Parallel vs antiparallel beta-sheets

Pauling and Corey derived a model for the conformation of fibrous proteins known as beta-keratins. In this conformation the polypeptide does not form a coil. Instead, it zig-zags in a more extended conformation than the alpha-helix. Amino acid residues in the beta-conformation have negative phi angles and the psi angles are positive. Typical values are phi = -140 degrees and psi = 130 degrees. In contrast, alpha-helical residues have both phi and psi negative. 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 a beta-sheet two or more polypeptide chains run alongside each other and are linked in a regular manner by hydrogen bonds between the main chain C=O and N-H groups. Therefore all hydrogen bonds in a beta-sheet are between different segments of polypeptide. This contrasts with the alpha-helix where all hydrogen bonds involve the same element of secondary structure. The R-groups (side chains) of neighbouring residues in a beta-strand point in opposite directions. 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. Note that peptide groups of adjacent residues point in opposite directions whereas with alpha-helices the peptide bonds all point one way. The axial distance between adjacent residues is 3.5 Angstroms. There are two residues per repeat unit which gives the beta-strand a 7 Angstrom pitch. This compares with the alpha-helix where the axial distance between adjacent residues is only 1.5 Angstroms. Clearly, polypeptides in the beta-conformation are far more extended than those in the alpha-helical conformation. Parallel, Antiparallel and Mixed Beta-Sheets. In parallel beta-sheets the strands all run in one direction, whereas in antiparallel sheets they all run in opposite directions. In mixed sheets some strands are parallel and others are antiparallel. Problem: Which of the following are true of beta sheet structures in a protein? A) Beta sheets are formed by one or multiple polypeptide chains B) Beta sheets are secondary structures in proteins C) In a beta sheet, polypeptide strands can be parallel or antiparallel D) In beta sheets hydrogen bonds connect polypeptide chains #parallelBetasheets #antiparallelBetasheets #secondaryStructure #protein #aminoAcid #polypeptide