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snake venom
Which is the world's most dangerous snake?
It depends on what you mean by dangerous. One of the large African black mambas secretes enough venom to kill 10 men. Spitting
cobras like the South African ringhals can hit the eyes of a victim from 3 m away.
The Australian tiger snake kills more people than all the other Australian snakes put together, which is saying a lot since the 10
most venomous snakes in the world are all Australian; it can kill people within an hour.
It has the most lethal venom of any snake. The most spectacular killer is probably the boomslang, an African treesnake, whose
venom contains a range of enzymes which result in the victim bleeding from all body orifices.
What's in these venoms?
Most contain a number of enzymes, such as cholinesterase, phospholipase, lecithinase and L-amino acid oxidase. They also have
toxins, such as bungaratoxin, found in the venom of the krait (Bungarus multicinctus)
a relatively small polypeptide, containing around 70 amino acids, with a molecular mass about 8000. Cobratoxin is a molecule of a
similar size.
Why are they toxic?
Communication between nerve and muscle occurs at the synaptic junction. The end of the nerve contains cells that make
acetylcholine. The arrival of a nerve impulse stimulates release of the acetylcholine. This binds to
CH3
H3C
O
Acetylcholine
N CH2 CH2 O C CH3
CH3
+
+
receptor sites in the postsynaptic cell, and causes the channel to open so that Na and K ions can dash in. This changes the
electrical potential and makes the muscle contract. The acetylcholine is rapidly degraded into acetate and choline by an enzyme
called acetylcholinesterase, which restores the normal potential of the membrane. The action is all over in a few microseconds.
2 m
Nerve
Presynaptic Membrane
Receptor
Postsynaptic
Membrane
0.1 m Cleft
Muscle
Bungaratoxin binds to the acetylcholine receptor and stops acetylcholine binding so that the channels in the receptors are blocked.
It fulfils the role of immobilising the prey. The limbs rapidly become paralysed and this soon spreads to the respiratory system.
Some small proteins (molecular mass 5000-9000) from viper venom bind to a receptor on blood platelets and stop blood clotting by
preventing the interaction between platelets and fibrinogen. Protein toxins called dendrotoxins in mamba venom have a single
polypeptide chain (with three cysteine-cysteine crosslinks) with 57-60 aminoacids; these block some potassium ion channels.
What do the enzymes do?
When the snake bites its victim, the enzymes enter the circulatory system and get spread through the body. They dissolve body
tissues so they start to dissolve the victim's body from within.
© Simon Cotton 1996
Most are neurotoxins and various enzymes, or haemotoxins.
Neurotoxins interfere with nerve conduction and cause paralysis (immobilise prey?) including paralysis of respiratory muscles.
haemotoxins hasten absorption of venom, attack and destroy cells and cause haemorrhaging by interfering with blood coagulation
zootoxins
Most venoms are a protein or closely related molecule.
Enzymes.
Popular enzymes cholinesterase, phospholipase, lecithinase, L-amino acid oxidase.
Sometimes coagulate blood. produce paralysis.
Indian cobra has particularly complex makeup to venom.
Mike Byford says.
Some block neurotransmitters
Protein enzyme inhibitor.
Sources of Information
Check Biol. Sciences review November 1995 V.Schwarz, "Snake Venom and Muscle Paralysis"
A.T.Yu, Venoms: Chemistry and Molecular Biology, John Wiley 1977.
A.L.Harvey, Chem.Ind., 20 Nov. 1995, 914-916. also Encyclopaedia Britannica.
Mike Byford says.
Some block neurotransmitters
Protein enzyme inhibitor.
Snake venom