Some varients of botulinum poison are so deadly that even a few nanograms can kill a person. How does a few nanograms of anything kill someone?
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First of all, your instinct is very good. It’s not average of any poison to be so toxic that a few nanograms kill someone. Let me walk through how poisoning works in terms of biochemistry.
Let’s first imagine a crowd, like a concert full of people. Imagine a man and wife arriving separate and not finding each other (not even knowing the both are there). What’s the chance of them finding each other? Assuming that they both walk around randomly at their usual speed, it only depends on how big the entire crowd is. In a small crowd they have better chances to bump into each other than in the huge stadium .
Now imagine that we clone them, so we have several copies of both the man and the wife. The more copies there are, the bigger the chance is that one copy of the man bumps into one copy of the wife. But it also depends on the crowd size, in a bigger crowd you need more copies for the same odds of meeting. The ratio of the crowd and the number of the person of interest is called *concentration* in chemistry.
Very similarly, in biochemistry, molecules and their reaction partners (such as toxins and their partners) wander around in our body, cluelessly. And they find each other based on the concentrations.
But there’s one more thing to add to the equation. Let’s get back to the man and wife at the concert. Let’s assume if they meet, they give each other a quick hug and then they go on. Yet another meeting between another copy of them, hug, repeat. But what if the man has his lover around the concert too, and when they meet, they keep hugging? Those husband clones that are in the hug with the lover, are out of the equation from the perspective of the wife.
Molecules in biochemistry are basically the same. Some partners like to stick together longer than others. And it has a very important consequence. You see, for each partner there are two ways of spending the time: alone (wandering around) or together (hug). If the hug time is long, then for the same amount wandering time you get much more hug time.
In other words, if you want to have a given amount of hug time happening, then you can either mix short-huggers in large numbers (large concentration), then they will *often* meet, hug a bit and go on. Or, you can mix long-huggers in a low concentration, they will rarely meet, but once eventually they do, they will hug forever. In chemistry, the length of hug is called affinity. High affinity means two molecules love each other a lot.
Now in life, there’s a lot of reaction that must happen at the exact amount it needed. Low affinity is not a bad thing, if that reaction needs to go at low rates. We have all of the different things in our body some work at low concentration and low affinity (because we need just a very little of that reaction) some with high concentration and low affinity etc.
So for something to be extremely poisonous you need two criteria.
One is that the poison must have high affinity to the target. It means that if you have a very few molecules of poison wandering around in your body, once it bumps into the target, they stay together forever, blocking the whatever thing the target would normally do. If the affinity is lower, you need more poison for the same effect, to compensate the shorter hug times with more meeting events.
And the other criteria is that the target molecule must be rare. With a 100 molecules of poison, you can occupy 100 molecules of target. If you have 1000 or 5000 target molecules in the body, then no matter how high the affinity is, you need much more of the poison.
In case of botox, both criteria are met. The botox acts on things called neuromuscular junctions, the points where the brain controls the muscles. Even though there are things that are more rare in our body, but these are also sort of few. Also we can’t afford losing too many of them because they make our muscles move. The more we lose the more muscle go paralyzed. Imagine losing 10% of heart muscle control due to losing 10% of overall neuromuscular junctions.
As well, botox has a very high affinity to the target which means you need very few molecules to completely occupy a lot of the target.
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