Beta helix


A beta helix is a tandem protein repeat structure formed by the association of parallel beta strands 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. Double stranded beta-helices are also very common features of proteins and are generally synonymous with jelly roll folds.
The first beta-helix was observed in the enzyme pectate lyase, which contains a seven-turn helix that reaches 34 Å long. The P22 phage tail spike protein, a component of the P22 bacteriophage, has 13 turns and in its assembled homotrimer is 200 Å 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 and from the spruce budworm, Choristoneura fumiferana, 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 or end-to-end. 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.