Last week on Science Sunday, I wrote about why the retina is cool. This week, I want to talk about why Botox is the coolest protein known to man, and by man I mean this particular man, as other scientists may well know of cooler proteins out there.

First: what is Botox? Most people know it as an accusation hurled at celebrities, to imply that they’re doing a really lame job of looking younger than they actually are. At a little deeper level, it’s a chemical that doctors can inject into your face to paralyze nerves that make you wrinkly. I have no idea how a nerve could be so utterly responsible for wrinkliness that paralyzing it could make you unwrinkly, but apparently that’s how things work. That’s not the point of this post.

But going to an even deeper level, Botox is botulin toxin, a deadly chemical that causes food poisoning. It is, as noted above, a protein, a particularly large molecule. How does it work?

The first thing that’s important to understand is that poisons don’t function by their inherent poisonousness. Oxygen is necessary to us but poisonous to some life form: there’s nothing inherent in either function it has. This may seem trivial, but there’s a tendency to think the other way. In fantastic fiction, the most efficient way to kill someone is often something called “death magic,” which seems to involve clubbing people over the head with distilled essence of DEATH. Alternatively, you can drain their life force, or sometimes drain their life form /with/ distilled essence of death. At any rate, the concept gets used enough in fiction that it must have some intuitive hold for people. Similarly, as a kid I remember seeing cartoons where poisons were always green or black, came with skull marks on them, and had evil-looking fumes coming out the top, generally making them look like distilled essences of death. It may be hard to believe that anyone actually thought this way, but people used to talk about sleep-causing drugs having a “dormitive virtue.” Today this is held up as a meaningless non-explanation, but there’s actually a quite substantial and incorrect idea here: that there’s something inherently sleep-causing in the nature of those drugs. In reality, though, there is no distilled essence of death, no life force, no dormitive virtue. Instead, there are very complicated biological systems whose parts can be altered in many different ways.

Botox, to return to our example, affects the nervous system. Last Sunday I mentioned how for neurons to communicate to one another, they release chemicals called neurotransmitters. The mechanisms for releasing these transmitters are based on the nature of cell membranes: they’re not solid, but rather made of a fatty liquid that holds its shape for reasons related to the reasons soap bubbles exist. Inside neurons, there are smaller structures made of the same stuff called vesicles, which contain the neurotransmitters. The fluid nature of cell/vesicle membranes means it isn’t too hard for them to fuse together, putting the contents of the vesicle outside the cell, where they can effect an adjacent neuron. However, in order for fusion to happen there needs to be interaction between proteins embedded in the membrane of the vesicle and proteins embedded in the cell membrane. This is the reason neurotransmitter release can be controlled; apparently it’s the entrance of calcium during neuron firing that activates these proteins.

Once you’ve got this background, it’s easy to see how botulin kills you: it destroys the proteins responsible for vesicle fusion, preventing neuron firing. Do this to the neurons that control the lungs or heart, you can’t get oxygen to where it’s needed. You might survive such an event if you could magically turn off cellular activity until oxygen was restored, but as it is your cells keep chugging, running themselves into the ground. What’s amazing about botulin is that it’s an enzyme: it facilitates chemical change (the destruction of vesicle-fusion proteins) without itself being altered. That means one molecule of botulin can take out many, many vesicle-fusion proteins. According to Wikipedia, it only takes about a microgram (one billionth of a kilogram) to kill you, making botulin a contender for the most deadly substance on Earth.

It should also be noted that making botulin is the exclusive preserve of bacteria. It is not an example of better living through chemistry, even if you think paralyzed facial nerves count as better living. In a way, it is an example of how natural products really are better, though in this case “better” means “better at killing you.” Evolution had millions of years to tinker out sophisticated systems for producing all kinds of chemicals, and for figuring out which chemicals are worth producing. This is why we look to nature for drugs to treat bacterial infections: antibiotics are basically evolved chemical warfare mechanisms for fungi to kill competing bacteria.

So, now you know: Botox isn’t just something injected by old celebrities trying to look younger. It is the greatest poison /ever/.