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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. A β-strand is a stretch of polypeptide chain typically 3 to 10 amino acids long with backbone in an extended conformation .
All beta-barrels can be classified in terms of two integer parameters: the number of strands in the beta-sheet, n, and the "shear number", S, a measure of the stagger of the strands in the beta-sheet. [3] These two parameters (n and S) are related to the inclination angle of the beta strands relative to the axis of the barrel. [4] [5] [6]
In biology literature, the term topology is also used to refer to mutual orientation of regular secondary structures, such as alpha-helices and beta strands in protein structure [3]. For example, two adjacent interacting alpha-helices or beta-strands can go in the same or in opposite directions.
The two most common secondary structural elements are alpha helices and beta sheets, though beta turns and omega loops occur as well. Secondary structure elements typically spontaneously form as an intermediate before the protein folds into its three dimensional tertiary structure.
Porins are composed of beta sheets (β sheets) made up of beta strands (β strands) which are linked together by beta turns (β turns) on the cytoplasmic side and long loops of amino acids on the other. The β strands lie in an antiparallel fashion and form a cylindrical tube, called a beta barrel (β barrel). [2]
The individual beta strands are labeled with their traditional designations (for historical reasons, sheet A is not used), highlighting the packing of the BIDG and CHEF four-stranded sheets. [ 1 ] The jelly roll or Swiss roll fold is a protein fold or supersecondary structure composed of eight beta strands arranged in two four-stranded sheets.
The Rossmann fold is a tertiary fold found in proteins that bind nucleotides, such as enzyme cofactors FAD, NAD +, and NADP +.This fold is composed of alternating beta strands and alpha helical segments where the beta strands are hydrogen bonded to each other forming an extended beta sheet and the alpha helices surround both faces of the sheet to produce a three-layered sandwich.
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