Tamulotoxin

The toxin has been classified as a short-chain scorpion toxin.{{cite journal|last1=Strong|first1=PN|last2=Clark|first2=GS|last3=Armugam|first3=A|last4=De-Allie|first4=FA|last5=Joseph|first5=JS|last6=Yemul|first6=V|last7=Deshpande|first7=JM|last8=Kamat|first8=R|last9=Gadre|first9=SV|last10=Gopalakrisnakone|first10=P|last11=Kini|first11=RM|last12=Owen|first12=DG|last13=Jeyaseelan|first13=K|title=Tamulustoxin: A Novel Potassium Channel Blocker from the Venom of the Indian Red Scorpion Mesobuthus tamulus|journal=Archives of Biochemistry and Biophysics|date=1 January 2001|volume=385|issue=1|pages=138–144|doi=10.1006/abbi.2000.2135|pmid=11361010}} It consists of 36 amino acids and is referred to as TmTx1.{{cite journal|last1=Rodríguez de la Vega|first1=RC|last2=Possani|first2=LD|title=Current views on scorpion toxins specific for K+-channels|journal=Toxicon|date=15 June 2004|volume=43|issue=8|pages=865–875|doi=10.1016/j.toxicon.2004.03.022|pmid=15208019}} A peptide consisting of 35 amino acids has also been identified, referred to as TmTx2. It possesses three intra-molecular disulphide bonds (S-S), leading to a highly stabilized conformation. It also has six cysteine residues which is a characteristic shared by many short-chain scorpion toxins.

TmTx belongs to the short scorpion toxin superfamily and the potassium channel inhibitor family.{{cite web|title=Animal Toxin Database|url=http://protchem.hunnu.edu.cn/toxin/Browse/SingleProtein.jsp?Type=SingleProtein&Filter=ToxinID&QS=AT0000618&Page=0|access-date=13 October 2014}} Adhering to the nomenclature of Tytgat et al.,{{cite journal|last1=Tytgat|first1=J|last2=Chandy|first2=KG|last3=Garcia|first3=ML|last4=Gutman|first4=GA|last5=Martin-Eauclaire|first5=MF|last6=Van der Walt|first6=JJ|last7=Possani|first7=LD|title=A unified nomenclature for short-chain peptides isolated from scorpion venoms: α-KTx molecular subfamilies|journal=Trends in Pharmacological Sciences|date=1 November 1999|volume=20|issue=11|pages=444–447|doi=10.1016/S0165-6147(99)01398-X|pmid=10542442}} potassium toxins can be divided into four subgroups: alpha, beta, gamma and kappa. It belongs to the group of alpha potassium toxins (α-KTx: alpha toxin affecting potassium channels). This group contains short-chain peptides of 23-42 acids with three or four disulphide bridges. The primary targets consist of voltage-gated Shaker-related potassium channels, ether-a-go-go related gene (HERG) potassium channels in the heart and calcium activated potassium channels. Within this family, TmTx belongs to the α-KTx 16 subfamily.

TmTx shows no homology with other species of scorpion toxins in BLAST of the TmTx sequence, apart from the position of its six cysteine residues. It is nevertheless categorized with other potassium channel scorpion toxins, because it shares the position of its six cysteine residues with other toxins. In phylogeny, TmTx does have similarities with other scorpion neurotoxins.

A comparative model has been suggested for the 3D protein structure of TmTx by using information from homologous proteins with known structures.{{cite journal|last1=Kumar|first1=RB|last2=Suresh|first2=MX|title=Homology modeling, molecular dynamics simulation and protein-protein interaction studies on calcium activated potassium channel blocker, Tamulotoxin from Buthus tamulus|journal=Advanced BioTech|date=10 January 2013|volume=12|issue=7|pages=11–14|url=http://www.advancedbiotech.in/archives_jan%2013%20_Homology%20Modeling.html|access-date=13 October 2014|url-status=dead|archive-url=https://archive.today/20141013133601/http://www.advancedbiotech.in/archives_jan%2013%20_Homology%20Modeling.html|archive-date=13 October 2014}} Based on this model, it is highly likely that TmTx blocks calcium activated potassium channels by binding to the S5-S6 segment and thus blocking its pore. The active site of TmTx in this model consists of 5 amino acids, which is essential for the activity of TmTx. These amino acids would be responsible for inhibiting transport of ions. On the other hand, TmTx does not seem to inhibit [¹²⁵I] apamin binding to synaptic membranes in the rat brain or ionomycin-induced 86Rb⁺ fluxes in C6 cells in vitro. This suggests that TmTx does not have an effect on SK channels or charybdotoxin-sensitive IK channels (calcium-activated potassium channel), respectively.

