4. Protein Structure
Beta Turns
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So now that we've talked about alpha helix and beta sheet secondary structures, in this video we're gonna talk about another type of secondary structure. The beta turns and loops. So beta turns and loops are a type of non repetitive secondary structure. And by non repetitive, all we mean is that they don't have that same repetitive periodic structure that alpha helices and beta sheets have. But beta turns and loops do cause the peptide backbone to change directions. And so beta turns and loops are usually found on the surface of proteins with hydrophilic amino acid residues. And so by being on the surface of proteins, that allows the hydrophilic amino acid residues to easily interact with their aqueous environment that most proteins are sitting in. And also beta turns and loops, they allow for the proteins to take on a folded compact shape. And that's because you can imagine a protein that has a folded compact shape is going to change the backbone direction a lot. Otherwise, it would continue in one direction and it would not have a compact shape. And so, thankfully to beta turns and loops that allow for changes in backbone direction, that allows us to have proteins that have folded compact shapes. Now down below, we're going to distinguish between the loops and beta turns. And so loops are essentially large links of amino acid, of there are large links of amino acids that cause changes in backbone direction without fixed internal hydrogen bonds. So, they do not use fixed internal hydrogen bonds. Now, beta turns on the other hand, which are also known as reverse turns, they are small loops, which have less than 4 amino acid residues, and they cause abrupt changes in backbone direction. And they're able to do that because they're stabilized by fixed internal hydrogen bonds unlike the loops. And so down below in our example, we're going to identify all the loops and beta turns in the figures. And so with our first figure over here on the left, notice we have a single polypeptide chain, and this polypeptide chain has an n terminal n and a c terminal n. And you'll see we have beta sheets, alpha helices, and more beta sheets here, or beta strands. And so notice that this beta strand is connected to this alpha helix here via a loop. And we know that it's a loop because it has 20 amino acids in it, which is greater than 4 amino acids. So this is a large link of amino acids causing a change in the backbone direction. So notice the backbone direction is going in this direction here and then it's changing through this loop. So this is a loop here. And so in our next example over here, notice what we have is a beta turn and we know that this is a beta turn because we can see that the backbone is abruptly changing directions. It's doing a quick u-turn. And, we have a small link of amino acids here, less than 4 amino acids, and we can see that it's stabilized by this hydrogen bond here, an internal hydrogen bond, and so, that's a way to identify as a beta turn as well. And notice over here with the loop, there are no internal hydrogen bonds. And so over here in our image on the the far right, we have a single polypeptide chain that has a bunch of secondary structures in it. It has the yellow structures, which are beta sheets, it has the pink structures in here, which are alpha helices, and it has beta turns and loops. So notice that these grayish links here of long amino acids are the loops, so those are our loops that are causing changes in the backbone direction. And then these blue, these shorter blue, structures that you see throughout are our beta sheets and that's because they're causing abrupt changes in backbone direction with a small amount of amino acid residues. So we can label them as beta turns. And so moving forward, we'll be able to get some more practice with beta turns, and we'll talk about different types of beta turns that exist. So I'll see you guys in those videos.
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