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A beta helix is a protein structure formed by the association of parallel beta strands in a helical pattern with either two or three faces. The structure is stabilized by inter-strand hydrogen bonds, protein-protein interactions, and sometimes bound metal ions. Both left- and right-handed beta helices have been identified.
Additional recommended knowledge
The simplest beta helix contains two "layers" of beta sheets connected by glycine-rich six-residue loops that invariably contain an aspartate to bind one calcium ion per loop. Each layer consists of a nearly-planar series of parallel hydrogen-bonded beta strands and the two layers together enclose a hydrophobic core.
Three-stranded beta helices form a distorted triangular prism shape in which each face exhibits parallel inter-strand hydrogen bonding. One of the three sheets that form the repeating structural motif can appear "bent" relative to the other two, which face each other as in the two-stranded helix. Two of the three linking loops between the sheets can be of arbitrary length and can even contain other structural domains; the third is restricted to two resides. A characteristic common hexapeptide repeat found in both left- and right-handed helices is the sequence [LIV] − [GAED] − X2 − [STAV] − X. Known three-stranded helices are appreciably longer than their two-stranded counterparts.
The first beta-helix was observed in the enzyme pectate lyase, which contains a seven-turn helix that reaches 34 Å (3.4 nm) long. The P22 phage tailspike protein, a component of the P22 bacteriophage, has 13 turns and in its assembled homotrimer is 200 Å (20 nm) in length. Its interior is close-packed with no central pore and contains both hydrophobic residues and charged residues neutralized by salt bridges.
Both pectate lyase and P22 tailspike protein contain right-handed helices; left-handed versions have been observed in enzymes such as UDP-N-acetylglucosamine acyltransferase and archaeal carbonic anhydrase. Other proteins that contain beta helices include the antifreeze proteins from the beetle Tenebrio molitor (right-handed) and from the spruce budworm, Choristoneura fumiferana (left-handed), where regularly spaced threonines on the β-helices bind to the surface of ice crystals and inhibit their growth.
Beta helices can associate with each other effectively, either face-to-face (mating the faces of their triangular prisms) or end-to-end (forming hydrogen bonds). Hence, β-helices can be used as "tags" to induce other proteins to associate, similar to coiled coil segments.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Beta_helix". A list of authors is available in Wikipedia.|