page_title: Beta sheet
section_title: Geometry
hierarchical_section_title: Beta sheet / Structure and orientation / Geometry
caption_attribution_description: English: φ,ψ plot ("Ramachandran diagram") of the torsion angles that specify protein backbone conformation, with the alpha-helix and beta-strand regions labeled. Data as in Lovell et al. 2003 Proteins 50:437, showing about 100,000 data points for general amino-acid types (not Gly, Pro, or pre-Pro) in high-resolution crystal structures.
context_page_description: The β-sheet is a common motif of regular secondary structure in proteins. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet. A β-strand is a stretch of polypeptide chain typically 3 to 10 amino acids long with backbone in an extended conformation. The supramolecular association of β-sheets has been implicated in formation of the protein aggregates and fibrils observed in many human diseases, notably the amyloidoses such as Alzheimer's disease.
context_section_description: 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. In the fully extended β-strand, successive side chains point straight up, then straight down, then straight up, etc. Adjacent β-strands in a β-sheet are aligned so that their Cα atoms are adjacent and their side chains point in the same direction. The "pleated" appearance of β-strands arises from tetrahedral chemical bonding at the Cα 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°. The pleating causes the distance between Cα
  ᵢ and Cα
  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 "sideways" distance between adjacent Cα atoms in hydrogen-bonded β-strands is roughly 5 Å (0.50 nm).
  However, β-strands are rarely perfectly extended; rather, they exhibit a twist. The energetically preferred dihedral angles near (φ, ψ) = (–135°, 135°) (broadly, the upper left region of the Ramachandran plot) diverge significantly from the fully extended conformation (φ, ψ) = (–180°, 180°). The twist is often associated with alternating fluctuations in the dihedral angles to prevent the individual β-strands in a larger sheet from splaying apart. A good example of a strongly twisted β-hairpin can be seen in the protein BPTI.
  The side chains point outwards from the folds of the pleats, roughly perpendicularly to the plane of the sheet; successive amino acid residues point outwards on alternating faces of the sheet.
caption: Ramachandran (φ, ψ) plot of about 100,000 high-resolution data points, showing the broad, favorable region around the conformation typical for β-sheet amino acid residues.
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