User:MNEU1.2Elisabeth/Hainantoxin

Hainantoxin (HNTX) is a neurotoxic protein, part of the venom from the Chinese bird spider Ornithoctonus hainana, that specifically inhibits the tetrodotoxin-sensitive Voltage-gated sodium channels, thereby causing blockage of neuromuscular transmission which results in paralyzing its pray. There are thirteen subgroups discovered (HNTX-I – HNTX-XIII), but only HNTX-I, -II, -III, -IV and -V are well described.

Etymology
The hainantoxin is a toxin from the Chinese bird spider O. hainana. This spider lives in a province in southern China called Hainan.

Source
HNTX-I, HNTX-III, HNTX-IV and HNTX-V are isolated from the Chinese bird spider Ornithoctonus hainana. HNTX-II is obtained from the spider Haplopelma hainanum.

HNTX-I
There are 33 amino acid residues in HNTX-I, with a total molecular weight of 3605-3608 Da. HNTX-I further contains six cysteines, three disulfide bonds (Cys2 and Cys17, Cys9 and Cys22, and Cys16 and Cys29) and amidated c-terminal. HNTX-I is also part of the inhibitor cystine knot structural family and contains a short triple-stranded anti-parallel beta-sheet and four beta-turns. The amino acid residues His28 and Asp26 are responsible for the bioactive part of HNTX-I.

HNTX-II
HNTX-II has a molecular weight of 4253 Da and contains 37 amino acid residues with six cysteines that form three disulfide bonds. The complete sequence of amino acids in HNTX-II is NH2-LFECSV SCEIEK EGNKD CKKKK CKGGW KCKFN MCVKV-COOH.

HNTX-III
The structure of HNTX-III consists of 33-35 amino acid residues, which form a beta-sheet with connections between Asp7 and Cys9, Tyr21 and Ser23, and Lys27 and Val30. The six cysteine amino acids form three disulfide bonds. Furthermore, an amidated c-terminal is present in HNTX-III.

HNTX-IV
HNTX-IV has 35 amino acid residues with a total molecular weight of 3989 Da and the first strand consists of an antiparallel beta-sheet. Furthermore HNTX-IV contains three disulfide bonds arranged in a cystine knot motif. The complete sequence of amino acids in HNTX-IV is NH2-ECLGFG KGCNPS NDQCCK SSNLVC SRKHRW CKYEI-CONH2. Lys 27, His28, Arg29 and Lys 32 are the neuroactive amino acid residues.

HNTX-V
HNTX-V consists of 35 amino acid residues, six cysteines and three disulfide bonds. The whole amino acid residue sequence of HNTX-V is NH2-ECLGFG KGCNPS NDQCCK SANLVC SRKHRW CKYEI-COOH. At the active binding site of HNTX-V, Lys27 and Arg 29 are the most important.

Family
HNTX-III and HNTX-IV are part of the Huwentoxin-I family. Toxins from the Huwentoxin-I family bind to site 1 on the sodium channels. Other subgroups of the Hainantoxin showed to bind at site 3 of the sodium channels.

Homology
The main component of the venom of O. hainana is HNTX-I. HNTX-I – HNTX-V all contain three disulfide bonds. HNTX-I, HNTX-II and HNTX-V have six cysteines. The cystine knot motif is present in HNTX-I and HNTX-IV. Amidated c-terminals are present in HNTX-I and HNTX-III.

Table 1: Homology between five subtypes of the Hainantoxin

Channel
The Hainantoxin inhibits selectively the tetrodotoxin-sensitive (TTX-S) voltage-gated sodium channels (VGSCs). Voltage-gated Ca2+ channels (VGCCs), tetrodotoxin-resistant (TTX-R) VGSCs and rectifier-delayed potassium channels are not affected by the Hainantoxin. HNTX-I specifically blocks Nav1.2 and para/tipE channels expressed in Xenopus laevis oocytes, the African clawed frog. HNTX-I is a weak antagonist of the vertebrate TTX-S VGSCs, but it is more potent on insect VGSCs.

Affinity
For the blockage of sodium channels, electrostatic interactions or hydrogen bonds are needed. Important for the electrostatic interaction is the presence of a positively charged region in the toxin, because the receptor site of the sodium channel contains a lot of negatively charged residues. In HNTX-I, the positively charged residues and a vicinal hydrophobic patch have most influence on the binding to the sodium channels. HNTX-IV has a positively charged patch containing the amino acids Arg26, Lys27, His28, Arg29 and Lys32, of which Lys27, Arg29 and Lys32 are the most important for interaction with the TTX-S VGSCs. Also HNTX-V shows an interface of positively charged amino acids that are responsible for the binding with the TTX-S VGSCs, where also Lys27 and Arg29 are the most important. Subtle differences in the positively charged patch can result in altered electro-static properties, causing altered pharmacological effects.

Table 2: IC50 values of five subgroups of the Hainantoxin

Mode of action
The Hainantoxin has a mode of action similar to α-scorpion toxins. These toxins bind to site 3 on sodium channels and slow or block the inactivation of the sodium channels to prolong the time course of an action potential. HNTX-II, HNTX-III and HNTX-V bind to site 3 and show to reduce the peak amplitude of sodium currents, but did not change the activation and inactivation kinetics of the VGSCs or the voltage threshold of inward currents. HNTX-III and HNTX-IV cause a hyperpolarizing shift during steady-state and differ the rate of recovery from inactivation. HNTX-I and HNTX-IV bind to site 1, similar to TTX, and thereby blocking the channel conductance. They do not alter the activation and inactivation kinetics. Ion selectivity of the VGSCs is not changed by the hainantoxin.

Symptoms
The Hainantoxin causes symptoms in mice, rats and cockroaches. HNTX-I has no significant effect on insects or rats. HNTX-III and HNTX-IV cause spontaneous contractions of the diaphragm muscle and the vas deferens smooth muscle of the rat. HNTX-III and HNTX-IV are able to paralyze cockroaches, and HNTX-IV can even paralyze rats.

LD50
Intracerebroventricular injection in mice with HNTX-II has a LD50 of 1.41 μg/g. The intraperitoneal LD50 value of HNTX-IV in mice is 0.2mg/kg. HNTX-III is 40 times more potent that HNTX-IV.

Therapeutic use
HNTX-III and HNTX-IV have an antagonistic effect on the toxin BMK-I, a toxic protein in the venom of the scorpion Buthus martensii.