The Beta-pleated sheet is a series of anti-parallel chains of covalently-linked amino acids, with adjacent chains linked by hydrogen bonds. The β-sheet is a common motif of regular secondary structure in proteins.
- The regular folding of each amino acid chain leads to a regular pleated pattern across chains
- The R-groups are directed perpendicularly to the plane of the sheet
- Beta sheets consist of beta strands (also β-strand) 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 parallel arrangement is less stable because the geometry of the individual amino acid molecules forces the hydrogen bonds to occur at an angle, making them longer and thus weaker
- Strands are not fully extended but have a zig-zag shape, which gives the sheet formation, in both parallel and antiparallel structures, a pleated appearance when viewed edge-on
- The side chains from the amino acid residues found in a β-sheet structure may also be arranged such that many of the adjacent side chains on one side of the sheet are hydrophobic, while many of those adjacent to each other on the alternate side of the sheet are polar or charged
Parallel and antiparallel arrangement is the direct consequence of the directionality of the polypeptide chain. In antiparallel arrangement, the C-terminus end of one segment is on the same side as the N-terminus end of the other segment. In parallel arrangement, the C-terminus end and the N-terminus end are on the same sides for both segments. The “pleat” occurs because of the alternating planes of the peptide bonds between amino acids; the aligned amino and carbonyl group of each opposite segment alternate their orientation from facing towards each other to facing opposite directions.
Commonly, an antiparallel beta-pleated sheet forms when a polypeptide chain sharply reverses direction. This can occur in the presence of two consecutive proline residues, which create an angled kink in the polypeptide chain and bend it back upon itself.
β-Strands paired together to form small β-sheets are also found in sequence-specific DNA interfaces. In these complexes, the β-sheets are present flat within the major groove, where side chains on the exposed surface of the sheet interact with functional groups on the edges of neighboring base pairs.
- Arc repressor–DNA complex
- TATA-box binding protein (TBP)
Larger β-sheets take on a rigid, slightly twisted structure and their insertion into either the major or minor groove requires considerable distortion of the DNA helix.
Some proteins that are disordered or helical as monomers, such as amyloid β (see amyloid plaque) can form β-sheet-rich oligomeric structures associated with pathological states. The amyloid β protein’s oligomeric form is implicated as a cause of Alzheimer’s. Its structure has yet to be determined in full, but recent data suggest that it may resemble an unusual two-strand β-helix.