Scorpion venom contains two large sets of peptides that target ion channels. One consists of short-chain peptides directed at potassium channels; the other of longer, larger molecules that target sodium channels. The second group makes up a substantial part of the venom by weight.
The distinction is not only about length. The two groups bind to different channel families, produce different outcomes and are studied on different set-ups in the laboratory. When a venom profile is discussed, what is usually meant is the ratio and diversity of these two sets.
These channels are not all of one type. Different subtypes occur in different tissues, and each differs in its activation threshold and closing speed. How strongly a toxin binds to which subtype is therefore measured separately for each.
What does the sodium channel do?
Voltage-gated sodium channels initiate the electrical signal in nerve and muscle cells. When the membrane voltage passes a certain threshold, the channel opens, sodium ions flow rapidly into the cell and the membrane quickly becomes positive. A short time later the channel closes of its own accord; this phase is called inactivation and is needed for the signal to end.
The working of the channel is therefore described in terms of three states: closed but ready to open, open, and inactivated. The timing of the transitions between these states determines how quickly and how often the cell can produce a signal. The effect of the toxins is explained by their interference with this timing.
The channel itself consists of a single large protein. Within the membrane this protein folds into four similar domains, which leave a pore in the middle for the ion to pass through. At the edge of each domain there is a region that senses changes in voltage. The points where the toxins bind are the areas adjacent to these regions on the outer surface of the channel.
The alpha and beta distinction
| Property | Alpha type | Beta type |
|---|---|---|
| Binding site | Close to the region of the channel responsible for inactivation | Close to the voltage-sensing region that controls the opening of the channel |
| Main effect | Slows inactivation, so the channel stays open longer than expected | Shifts the channel's activation threshold to lower voltages |
| Outcome | The signal takes longer to die away | The channel opens more readily than it should |
| Length | Long-chain group, mostly around sixty amino acids | |
The distinction between these two groups rests on where the site they bind to sits on the channel. Peptides of both types are commonly found together in the same venom. Because their effects complement one another, it is also hard to explain the behaviour of the venom as a whole through any single component.
The literature uses a numbering system for these peptides based on their binding areas on the channel. The alpha and beta distinction corresponds to two different areas of that system. Molecules binding at different points on the same channel can alter each other's effect, which explains why this map was drawn up.
Structure: a long chain and four bridges
These peptides are longer than their short-chain relatives and typically carry four disulfide bridges. The bridges hold the chain together as a tight core; the resulting fold is a shared structural theme that recurs in peptides from different scorpion species. The function of the bridges is described in the article on disulfide bridges.
This tight structure gives the molecule considerable resistance to heat and enzymes. The same structure makes the peptide harder to synthesize under laboratory conditions or to produce recombinantly; bridges that fail to pair correctly leave the molecule not behaving as expected.
The short-chain group that targets potassium channels is smaller and mostly carries three bridges; the features of that family are covered in the article on α-KTx peptides and the channels they target in the article on Kv1 channels.
The behaviour of crude venom as a whole cannot be read as the sum of the separate effects of the components within it. Different components may bind to different regions of the same channel, and one may make another's binding easier or harder. This is one of the reasons why components are separated and examined one by one in research.
Species selectivity
Some of the sodium channel toxins in scorpion venom bind markedly more strongly to vertebrate channels and others to insect channels. This distinction is linked to the animal using its venom both in hunting and in defence: prey are mostly arthropods, while threats are mostly vertebrates.
Selectivity is a subject of interest in basic research. Which small structural differences, and in which region of the channel, change the binding tells us something about both channel biology and peptide design. This work is carried out at laboratory scale and does not amount to a recommendation for any product or application.
Peptides that bind selectively to insect channels form a research topic of their own. Which structural features of these molecules determine selectivity is examined in both comparative biology and biotechnology studies. Work in this field is at laboratory scale and is not presented here as a recommendation for any product or application.
How do they appear in analysis?
In crude venom these peptides are not found on their own but among dozens of components. When the fractions obtained after chromatographic separation are assessed by molecular weight, the long-chain group appears in a higher weight region than the short-chain one. Batch data such as the weight distribution and the number of components are published on the analysis page.
There is no way to say which channel family a peptide targets by looking at its weight alone. That calls for sequence information and functional measurement; chromatography and electrophoresis are the steps that pave the way to this stage. Crude venom is the source material at the start of that chain and is intended for research and industrial use only.