Another suggested target is the Kv1.6 channel, a voltage-gated potassium channel. There are two suggestions for the mode of action. Either it works via blocking the open channel, or there could be a modulation of slow inactivation of this channel. Upon wash, a complete reversal of the block occurred, suggesting that the binding of the toxin to the channel is not very strong.

Injection of the venom of H. tamulus in rats induces hyperventilatory and hypertensive responses {{cite journal|last1=Singh|first1=SK|last2=Deshpande|first2=SB|title=Injection of Mesobuthus tamulus venom in distal segment of femoral artery evokes hyperventilatory and hypertensive responses in anaesthetised rats|journal=Neuroscience Letters|date=13 June 2008|volume=438|issue=1|pages=64–66|doi=10.1016/j.neulet.2008.04.037|pmid=18472330|s2cid=25722401}}{{cite journal|last1=Pandey|first1=R|last2=Deshpande|first2=SB|title=Protective effects of aprotinin on respiratory and cardiac abnormalities induced by Mesobuthus tamulus venom in adult rats|journal=Toxicon|date=August 2004|volume=44|issue=2|pages=201–205|doi=10.1016/j.toxicon.2004.05.025|pmid=15246770|bibcode=2004Txcn...44..201P}} and in humans.{{cite journal|last1=Bawaskar|first1=HS|last2=Bawaskar|first2=PH|title=Scorpion sting: update|journal=The Journal of Association of Physicians in India|date=January 2012|volume=60|pages=46–55|pmid=22715546|url=http://www.japi.org/january_2012/08_scorpion_sting_update.html|access-date=13 October 2014}} The toxicity of the venom varies with age and species.{{cite journal|last1=Tiwari|first1=AK|last2=Deshpande|first2=SB|title=Toxicity of scorpion (Buthus tamulus) venom in mammals is influenced by the age and species|journal=Toxicon|date=December 1993|volume=31|issue=12|pages=1619–1622|doi=10.1016/0041-0101(93)90346-K|pmid=8146875|bibcode=1993Txcn...31.1619T }}

Based on the structure, biological compounds can be identified which could have a maximum binding affinity to the active site of TmTx toxin protein and thereby preventing the toxin to bind to the ionic pore of the channel.{{cite journal|last1=Kumar|first1=RB|last2=Suresh|first2=MX|title=Computational analysis of bioactive phytochemicals as potential inhibitors for calcium activated potassium channel blocker, tamulotoxin from Mesobuthus tamulus|journal=Pharmacognosy Journal|date=April 2013|volume=5|issue=2|pages=41–45|doi=10.1016/j.phcgj.2013.02.001}} Therefore, these compounds could in future be used as an antidote for TmTx. Three bioactive compounds have been identified from the plants Andrographis paniculata and Ocimum basilicum. Based on computer models, separate ligands have also been identified, which could block TmTx.{{cite journal|last1=Kumar|first1=RB|last2=Suresh|first2=MX|title=Pharmacophore mapping based inhibitor selection and molecular interaction studies for identification of potential drugs on calcium activated potassium channel blockers, tamulotoxin|journal=Pharmacognosy Magazine|date=April 2013|volume=9|issue=34|pages=89–95|doi=10.4103/0973-1296.111239|pmid=23772102|pmc=3680861 |doi-access=free }}

{{Reflist}}

{{Ion channel modulators}}

Category:HERG blocker

Category:Ion channel toxins