Oxygen atoms are colored red and nitrogen atoms colored blue.īecause peptide chains have a directionality conferred by their N-terminus and C-terminus, β-strands too can be said to be directional. Parallel β-sheet hydrogen bonding patterns, represented by dotted lines. The "sideways" distance between adjacent C α atoms in hydrogen-bonded β-strands is roughly 5 Å (0.50 nm). I + 2 to be approximately 6 Å (0.60 nm), rather than the 7.6 Å (0.76 nm) expected from two fully extended trans peptides. The pleating causes the distance between C α The "pleated" appearance of β-strands arises from tetrahedral chemical bonding at the C α atom for example, if a side chain points straight up, then the bonds to the C′ must point slightly downwards, since its bond angle is approximately 109.5°. Adjacent β-strands in a β-sheet are aligned so that their C α atoms are adjacent and their side chains point in the same direction. In the fully extended β-strand, successive side chains point straight up and straight down in an alternating pattern. The majority of β-strands are arranged adjacent to other strands and form an extensive hydrogen bond network with their neighbors in which the N−H groups in the backbone of one strand establish hydrogen bonds with the C=O groups in the backbone of the adjacent strands. Their model incorporated the planarity of the peptide bond which they previously explained as resulting from keto-enol tautomerization. A refined version was proposed by Linus Pauling and Robert Corey in 1951. However, Astbury did not have the necessary data on the bond geometry of the amino acids in order to build accurate models, especially since he did not then know that the peptide bond was planar. He proposed the idea of hydrogen bonding between the peptide bonds of parallel or antiparallel extended β-strands. The first β-sheet structure was proposed by William Astbury in the 1930s. b) Edge-on view of the central two β-strands in a, showing the righthanded twist and the pleat of C αs and sidechains that alternately stick out in opposite directions from the sheet. Oxygen atoms are red balls, nitrogen atoms are blue, and hydrogen atoms are omitted for simplicity sidechains are shown only out to the first sidechain carbon atom (green). Arrows indicate chain direction, and electron density contours outline the non-hydrogen atoms. a) Front view, showing the antiparallel hydrogen bonds (dotted) between peptide NH and CO groups on adjacent strands. An example of a 4-stranded antiparallel β-sheet fragment from a crystal structure of the enzyme catalase ( PDB file 1GWE at 0.88 Å resolution).
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