Nomenclature
The vast majority of snake venom neurotoxic peptides competitively
bind to the nicotinic acetylcholine receptor and, following the
same nomenclature as used for cone snail toxins, are termed alpha-neurotoxins.
These alpha-neurotoxins are comprised of two main structural forms,
Type I (formerly short-chain) and Type II (formerly long-chain)
although the actualy diversity is much greater. The two main groups
of toxins are similar in action but differ in size, from an average
of slightly above sixty amino acids to an average of seventy-three
amino acids having either four or five disulfide bridges respectively.
These structural differences are due to the Type I neurotoxins
having a primary structure similar to the Type II but with the
latter having a C-terminal extension. The toxins bind with high
affinity to alpha2 forms of skeletal nicotinic acetylcholine
receptors but only the Type II neurotoxins bind to alpha7 neuronal
acetylcholine receptors. In contrast kappa-neurotoxins, such as
kappa-bungarotoxin, target the alpha3beta2 neuronal AChRs. A number
of other alpha-neurotoxic forms exist, with the ancestral structural
condition being a particular ten cysteine arrangement as typified
by alpha-colubritoxin isolated from the venom of Coelognathus
radiatus (radiated rat snake). The receptor subtype affinities
for these ancestral toxins is currently being elucidated by us.
Muscarinic Toxins
Snake venom neurotoxic 3FTx are typically alpha-neurotoxic
and this is the ancestral condition, this activity being widespread
not only in elapid venoms but also in the venoms of the various
'colubrid' families. However, a number of neurotoxic peptides
have been isolated that are active against different postsynaptic
receptors or are presynaptically active. Mambas, elapids from
Africa, are unique in having a number of toxins targeting other
receptors or binding presynaptically. Targets include the muscarinic
receptors, the enzyme acetylcholinesterase and potassium channels.
The muscarinic toxins were first isolated from the venom of the
Eastern green mamba (Dendroaspis angusticeps) with other
toxins following from other members of this genus. Muscarinic
toxins can be broken into two groups, A and B. Group A toxins
have 65 or 66 amino acids, four disulfide bonds and a number of
invariant residues. All of the group A toxins have a Lys or Arg
at residue 34, differing from loop two of the a-neurotoxins and
Trp 28, Tyr 30 and Tyr 36 are other essential and invariant residues.
Only one group B toxin has been sequenced, this potent M2-specific
toxin having only partial homology to the other muscarinic toxins
but interestingly. has a high degree of homology with Type I alpha-neurotoxins.
Structurally, muscarinic toxins are similar to other members of the large group of 3FTx (three-finger toxins) such as alpha-neurotoxins, fasciculins, calciseptines, mambins and cardiotoxins/cytotoxins but are more variable in primary structure. The higher degree of variability amongst muscarinic toxins than amongst comparable alpha-neurotoxins is not surprising as these toxins target five different receptors versus the two targeted by the alpha-neurotoxins.
Due to specificity of binding, muscarinic toxins have shown
to be useful in the characterization of receptor sub-types found
in particular tissues. MT-3, as a case point, was shown to be
a potent yet reversible competitive antagonist at the M4 subtype
muscarinic receptor. This toxin was used to identify the presence
of M4 receptors in the rat brain through the selective inhibition
of adenylate cyclase activity. Similarly, M1 toxins were used
to bind the M1 receptor on rat striated muscle tissue and M4-specific
toxins were used to show that M4 subtype muscarinic receptors
make up the vast majority of the remaining receptors. These receptors
are of great interest in the treatment of Parkinson's disease,
as it is hoped that selectively blocking the M4 receptors would
greatly aid in restoring normal movement.
Other toxins have been shown to inhibit the binding of muscarinic ligands, specifically a 13,600 MW component from Naja sputatrix and a 13,800 MW component from Crotalus atrox. However, rather than being peptides such as in the mamba muscarinic toxins, these components are larger phospholipase A2(PLA2) enzymes. These toxins were significantly less potent than those isolated from the mambas and displayed none of the useful specificity.
Dendrotoxins
The mambas are quite unique amongst the elapids in possessing
a class of toxins, the dendrotoxins, which are active upon voltage
gated potassium channels. Dendrotoxins are approximately 59 amino
acid single chain peptides with three disulfide bonds and are
part of the snake toxin family that utilises the BPTI/Kunitz-type
protease inhibitor scaffold. A structural key to the differentiation
between the potassium channel blocking dendrotoxins and the protease
inhibiting toxins is the Lys 5 residue of the dendrotoxins lying
in close association with a hydrophobic residue (Leu, Tyr or Phe
9).
Through the use of a dendrotoxin as an investigational tool, a new class of potassium channel toxin was discovered in sea anemone venom. As a number of invertebrate toxins such as those from scorpions also target the same sight but the toxins are structurally distinct, this allows the use of the toxins as investigational ligands of receptor binding sites.
Structure-Function Relationships Of Snake Toxins
Despite there being significant variations in amino acid
sequences and biological targets, most medium sized elapid snake
venom peptides share a similar molecular scaffold. These venom
components are 6-8 kDa with a disulfide-linked core resulting
in three loops or three 'fingers', hence the name three-finger
toxins or 3FTx. A comparison of the structure of the cardiotoxin-gamma
from Naja nigricollis (black spitting cobra) and that of
erabutoxin-b from Laticauda semifasciata (banded sea krait)
shows the tremendous homology of secondary and tertiary structure.
The conservation of structure, however, belies the diversity of
action. While the carbon backbone and general 3D structures are
virtually identical, differences in structure are evident in the
spacefilling models. These changes have profound effects on bioactivity,
acting upon membrane lipids and proteins of blood cells or heart
cells (cardiotoxins) or block the acetylcholine receptor (alpha-neurotoxins
like erabutoxin-b).
However, just as conservation of structure does not automatically
translate to conservation of function, differentiation of structure
does not necessarily mean differentiation of function. Erabutoxin-b
and LS-III, both from the same venom, would appear to have even
less in common than erabutoxin-b and cardiotoxin-gamma and thus
be even more divergent in activity. However, both are alpha-neurotoxins,
with the former being a Type I alpha-neurotoxin and the latter
a Type II alpha-neurotoxin. This reinforces a theme repeated throughout
venoms that similar toxins can have widely divergent activities,
just as divergent toxins can have very similar activities.
