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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 .
A piece of paper can be formed into a cylinder by bringing opposite sides together. The two edges come together to form a line. Shear can be created by sliding the two edges parallel to that line. Likewise, a beta barrel can be formed by bringing the edges of a beta sheet together to form a cylinder. If those edges are displaced, shear is created.
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.
The beta-propeller structure is stabilized mainly through hydrophobic interactions of the beta-sheets, while additional stability may come from hydrogen bonds formed between the beta-sheets of the C- and N-terminal ends. In effect this closes the circle which can occur even more strongly in 4-bladed proteins via a disulfide bond. [2]
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]
Beta-barrel proteins are so far found only in outer membranes of gram-negative bacteria, cell walls of gram-positive bacteria, outer membranes of mitochondria and chloroplasts, or can be secreted as pore-forming toxins. All beta-barrel transmembrane proteins have simplest up-and-down topology, which may reflect their common evolutionary origin ...
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The backbone switches repeatedly between the two β-sheets. Typically, the pattern is (N-terminal β-hairpin in sheet 1)-(β-hairpin in sheet 2)-(β-strand in sheet 1)-(C-terminal β-hairpin in sheet 2). The cross-overs between sheets form an "X", so that the N- and C-terminal hairpins are facing each other.