12/14/2023 0 Comments Beta sheet peptide backbone![]() ![]() Parallel sheets are less twisted than antiparallel and are always buried. The overall twisting of the sheet results from a relative rotation of each residue in the strands by 30 degrees per amino acid in a right-handed sense. This twist is left-handed as shown below. In globular protein X-ray structures are twisted. The Pauling-Corey model of the beta-sheet is planar. ![]() Sheets tend to be either all parallel or all antiparallel, but mixed sheets do occur. No preference for parallel or antiparallel beta-sheets is observed, but parallel sheets with less than four strands are rare, perhaps reflecting their lower stability. The width of a six-stranded beta-sheet is approximately 25 Angstroms. In the classical Pauling-Corey models the parallel beta-sheet has somewhat more distorted and consequently weaker hydrogen bonds between the strands.īeta-sheets are very common in globular proteins and most contain less than six It emphasises the highly regular pattern of hydrogen bonds between the main chain NH and CO groups of the constituent strands. In mixed sheets some strands are parallel and others are antiparallel.īelow is a diagram of a three-stranded antiparallel beta-sheet. In parallel beta-sheets the strands all run in one direction, whereas in antiparallel sheets they all run in opposite directions. Parallel, Antiparallel and Mixed Beta-Sheets. Clearly, polypeptides in the beta-conformation are far more extended than those in the alpha-helical conformation. This compares with the alpha-helix where the axial distance between adjacent residues is only 1.5 Angstroms. There are two residues per repeat unit which gives the beta-strand a 7 Angstrom pitch. The axial distance between adjacent residues is 3.5 Angstroms. Note that peptide groups of adjacent residues point in opposite directions whereas with alpha-helices the peptide bonds all point one way. ![]() Imagining two strands parallel to this, one above the plane of the screen and one behind, it is possible to grasp how the pleated appearance of the beta-sheet arises. With RasMol installed you can click here for the coordinate file of a single strand. The R-groups (side chains) of neighbouring residues in a beta-strand point in opposite directions. This contrasts with the alpha-helix where all hydrogen bonds involve the same element of secondary structure. Therefore all hydrogen bonds in a beta-sheet are between different segments of polypeptide. In a regular manner by hydrogen bonds between the main chain C=O and N-H groups. In a beta-sheet two or more polypeptide chains run alongside each other and are linked A section of polypeptide with residues in the beta-conformation is refered to as a beta-strand and these strands can associate by main chain hydrogen bonding interactions to form a beta sheet. In contrast, alpha-helical residues have both phi and psi negative. Typical values are phi = -140 degrees and psi = 130 degrees. Amino acid residues in the beta-conformation have negative phi angles and the psi angles are positive. Instead, it zig-zags in a more extended conformation than the alpha-helix. In this conformation the polypeptide does not form a coil. Pauling and Corey derived a model for the conformation of fibrous proteins known as beta-keratins. Beta-Sheet Geometry Beta-Sheet Geometry Back to Index ![]()
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