 Beta sheets are actually simple in many ways, because there is almost only one or two ways to organize beta sheets. It might not look at it if you see the structure this way, but again, that's why I love to have these simplified cartoon representations. So I'm not going to draw this particular protein, but what makes this complicated is that beta sheets, like helix is in practice, they're not going to be perfectly ideal. But when a helix is not ideal, it's going to be slightly tilted or something. It's hard to, if you're actually significantly distorting a helix, you're going to start breaking the hydrogen bonds, and then it's an obvious kink in it. But what can happen with beta sheet, since it's a sheet, right? You can twist it a little bit or twist it like that, and then you also have all these turns between them. So the reason why this might look like a spaghetti-like mess, trust me, it's not. But it's some of these, you see the turns, and they're of different lengths and everything. If I were to show these proteins with every single atom, you would see a beautifully packed system there, not going to be holes. There might be some water in the binding site here in the middle or something, but each domain here is going to be a beautiful and rigid structure. So avoid getting too confused by the loops. That's, on the other hand, why it frequently makes much more sense to draw beta sheets with these strands. So we talked about this before, but when I draw arrows as sheets, they go from the N terminal side to the C terminal side. So this is really in the direction of the sequence. And then I have my next sheet and maybe a, sorry, not sheet, I call the individual component strand. So N to C, N to C, N to C, etc. The reason for drawing these schematic things is that you avoid the problems where you don't see the forest for all the trees. You don't really care about the specific information about the loops. You want to study the relative orientations of the sheets, and in particular what strand here is binding to what other strand. We'll be returning to that. Beta sheets are simple though, because there are pretty much only two ways we can organize them. We can have them like that, anti-parallel, or I can have them that they're parallel. But I start with one strand, N to C, and then they go on the inside where there could be something, either a loop or a helix or something. And then I have a second strand next to the first, and they're, yes, N to C in both cases. And they're, again, forming hydrogen bonds, but the direction of the chain is parallel for both of them. That's it. There are no other ways. It's a two-dimensional structure. It's not three-dimensional. Sure, the size might be different. One strand might be slightly longer. They might be twisted. That's why it looks complicated in practice. But beta sheets are actually way simpler in terms of domains than helices, even if you're not going to think so when you look at them in PDB. They're also continuous. Typically, you tend to have two stacks. It's actually pretty rare. You virtually never have an isolated beta sheet. Things get more complicated if you're mixing helices and sheets. But if it's pure beta sheets, they tend to be grouped in two or two. Of these, the anti-parallel sheets are more common. Why is the anti-parallel sheet more common? Well, it's easy to have something here and then have a short loop, and then something, and then a short loop. Here, the dashed line here has to be something. It's not going to be advantageous to have long loops all the time, and then you need to create a helix or something. So anti-parallel is more common, but it doesn't mean it's not that parallel is rare. It's just not quite as common. The other thing that you see that makes life more complicated, looking at this structure, do you see that this sheet in particular, it's kind of twisted, right? So you're never going to see a beautiful parallel sheet if I grab a piece of paper here. They will never look like that. They're rather going to look something like that, twisted. That is not because it's imperfect, or rather it is because it's imperfect, but that's because the Ramachandran diagram tells us they should be that way. I know I'm lying a bit now. Remember when I drew all these structures and say that beta sheets are pleated and just stretched out? Sorry, it was kind of a lie. How was that a lie? Let me show you the Ramachandran diagram. You remember that you had the phi angle here and the psi angle there in the amino acid backbone? Well, to be perfectly stretched out, both phi and psi should be at 180 degrees, right? Then you would literally have a zigzag chain. But look at that region up there where you have the beta sheets. That is not up in the exact corner where both phi and psi are exactly 180 degrees. It's slightly offset. And when this is slightly offset, you're still going to have something that is mostly zigzag. But in each repeating unit here, we're going to be deviating a little bit, a couple of degrees. And when that happens in all the chains, that's when you're done with this effect that we're gradually going to twist the sheet as we go along in each strand here. That has a couple of effects. First, it means that all these sheets that appear to be distorted, they are distorted, but they're not distorted because it's a bad protein. They are distorted from the planar case because that's going to be the most stable form for any beta sheet. Perfectly planar would be rare. I wouldn't actually believe a large planar beta sheet if I saw it. Second, virtually all helices have... Sorry, helices. Virtually all sheets have a right-handed crossing that you see on the upper part here. So that I go there, it's not entirely... Well, the right-handed part, imagine looking at it from my side here so that you're going to turn over and then you come back and that turn has happened in a right-handed fashion when you're seeing it from the first strand to the second strand. It can be left-handed, as you see in the lower example, but that's going to be very rare. Much more rare than seeing parallel strands. The reason for that is, again, this turn. It's not random. It's not that they're turning 15 degrees in either direction. This means that it's always offset in the same direction and that simply means that this part is going to be less stressed because if the each strand is anyway turning a bit, it's going to be easier to make a crossover that is compatible with that turn rather than making one that is not. The one that is anti-compatible with that one will effectively have to turn a bit more than 180 degrees, say 195, while the first one will only have to turn roughly 170 or 165 degrees. But, again, that's all we needed when they are parallel. If they are anti-parallel, this effect is less strong. So as a result of that, you're always seeing a bit of twist to sheets, but that's because it's the most stable structure. Can you imagine anything else? Well, this is for one sheet. If I combine two sheets, there are going to be at least two different ways to put them together. Let's look at it.