Beta helix

A beta helix is a tandem protein repeat structure formed by the association of parallel beta sheet in a helical pattern with either two or three faces. The beta helix is a type of solenoid protein domain. 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. These structures are distinct from jelly roll folds, a different protein structure sometimes known as a "double stranded beta helix". 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 tail spike 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. Members of the pentapeptide repeat family have been shown to possess a quadrilateral beta helix structure.