In our example, we're going to identify all the loops and beta turns in the figures. And they're able to do that because they're stabilized by fixed internal hydrogen bonds, unlike the loops and so down below. On the other hand, which are also known as reverse turns, they are small loops which have less than four amino acid residues, and they cause abrupt changes and backbone direction. Now down below, we're going to distinguish between the loops and beta turns, and so loops are essentially large links of amino acid, um, of their large lengths of amino acids that cause changes and backbone direction without fixed internal hydrogen bonds so they do not use fix internal hydrogen bonds. And so, thankfully to beta turns and loops that allow for changes in backbone direction that allows us toe have proteins that have folded compact shapes. Otherwise it would continue in one direction, and it would not have a compact shape. They allow for the proteins to take on a folded compact shape, and that's because you could imagine a protein that has a folded compact shape is going to change the backbone direction. And so, by being on the surface of proteins that allows the Hydra Filic amino acid residues, Thio easily interact with their acquis environment that most proteins are sitting in and also beta turns and loops. And so beta turns and loops are usually found on the surface of proteins with hydro filic, amino acid residues. But beta turns and loops do cause the peptide backbone to change directions. All we mean is that they don't have that same repetitive, periodic structure that Alfa Ulysses and beta sheets have. So beta turns and loops are a type of non repetitive secondary structure and by non repetitive. These results suggest that backbone hydrogen bonding is critical for the assembly of amyloid fibrils.So now that we've talked about Alfa Helix and beta sheet secondary structures in this video, we're going to talk about another type of secondary structure the beta turns and loops. Abeta16-20e also inhibits the aggregation of the Abeta1-40 peptide and disassembles preformed Abeta1-40 fibrils. Analytical ultracentrifugation experiments demonstrate that this ester peptide, Abeta16-20e, is predominantly monomeric under solution conditions, unlike the fibril-forming Abeta16-20 peptide. The ester bonds were incorporated in an alternating fashion so that the peptide presents two unique hydrogen bonding faces when arrayed in an extended, beta-strand conformation one face of the peptide has normal hydrogen bonding capabilities, but the other face is missing amide protons and its ability to hydrogen bond is severely limited. In this paper, we report that interfering with the backbone hydrogen bonding of an amyloidgenic peptide (Abeta16-20) by replacing amide bonds with ester bonds prevents the aggregation of the peptide. Evidence also suggests that the macromolecular assemblies of peptides and proteins in amyloid fibrils are stabilized by intermolecular beta-sheets. A number of cytokines and the HIV Protease, for example, dimerize through beta-sheet motifs. Protein-protein interactions are frequently mediated by stable, intermolecular beta-sheets.
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