 In the late 1990s, Rod McKinnon was able to crystallize an iron channel from a small bacterium. And this channel is called KCSA. So this is a tetramir. It might be, if I show it from the top. This is from the side and this would be the membrane. And from the top, this consists of four subunits that have the same sequence. And these four subunits, they have both long and short helices, where the long helices create some sort of nice packed environment so that the lipids would pack here. But on this actual pore here in the middle, we will have water. There's going to be a hole filled with water here in the middle. The water can't reach the lipid and the lipids can't reach the water because the iron channel is shielding them. That's a great way to achieve exactly that type of pore that Adrian Prasidian had proposed. This might be a bit too much information. So typically to understand what these do, we will have to simplify a bit. So I'm going to hide all the details and just show two of these subunits next to each other. And then it looks like this. So what happens here is that you have a large distributor of water and everything roughly here. Do you see the top here? So there you have a relatively unstructured part of the amino acid backbone. But that is turning all the oxygens on the amino acids, the CO groups, facing each other. That's going to create a series of small binding sites where we have negative groups pointing to the inside, which is going to stabilize a positively charged group. That sounds great. With this way, we'll be able to take a positively charged ion and transport it. And this particular channel transports potassium ions. That's why it's called KCSA. So it's K-C-S-A. And that K is because it's transporting potassium ions. Sorry about that. In fact, this channel is so great that for every one million or so of potassium ions it lets through. It's only lets through one sodium ion. Have a look at those ions in the periodic system and think about that for a second. Do you have no idea what I'm talking about? I'm going to draw them just schematically. Here I will draw a potassium ion for you. And here I will draw a sodium ion for you. And now I ask you, please design a hole through the membrane. That hole should let through virtually all of these ions, but none of that. That's a bit hard, right? Any hole that's large enough for potassium will let through sodium, too. So how on earth do we create a hole that only lets through the large radius, but not the small radius? It turns out that proteins, sorry, ions do not exist like this in solution. So ions have a layer of so-called hydration water. Remember I talked about water screening? So these are positively charged ions. So water molecules will turn their oxygens facing the ion. And you can actually show that when all these waters turn, they will bind. And they will bind reasonably hard, actually. They're not going to let go of all these waters in the first place. The charge, however, is the same. And on average, these waters are slightly further away from the charge. These waters are going to be closer to the charge. And if you know your physics, you can actually calculate that these waters will be bound harder. Because it's the same charge, but they're closer to the charge. That creates a beautiful effect that nature has utilized. How has nature used this? Now, I'm going to need to show you something even more schematic. If I take that structure and I idealize the helices. So I show the helices as rods here. The four gray helices here, they're pointing kind of like arrows into a reactor or something. And do you see that there is a density here in yellow? I think that's a cobalt ion, actually. But the whole idea is that that density corresponds to an ion. Remember when I spoke about helix dipoles and helix capping? So this is a positively charged ion. What if all those helices are oriented such that the dipole is pointing away, meaning that they turn this effective negative charge against this? That means that the four helices now is going to create a cradle or something where they would love for something to bind. And if this now is potassium, potassium does not have its hydration water bound as tightly. So the potassium ion will come here and then the potassium will let go of its hydration water. Because instead of the oxygens here, it will be stabilized by these four helices. Now, once the potassium is there, it will gradually move up along this ladder to the first, second and the third binding site. And then it will go through the channel. In fact, this is so efficient that the diffusion of potassium is almost the same as if it was literally just a hole. The potassium will hardly feel any resistance whatsoever. The sodium, on the other hand, has its water much tighter bound. And nature, again, has through evolution arranged this in effect so that the potassium will never be able to let go of its water here. The sodium will never let go of its water here. The sodium is going to be stuck here because the sodium, together with all its crystal water, is larger than the potassium without its water, even if I might not have drawn them to scale here. So that's a beautiful way how the structure of a protein stabilizes potassium and lets an ion that's large through while the seemingly smaller ion without the hydration water can't get through because together with its hydration water, it's too large.