Beta helix

    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.

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Two-stranded helices

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 helices

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.

References

• Branden C, Tooze J. (1999). Introduction to Protein Structure 2nd ed. Garland Publishing: New York, NY. pp 84-6.
• Dicker IB and Seetharam S. (1992) "What is Known about the Structure and Function of the Eschericia-coli Protein FirA" Mol. Microbiol., 6, 817-823.
• Kisker C, Schindelin H, Alber BE, Ferry JG and Rees DC. (1996) "A left-handed β-helix revealed by the crystal structure of a carbonic anhydrase from the achaeon Methanosarcina thermophile", EMBO J., 15, 2323-2330. (Left-handed)
• Liou YC, Tocilj A, Davies PL and Jia Z. (2000) Mimicry of ice structure by surface hydroxyls and water of a beta-helix antifreeze protein." Nature, 406, 322-324.
• Leinala EK, Davies PL and Jia Z. (2002) "Crystal Structure of β-Helical Anitfreeze Protein Points to a General Ice Binding Model", Structure, 10, 619-627.
• Raetz CRH and Roderick SL. (1995) "A Left-Handed Parallel β Helix in the Structure of UDP-N-Acetylglucosamine Acyltransferase", Science, 270, 997-1000. (Left-handed)
• Steinbacher S, Seckler R, Miller S, Steipe B, Huber R and Reinemer P. (1994) "Crystal structure of P22 tailspike protein: interdigitated subunits in a thermostable trimer", Science, 265, 383-386. (Right-handed)
• Vaara M. (1992) "Eight bacterial proteins, including UDP-N-acetylglucosamine acyltransferase (LpxA) and three other transferases of Escherichia coli, consist of a six-residue periodicity theme", FEMS Microbiol. Lett, 97, 249-254.
• Yoder MD, Keen NT and Jurnak F. (1993) "New domain motif:the structure of pectate lyase C, a secreted plant virulence factor", Science, 260, 1503-1507. (Right-handed)