 So that second protein had another important motif or super secondary structure that I'm going to show you. This was this layer of beta sheet where all the strands are parallel and it's kind of wedged between two layers of helices. We can draw that schematically. So I had one, two, three, four, five, six beta strands and then I had two helices up here, and I have three helices down here. I happen to know this one. That's why I can do it fairly quickly. We're starting to go in the let's see, we're coming in from the bottom and then we're going out in the top, the bottom, top, bottom, top, bottom up there, top, bottom, top, bottom, and then we're going out there in the C-terminus. This has a special name. This is a so-called Rossman fold. It is very efficient at binding nucleotides in particular that has to do with this pattern of the edge of a beta sheet. And in particular in this case, we have two hydrophobic regions on both sides of the sheet between that and the corresponding alpha helices. This was discovered by Michael Rossman in 1973. Do you see here that nature appears to reuse things patterns? This is kind of a Tim barrel, right? But we've just cut it open. So you have the Rossman fold, we have the Tim barrel. They're kind of secondary structure elements, but they're so common that they're so commonly occurs an entire domain in a protein that we've given them a name. They're really folds. So Rossman fold and a Tim barrel fold. In the Rossman fold, this particular alteration between beta strand and helix actually solves another problem. Remember when I told you about these right-handed crossovers being very common in beta strands, right? So you're having one strand, the green there, and then I need to go back to the beginning so I can start on the second strand. It's very disadvantageous to just put a loop there because helices, sorry, residues that are just in a loop they would lose a ton of entropy in that they can't move, but they're not really gaining in the enthalpy because I'm not putting them in a favorable structure with lots of hydrogen bonds. But instead of having a free loop, I can put a secondary structure element there such as a helix. It's going to become very stable. And that's why we get this pattern of right-handed turns so that sheet, helix, sheet, helix, etc. So the reason why folds occur, it's not just chance. It is evolution and evolution has selected for them because they're nice and stable and not too entropically unfavorable. In fact, this is true for this Tim barrel too. Look at the Tim barrel. Here too, I have this edge of a beta sheet, right? It's going to be great for binding things. Here too, I actually have two binding sites. I have one binding site between the helices and the sheet layer and another binding site roughly on the very inside of the sheet. So both these structures have kind of evolved to be efficient binders. That is very common in particular for beta sheet and even more so for combinations of beta sheets and helices. So you can think of the entire structure creating a small crevice or well, cradle, something that can either bind a foreign molecule or recognize part of a different protein. And in the middle here, you see that too, that between those sheets at the edge of secondary structures, that's typically where we find binding sites for ligands.