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Beta turns are especially common at the loop ends of beta hairpins; they have a different distribution of types from the others; type I' is the most common, followed by types II', I and II. Additional turn types have been defined by clustering turn conformations within very high-resolution protein structures. [11]
A hairpin is a special case of a turn, in which the direction of the protein backbone reverses and the flanking secondary structure elements interact. For example, a beta hairpin connects two hydrogen-bonded , antiparallel β-strands (a rather confusing name, since a β-hairpin may contain many types of turns – α, β, γ, etc.).
The secondary structure is assigned based on hydrogen bonding patterns as those initially proposed by Pauling et al. in 1951 (before any protein structure had ever been experimentally determined). There are eight types of secondary structure that DSSP defines: G = 3-turn helix (3 10 helix). Min length 3 residues. H = 4-turn helix . Minimum ...
Beta bend ribbons may be formed from any of these types but type I is the commonest in proteins, as it is for single beta turns. Beta bend ribbons made from type I or I’ turns are somewhat twisted, while beta bend ribbons made from type II or II’ beta turns are flat. Beta bend ribbons with mixtures of different beta turn types also occur.
The beta strands are parallel, and the helix is also almost parallel to the strands. This structure can be seen in almost all proteins with parallel strands. The loops connecting the beta strands and alpha helix can vary in length and often binds ligands. Beta-alpha-beta helices can be either left-handed or right-handed.
Beta hairpin Extremely common. Two antiparallel beta strands connected by a tight turn of a few amino acids between them. Greek key Four beta strands, three connected by hairpins, the fourth folded over the top. Omega loop A loop in which the residues that make up the beginning and end of the loop are very close together. [11] Helix-loop-helix
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The beta sheet (β-sheet, also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands ( β-strands ) connected laterally by at least two or three backbone hydrogen bonds , forming a generally twisted, pleated sheet.