The Clostridium botulinum neurotoxin produced by the bacteria Clostridium botulinum (and some strains of Clostridium butyricium and Clostridium baratii) is the most lethal toxin known today. Seven serotypically botulinum neurotoxins (BoNTs) have been found, each of them is categorized into subtypes depending on their amino acid sequence. The protein is initially synthesized as a single polypeptide chain of ≈150 kDa and is then cleaved by a protease to yield the mature toxin, which consists of a light chain (LC which is 50 kDa) and a heavy chain (HC which is 100 kDa). LC and HC are held together by a long peptide belt, non-covalent interactions and a single inter-chain disulphide bond [1]
The Clostridium botulinum neurotoxin serotype A light chain (BoNT/A-LC) shown below is composed of 425 residues. It was obtained with high resolution X-ray crystal structure using an inhibitory peptide and the catalytic Zn(II) ion. [2]
Structure
Global aspect
The Ramachandran diagram (see Figure 1) shows the distribution of the residues in the protein. Amino acids are globally in favored regions.
Figure 1 : Ramachandran diagram of C.botulinum neurotoxin serotype A light chain (made using Python by Rémi Pelletier)
The of BoNT/A-LC[3]
- The light chain of Clostridium botulinum neurotoxin serotype A has 11 α-helices :
- Helix 1 :
- Helix 2 :
- Helix 4 :
- Helix 5 :
- Helix 6 :
- Helix 7 :
- Helix 8 :
- Helix 9 :
- Helix 10 :
- Helix 11 :
- Helix 12 :
- It also has three 3-10 helices :
- Helix 3 :
- Helix 13 :
- Helix 14 :
- There are several most of them are anti parallel except
- An interesting structure is also a typical : indeed the chain makes a sharp reversal by 180° within 4 residues, moreover Cα from the i residue and the Cα from the i+3 residue are separated by less than 7 angstroms.
Structural highlights
Mechanism
The neurotoxin inactivates neurotransmitter release owing to its metalloprotease activity. It targets the presynaptic membrane of peripheral nerve terminals using a binding mode based on the use of two independent receptors: a polysialoganglioside (PSG) and a protein receptor in the lumen of synaptic vesicles[4]. Clostridium botulinum neurotoxin is initially synthesized as a single polypeptide chain of ≈150 kDa and is then cleaved by a protease to yield the mature toxin, which consists of a light chain (LC which is 50 kDa) and a heavy chain (HC which is 100 kDa). The heavy chain presents the
Figure 2 : Enzymatic mechanism (made using ChemDraw Ultra by Xavier Hartmann
Therapeutic applications
In the late 1960s, a Edward Schantz and a San Francisco opthalmologist, Alan Scott, start to work on the using of the botulinum toxin in therapeutic process.
First, they try to treat strabismus and in the late 1970s the neurotoxin is used in many therapeutic applications.[5]
Hyperhidrosis and CHARGE syndrome
We can now treat excessive sweating with botulinium toxin [6] thanks to its action on the receptors of the parasympathetic network. Excessive dribbling can be treated by the same way.
Cosmetic
Non-lethal amount of botulinum toxin can be injected localy to paralyse targeted muscles and reduce wrinkles temporaly[7] (5 to 6 months).
Blepharospesm, nystagmus, torticollis and strabismus
This pathologic can be treated by the same methode. Every anomalous behaviour of muscle tissue can be fixed thanks to the paralyzing effect of the toxin. It is based on the muscles relaxation.
Cervical dystonia
BTX-A is used against cervical dystonia. But it can become inefficient after a periode of use.
References
- ↑ Rossetto O, Pirazzini M, Montecucco C. Botulinum neurotoxins: genetic, structural and mechanistic insights. Nat Rev Microbiol. 2014 Aug;12(8):535-49. doi: 10.1038/nrmicro3295. Epub 2014 Jun, 30. PMID:24975322 doi:http://dx.doi.org/10.1038/nrmicro3295
- ↑ Silvaggi NR, Wilson D, Tzipori S, Allen KN. Catalytic features of the botulinum neurotoxin a light chain revealed by high resolution structure of an inhibitory peptide complex. Biochemistry. 2008 May 27;47(21):5736-45. Epub 2008 May 6. PMID:18457419 doi:10.1021/bi8001067
- ↑ http://www.rcsb.org/pdb/explore/remediatedSequence.do?structureId=3BON&bionumber=1
- ↑ Rummel A. Double receptor anchorage of botulinum neurotoxins accounts for their exquisite neurospecificity. Curr Top Microbiol Immunol. 2013;364:61-90. doi: 10.1007/978-3-642-33570-9_4. PMID:23239349 doi:http://dx.doi.org/10.1007/978-3-642-33570-9_4
- ↑ http://rms.medhyg.ch/numero-200-page-870.htm
- ↑ http://rms.medhyg.ch/numero-200-page-870.htm
- ↑ http://rms.medhyg.ch/numero-200-page-870.htm
