A beta helix is a tandem protein repeat structure formed by the association of parallel

beta strands Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labio ...

in a helical pattern with either two or three faces. The beta helix is a type of

solenoid protein domain Solenoid protein domains are a highly modular type of protein domain. They consist of a chain of nearly identical folds, often simply called tandem repeats. They are extremely common among all types of proteins, though exact figures are unknown. ...

. The structure is stabilized by inter-strand

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

s, protein-protein interactions, and sometimes bound metal

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s. Both left- and right-handed beta helices have been identified. These structures are distinct from

jelly-roll fold The jelly roll or Swiss roll fold is a protein fold or supersecondary structure composed of eight beta strands arranged in two four-stranded sheets. The name of the structure was introduced by Jane S. Richardson in 1981, reflecting its resemblanc ...

s, 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 ''Salmonella virus P22'' is a bacteriophage in the ''Podoviridae'' family that infects ''Salmonella typhimurium''. Like many phages, it has been used in molecular biology to induce mutations in cultured bacteria and to introduce foreign genet ...

tail spike protein, a component of the P22

bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacteri ...

, has 13 turns and in its assembled homo trimer 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 bridge In electrochemistry, a salt bridge or ion bridge is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell (voltaic cell), a type of electrochemical cell. It maintains electrical neutrality within the in ...

s. Both pectate lyase and P22 tailspike protein contain right-handed helices; left-handed versions have been observed in

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

s such as

UDP-N-acetylglucosamine acyltransferase Uridine diphosphate ''N''-acetylglucosamine or UDP-GlcNAc is a nucleotide sugar and a coenzyme in metabolism. It is used by glycosyltransferases to transfer ''N''-acetylglucosamine residues to substrates. D-Glucosamine is made naturally in the ...

and archaeal carbonic anhydrase. Other proteins that contain beta helices include the

antifreeze An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, all ...

proteins from the beetle ''

Tenebrio molitor Mealworms are the larval form of the yellow mealworm beetle, ''Tenebrio molitor'', a species of darkling beetle. Like all holometabolic insects, they go through four life stages: egg, larva, pupa, and adult. Larvae typically measure about or ...

'' (right-handed) and from the

spruce budworm ''Choristoneura'' is a genus of moths in the family Tortricidae. Several species are serious pests of conifers, such as spruce and are known as spruce budworms. Species *'' Choristoneura adumbratanus'' (Walsingham, 1900) *'' Choristoneura afri ...

, ''

Choristoneura fumiferana ''Choristoneura fumiferana'', the eastern spruce budworm, is a species of moth of the family Tortricidae native to the eastern United States and Canada. The caterpillars feed on the needles of spruce and fir trees. Eastern spruce budworm po ...

'' (left-handed), where regularly spaced

threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO� ...

s on the β-helices bind to the surface of

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...

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 A coiled coil is a structural motif in proteins in which 2–7 alpha-helices are coiled together like the strands of a rope. (Dimers and trimers are the most common types.) Many coiled coil-type proteins are involved in important biological fu ...

segments. Members of the

pentapeptide repeat Pentapeptide repeats are a family of sequence motifs found in multiple tandem copies in protein molecules. Pentapeptide repeat proteins are found in all species, but they are found in many copies in cyanobacterial genomes. The repeats were first ...

family have been shown to possess a quadrilateral beta-helix structure.

References

External links

SCOP family of right-handed β-helices

CATH β-helix protein family
{{Spirals Protein folds Helices