A beta helix is a
tandem protein repeat structure formed by the association of parallel
beta sheet
The beta sheet (β-sheet, also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gene ...
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 Protein structure, folds, often simply called Protein tandem repeats, tandem repeats. They are extremely common among all types of pr ...
. The structure is stabilized by inter-strand
hydrogen bond
In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...
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 convent ...
s. 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
A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...
, 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 an essential laboratory device discovered over 100 years ago. It contains an electrolyte solution, typically an inert solution, used to connect the Redox, oxidation and reduction Half cell, ...
s.
Both pectate lyase and
P22 tailspike protein contain right-handed helices; left-handed versions have been observed in
enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s such as
UDP-N-acetylglucosamine acyltransferase and archaeal
carbonic anhydrase
The carbonic anhydrases (or carbonate dehydratases) () form a family of enzymes that catalyst, catalyze the interconversion between carbon dioxide and water and the Dissociation (chemistry), dissociated ions of carbonic acid (i.e. bicarbonate a ...
.
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, allow ...
proteins from the beetle ''
Tenebrio molitor
Mealworms are the larval form of the yellow mealworm beetle, ''Tenebrio molitor'', a species of darkling beetle.
The yellow mealworm beetle prefers a warmer climate and higher humidity. Male mealworm beetles release a sex pheromone to attract ...
'' (right-handed)
and from the
spruce budworm, ''
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 populat ...
'' (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 when dissolved in water), a carboxyl group (which is in the deprotonated −COO− ...
s on the β-helices bind to the surface of
ice
Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...
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.
References
External links
SCOP family of right-handed β-helicesCATH β-helix protein family
{{Spirals
Protein folds
Helices