Ophanin
Encyclopedia
Ophanin is a toxin
found in the venom of the King Cobra (Ophiophagus hannah), which lives throughout South East Asia. This toxin
belongs to the cysteine-rich secretory protein
(CRISP) family. Ophanin weakly blocks the contraction of smooth muscles elicited by high potassium-induced depolarization
, suggesting that it inhibits voltage-dependent calcium channels.
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The toxin was named ophanin after the snake whose venom it is derived from, the King Cobra (Ophiophagus hannah).
Although the venom has relatively low toxicity
, this is compensated by the high amounts of it injected into the prey for each bite.
. Its molecular weight is 25 kDa (from positions 19 – 239), which conforms to the molecular mass predicted from its cDNA sequences.
These proteins possess 16 strictly conserved cysteines and contain 8 disulfide bonds.
Ten of the 16 cysteine residues are clustered at the C-terminal end of the protein.
Ophanin belongs to the “long” CRISPs subgroup, which consists of the 9 CRISPs with the longest sequences. Snake venom CRISPs belonging to different subgroups act on different biological targets, contributing in this way to the diversity of damaging effects of snake venoms.
branch than the Elapidae
branch even though O. Hannah belongs to the Elapidae snakes.
Ophanin, along with other specific snake toxins like triflin
and ablomin
, is also a helothermine-related venom protein (Helveprin) which was originally isolated from the skin of the Mexican beaded lizard.
The differences between the inhibitory activity of CRISPs may be explained through sequence comparisons that suggest a site that may be critical for inhibition of channel activity. Phe
189 and Glu
186 are the most likely functional residues: strong blockers of smooth muscle contraction (ablomin
, triflin
, and latisemin
) all have Phe189, and all blockers of smooth muscle contraction except ophanin have Glu186. The significance of this lack of the probable functional residues in ophanin has not yet been addressed. However, it is likely that the picture is more complex and other residues contribute to the inhibitory activity of CRISPs on smooth muscle contraction and some data supports this. For example pseudecin, while also having Phe189, does not affect depolarization-induced contraction.
Since ablomin only blocks contraction induced by depolarization, but not by caffeine, the effect of ablomin is likely to be caused by inhibition of voltage-gated ion channels. An activation of smooth muscle cells through caffeine activates ryanodine
receptors of the sarcoplasmic reticulum, whereas an activation through high levels of extracellular potassium depolarizes the membrane (due to the change of the reversal potential
for potassium towards more positive values) and would then activate voltage-gated calcium-ion channels leading to high levels of intracellular calcium ions. The intracellular calcium ion concentration correlates well with contraction force in the rat-tail artery. Thus, contraction following extracellular application of high-potassium solution depends on the influx of the extracellular calcium ions through voltage-gated calcium channels. Therefore ablomin (and by extension ophanin) most likely targets voltage-gated calcium channels on smooth muscle.
Toxin
A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...
found in the venom of the King Cobra (Ophiophagus hannah), which lives throughout South East Asia. This toxin
Toxin
A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...
belongs to the cysteine-rich secretory protein
Cysteine-rich secretory protein
Cysteine-rich secretory proteins, often abbreviated as CRISPs, are a group of glycoproteins found exclusively in vertebrates. They are a subgroup of the CRISP, antigen 5 and Pr-1 protein superfamily and are substantially implicated in the functioning of the mammalian reproductive system...
(CRISP) family. Ophanin weakly blocks the contraction of smooth muscles elicited by high potassium-induced depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
, suggesting that it inhibits voltage-dependent calcium channels.
Etymology
Source
Ophanin is produced in the venom glands of the King Cobra (O. Hannah).Although the venom has relatively low toxicity
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...
, this is compensated by the high amounts of it injected into the prey for each bite.
Structure
Ophanin was successfully isolated from O. Hannah venom by gel filtration and cation-exchange chromatographyChromatography
Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures....
. Its molecular weight is 25 kDa (from positions 19 – 239), which conforms to the molecular mass predicted from its cDNA sequences.
Homology
Ophanin is a cysteine-rich secretory protein and therefore belongs to the CRISP family.These proteins possess 16 strictly conserved cysteines and contain 8 disulfide bonds.
Ten of the 16 cysteine residues are clustered at the C-terminal end of the protein.
Ophanin belongs to the “long” CRISPs subgroup, which consists of the 9 CRISPs with the longest sequences. Snake venom CRISPs belonging to different subgroups act on different biological targets, contributing in this way to the diversity of damaging effects of snake venoms.
Family
The phylogenetic tree constructed from the nucleotide sequences of all known snake venom CRISPs shows that ophanin is more closely related to the ViperidaeViperidae
The Viperidae are a family of venomous snakes found all over the world, except in Antarctica, Australia, Ireland, Madagascar, Hawaii, various other isolated islands, and above the Arctic Circle. All have relatively long, hinged fangs that permit deep penetration and injection of venom. Four...
branch than the Elapidae
Elapidae
Elapidae is a family of venomous snakes found in tropical and subtropical regions around the world, terrestrially in Asia, Australia, Africa, North America and South America and aquatically in the Pacific and Indian Oceans...
branch even though O. Hannah belongs to the Elapidae snakes.
