 This might not be entirely obvious why they're nice, beautiful, and stable structures. I figured the best way to convince you about that is to draw them and we'll see if it makes more sense. This particular one is called Tim Barrel. The fact that it's a barrel you can probably guess. The Tim part stands for Trios Phosphatase Isomerase. So it's an ACE that means that it's an enzyme. And this particular one is involved in glycolysis, which is an important part of the energy turnover in our bodies. In addition, inside these AT-lases we have eight beta strands. And if I now draw the chain here, I'm going to use solid lines to draw loops facing us and dotted lines to draw loops that are under the structure. So we come in from the bottom here with the N-terminus that's on the lower side. And then we have upper side, lower, upper, lower, upper, lower, upper, lower, lower, upper, lower, upper, lower, upper. And then the C-terminus there. Do you see what has happened here? First, this is a parallel structure. All the beta strands here are parallel because I have one beta strand. Then I have an alpha helix in the other direction and then I have a beta strand in the same direction as the first one. In fact, mixing the secondary structures this way is a requirement to be able to have parallel structures. In fact, the alpha helixes are also parallel here because if all the beta strands are pointing up, all the alpha helixes will be pointing down or in. Second, is this going to be a nice and stable beta sheet? You bet it. It's as good as it gets. There are no edges on this beta sheet because even the first, even the last strand here is not going to have any unpaired bonds. Those are paired up with the first. So effectively it's an infinitely long beta sheet as far as the strand goes at least. The helixes are also going to be quite stable. And here you might see the difference between the cartoon which was great for me to visualize and understand the structure versus reality. You should know why the helixes are tilted. That was that crossover angle, right? You should also know that if you look at the strands here, they appear to almost be a bit splayed or something. That is because we had that built-in twisting feature of beta strands. So all these things that looks like it's ugly and non-perfect, well, in one way it is ugly and non-perfect, but that's just because we have rather achieved close to perfect packing. I'll tell you more about that structure in a second. This is another protein where you have a part here where we have helixes and sheets that alternate. And up here you have helixes and sheets separately. I'm not even going to try to draw the full diagram for this, but up in this part you could imagine something like being helix, helix, helix, helix. And then a number of sheets and then continue on to more helixes, right? Whenever you have alpha plus beta, the helixes here would need to be anti-parallel because they go up, down, up, down, up, down. The beta strands, they also have to be anti-parallel. They need to go up, down, up, down because the second, if they're going to be parallel, there has to be something between the first and the second strand. And the obvious thing to put between them is another secondary structure element. As always, there are exceptions, but I can't even remember ever seeing any structures while I had parallel beta strands and then just connected between long loops. It would be very disadvantageous for those loops. This is an interesting structure. I'm going to come back in a second to what we see here. The entire particular protein is called ADH, alcohol dehydrogenase. And we might talk a little bit more about that in the bioinformatics lecture. So it's a protein responsible for breaking down ethanol in your liver. And large parts of the population on earth actually has a slight deficiency in this, so they have a variant of the gene, which means that their bodies are not as efficient at breaking down alcohol, which is one reason why certain parts of the population in Asia are significantly more influenced by alcohol than, say, in Scandinavia. This particular part of the structure is interesting. It is a structure that has one layer of alpha helices, then one layer of beta strands, and then one layer of alpha helices. Let's look at that in a little bit more detail.