exocytosis accounts for flaccid paralysis.

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