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Tamulotoxin (or Tamulus toxin, Tamulustoxin, in short form: TmTx) is a venomous neurotoxin from the Indian Red Scorpion (Hottentotta tamulus, Mesobuthus tamulus or Buthus tamulus).

Structure
The toxin has been classified as a short-chain scorpion toxin. It consists of 36 amino acids and is referred to as TmTx1. 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.

Family
TmTx belongs to the short scorpion toxin superfamily and the potassium channel inhibitor family. Adhering to the nomenclature of Tytgat et al., potassium toxins can be divided into four subgroups: alpha, beta, gamma and kappa. Keeping the structure of TmTx in mind, 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. In this family, TmTx belongs to the α-KTx 16 subfamily.

Homology
TmTx appears to show no homology with other species of scorpion toxins in BLAST of the TmTx sequence, apart from the position of its six cysteine residues. Still, it is 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.

Target and mode of action
A comparative model has been suggested for the 3D protein structure of TmTx by using information from homologous proteins with known structures. 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 is a case of a mechanism that 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.

Toxicity
In theory blocking calcium activated potassium channels should lead to increased neurotransmitter release. This mechanism could lead to an over activation of cardiovascular cells, ultimately leading to death. Blocking voltage dependent potassium channels would have similar effects, such as over activation by prolonged depolarization or repeated firing of action potentials. However, both mechanisms have not been directly observed in vivo experiments with TmTx.

Clinical effects
TmTx has never been tested independently, although clinical symptoms of the venom of H. tamulus have been described in detail in rats and in humans. It has been investigated how biochemical and enzymatic parameters vary in rats, when different levels of the LD50 are administered. It has also been proven that the toxicity of the venom varies with age and species.

Treatment
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. 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.