Ophanin, along with other specific snake toxins like triflin
Triflin
Triflin is a cysteine-rich secretory protein , which is excreted by the venom gland of the Habu snake . Triflin reduces high potassium-induced smooth muscle contraction, suggesting a blocking effect on L-type calcium channels.- Source :Triflin is a toxin derived from snake venom...
and ablomin
Ablomin
Ablomin is a toxin present in the venom of the Japanese Mamushi snake, which blocks L-type voltage-gated calcium channels.- Etymology :The protein ablomin is a component of the venom of the Japanese Mamushi snake, Gloydius blomhoffi...
, is also a helothermine-related venom protein (Helveprin) which was originally isolated from the skin of the Mexican beaded lizard.
Target
Ophanin is a weak blocker of the high potassium-induced contraction of smooth muscles. Snake venom CRISP family proteins inhibit depolarization-induced smooth muscle contraction to different extents. Compared to the normal contraction of smooth muscle, ophanin is able to reduce their force of contractility to 84% ± 1%, which is less than most other CRISPs.The differences between the inhibitory activity of CRISPs may be explained through sequence comparisons that suggest a site that may be critical for inhibition of channel activity. Phe
PHE
PHE may refer to:* Population Health and Environment , an approach to development that integrates health or family planning with conservation efforts* Paramount Home Entertainment* BitTorrent protocol encryption...
189 and Glu
Glutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...
186 are the most likely functional residues: strong blockers of smooth muscle contraction (ablomin
Ablomin
Ablomin is a toxin present in the venom of the Japanese Mamushi snake, which blocks L-type voltage-gated calcium channels.- Etymology :The protein ablomin is a component of the venom of the Japanese Mamushi snake, Gloydius blomhoffi...
, triflin
Triflin
Triflin is a cysteine-rich secretory protein , which is excreted by the venom gland of the Habu snake . Triflin reduces high potassium-induced smooth muscle contraction, suggesting a blocking effect on L-type calcium channels.- Source :Triflin is a toxin derived from snake venom...
, and latisemin
Latisemin
Latisemin is a cysteine-rich secretory protein that can be isolated from the venom of the Black-banded sea krait, a sea snake indigineous to the warmer waters of the western Pacific Ocean...
) all have Phe189, and all blockers of smooth muscle contraction except ophanin have Glu186. The significance of this lack of the probable functional residues in ophanin has not yet been addressed. However, it is likely that the picture is more complex and other residues contribute to the inhibitory activity of CRISPs on smooth muscle contraction and some data supports this. For example pseudecin, while also having Phe189, does not affect depolarization-induced contraction.
Mode of action
There is no direct evidence of a particular mode of action of ophanin blocking depolarization-induced contractions of the smooth muscles. However, based on the hypothesis of Yamazaki and colleagues in regards to ablomin, another snake venom toxin from the CRISP family that also blocks depolarization-induced smooth muscle contraction, we can postulate a similar mechanism might be in place for ophanin.Since ablomin only blocks contraction induced by depolarization, but not by caffeine, the effect of ablomin is likely to be caused by inhibition of voltage-gated ion channels. An activation of smooth muscle cells through caffeine activates ryanodine
Ryanodine
Ryanodine is a poisonous alkaloid found in the South American plant Ryania speciosa . It was originally used as an insecticide....
receptors of the sarcoplasmic reticulum, whereas an activation through high levels of extracellular potassium depolarizes the membrane (due to the change of the reversal potential
Reversal potential
In a biological membrane, the reversal potential of an ion is the membrane potential at which there is no net flow of that particular ion from one side of the membrane to the other...
for potassium towards more positive values) and would then activate voltage-gated calcium-ion channels leading to high levels of intracellular calcium ions. The intracellular calcium ion concentration correlates well with contraction force in the rat-tail artery. Thus, contraction following extracellular application of high-potassium solution depends on the influx of the extracellular calcium ions through voltage-gated calcium channels. Therefore ablomin (and by extension ophanin) most likely targets voltage-gated calcium channels on smooth muscle.
Toxicity
The of the venom in mice is ~1.2 to 3.5 mg/kg via intravenous injection. The LD50 of ophanin is not yet known.See also
- Other snake venom proteins in the CRISP family:
- PiscivorinPiscivorinPiscivorin is a component of snake venom secreted by the Eastern Cottonmouth . It is a member of the cysteine-rich secretory protein family, which blocks voltage-dependent calcium channels.- Etymology :...
from the Eastern Cottonmouth - TriflinTriflinTriflin is a cysteine-rich secretory protein , which is excreted by the venom gland of the Habu snake . Triflin reduces high potassium-induced smooth muscle contraction, suggesting a blocking effect on L-type calcium channels.- Source :Triflin is a toxin derived from snake venom...
from the Habu snake - AblominAblominAblomin is a toxin present in the venom of the Japanese Mamushi snake, which blocks L-type voltage-gated calcium channels.- Etymology :The protein ablomin is a component of the venom of the Japanese Mamushi snake, Gloydius blomhoffi...
from the Mamushi snake - LatiseminLatiseminLatisemin is a cysteine-rich secretory protein that can be isolated from the venom of the Black-banded sea krait, a sea snake indigineous to the warmer waters of the western Pacific Ocean...
from the Erabu snake
- Piscivorin