Cone Snails and Disulphide Bonds
The textile cone snail (Conus textile) injects disulphide-rich conotoxins to paralyse its prey.
Back in October, I wrote about a group of researchers who analysed the peptides in glow worm venom in the hope of finding a new type of pain killer. They found an assortment of digestive enzymes, but in much greater quantities they found cysteine-rich peptides.
Since then, I have read again and again about animal venoms containing a large number of cysteine residues. Cone snails, reportedly one of the most venomous creatures in the world, hunt their prey, impale it with a harpoon, and through this hollow spike deliver their lethal venom. There are more than 700 species of cone snail and each has evolved its own cocktail of hundreds of different peptides. These conopeptides or conotoxins have been classified into families, the first division being whether the peptide contains a large number of cysteine residues or not.
The authors of the glow worm study remarked that cysteine is toxic in large quantities, so is it the presence of the cysteine that causes the toxicity, or something else? To answer this, let’s start at the beginning, with the chemistry.
Cysteine is an amino acid containing a thiol or sulfhydryl functional group (–SH), with one sulphur and one hydrogen atom. Cysteine isn’t an essential amino acid - it can be made by our bodies (and the bodies of other animals) from methionine – also a sulphur-containing amino acid – using a process of building that starts with serine.
What’s important about cysteine is that two cysteine residues in different parts of the same protein, or in different proteins, can come together to form a disulphide bond. This disulphide bond is a solid cross-linker – if you imagine making a model out of a flat piece of paper, a disulphide bond would be the glued flap that turns the paper from 2D into 3D.
What I hadn’t appreciated was how small conotoxins are – they’re tiny! Not more than 8-30 amino acids long. Thinking back to our paper model, adding a glued flap turns a small piece of paper from a scrap easily lost to the floor, to something that has rigidity. For venomous animals, disulphide bonds not only give structure to venom peptides, they also make the peptides incredibly stable, resistant to heat and resistant being broken apart by enzymes.
With their rigid 3D shape, conotoxins have evolved to be highly selective for their target – and it is for this reason that many conotoxins have been developed into therapeutics. The first was a ω-conotoxin developed as Ziconotide (brandname Prialt), approved by the FDA in 2004 for the treatment of pain that is so severe it is unmanageable using standard pain medications. Conus magus uses this ω-conotoxin to paralyze its fish prey. The toxin blocks voltage-gated calcium channels in the fish to prevent nerve transmission, resulting in paralysis. Humans have the same voltage-gated calcium channels, but what’s remarkable is that in humans, they are located in a different place. Instead of causing paralysis, this ω-conotoxin - Ziconotide - blocks the transmission of pain signals.
A nod to evolution here - in developing Ziconotide, Neurex Corp (now owned by Perrigo Company PLC), who did the development work, did a lot of structure-function characterisation. They changed every single one of its twenty-five amino acids to see if they could improve the molecule in any way. And you know what? In the end they went with the very same sequence of amino acids in Conus magus’s ω-conotoxin – much as we saw for Exenatide for the treatment of type 2 diabetes. Exenatide is identical to the Exendin-4 made by the Gila monster.
Disulphide-rich proteins are abundant and highly specific for their target in the body - they make fantastic medicines - but they’re not that easy to make. It can be difficult to get the right cysteine to find its correct partner cysteine to form a disulphide bond, especially when there are a number of them. Ziconotide was the first conotoxin approved for clinical use in part because it could be made in the lab reliably with the proper formation of disulphide bonds.
And as to the chemical nature of cysteine – is it really that toxic in large quantities? Most sources agree that taking large quantities of any amino acid can’t be good for you, and there are several conditions with which you should definitely not take cysteine supplements at all – if you are pregnant or breastfeeding, have diabetes, or the rare genetic condition cystinuria. For diabetes, too much cysteine can interfere with the activity of insulin – itself a cysteine-rich protein. (Insulin is formed from two peptide chains joined by a pair of disulphide bonds.) In cystinuria, the body is unable to reabsorb cystine (the oxidised form of cysteine) into the blood, so instead it forms crystals, which aggregate to form kidney stones.
But in general, cysteine itself - in its L-cysteine form - is not toxic, with some studies even suggesting that in high protein diets (those containing poultry, eggs, beef, whole grains), it might be the cysteine that is responsible for lowering blood pressure and reducing the risk of stroke [1].
You might remember, at the end of my post about jellyfish and the therapeutic potential of collagen, the discovery that blood pressure-lowering ACE inhibitors have been found in hydrolysates from plants, chickens, milk, and other marine animals. Whether cysteine is responsible for lowering blood pressure here too is something I’ll need to understand further.
Venom peptides are cysteine-rich, but the significance of this is unlikely to be due to cysteine itself being toxic, rather due to the 3D structure that disulphide bonding affords peptides. A rigid structure allows the peptide to be highly specific to its target – and also highly deadly.
Reference
Larsson, S. C., Håkansson, N. and Wolk, A. (2015) Dietary Cysteine and Other Amino Acids and Stroke Incidence in Women Stroke 46(4):922-926.
Further Reading
Pennington, M. W., Czerwinski, A. and Norton, R. S. (2018) Peptide therapeutics from venom: Current status and potential Bioorganic & Medicinal Chemistry 26(10):2738-2758.
Shaikh, N. Y. and Sunagar, K. (2023) The deep-rooted origin of disulfide-rich spider venom toxins eLife 12:e83761.
Resources
Build a paper model of insulin with resources provided by the Protein Data Bank - Protein 101 Training and Outreach Portal.
About the Illustration
Made using pencil, watercolour and charcoal pencil on paper – a textile cone snail (Conus textile) with disulphide bond structures worked into the triangular pattern on its shell.
Acknowledgements
A special thank you to Stephen H Kawai, chemist and shell collector, on Bluesky, for helping me to understand the significance of disulphide bonding in conotoxins - and for guiding me towards this video about cone snail venom complexity.