LmKTT-1a
LmKTT-1a is a bifunctional Kunitz-type toxin belonging to the ẟ-KTx subfamily, which can be found in the venom of Lychas ''mucronatus''. As a bifunctional toxin, it both inhibits trypsin activity and blocks Kv1 channels with a weak selectivity towards Kv1.3 channels.
Chemistry
Structure
The mature protein of LmKTT-1a is composed of 59 amino acids with a molecular mass of 8658.6 Da.These residues assemble into a unique Kunitz-type structural fold, which is typical for the δ-KTx subfamily. This characteristic fold consists of a short α-helix connected to two antiparallel β-sheets with six cysteines in the sequence to form three disulfide bridges. This ẟ-KTx subfamily's typical Kunitz-type fold represents a third possible structure of scorpion toxins specific to potassium channels.
MKSFLLIALVLFFLFVSYASAKKKCQLPSDVGKGKASFTRYYYNEESGKCETFIYGGVGGNSNNFLTKEDCCRECAQGSC Signal Sequence Mature Protein |
Cysteine framework
LmKTT-1a adopts a distinctive cysteine framework. While Kunitz-type toxins normally have a CysII-CysIV disulfide bridge, LmKTT-1a lacks this bridge and instead possesses two cysteine residues near the C-terminus, which form a new disulfide bridge. This disulfide bridge has little to no effect on the toxin's fold and ability to block potassium channels. Yet, after eliminating this Cys51-C59 disulfide bridge, trypsin was inhibited with five-fold lower Ki than wildtype LMKTT-1a.Targets
LmKTT-1a targets voltage-gated potassium channels and blocks them. The toxin has a weak selectivity for Kv1.3 channels with an IC50-value of 1.58±0.73 μM, with less effect on Kv1.1 and Kv1.2 channels.Moreover, LmKTT-1a also selectively inhibits trypsin with a Ki value of 0.14-0.16 μM at a 1:1 stoichiometric ratio, while not affecting chymotrypsin and elastase activity.