![Animated Beta sheet](https://upload.wikimedia.org/wikipedia/commons/5/5e/Animated_Beta_sheet.gif)
The beta sheet, (β-sheet) (also β-pleated sheet) is a common
motif of the regular
protein secondary structure
Protein secondary structure is the three dimensional form of ''local segments'' of proteins. The two most common secondary structural elements are alpha helices and beta sheets, though beta turns and omega loops occur as well. Secondary structure ...
. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three
backbone
The backbone is the vertebral column of a vertebrate.
Arts, entertainment, and media Film
* ''Backbone'' (1923 film), a 1923 lost silent film starring Alfred Lunt
* ''Backbone'' (1975 film), a 1975 Yugoslavian drama directed by Vlatko Gilić
M ...
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, forming a generally twisted, pleated sheet. A β-strand is a stretch of
polypeptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A p ...
chain typically 3 to 10
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s long with backbone in an extended
conformation. The supramolecular association of β-sheets has been implicated in the formation of the
fibrils
Fibrils (from the Latin ''fibra'') are structural biological materials found in nearly all living organisms. Not to be confused with fibers or filaments, fibrils tend to have diameters ranging from 10-100 nanometers (whereas fibers are micro ...
and
protein aggregates observed in
amyloidosis
Amyloidosis is a group of diseases in which abnormal proteins, known as amyloid fibrils, build up in tissue. There are several non-specific and vague signs and symptoms associated with amyloidosis. These include fatigue, peripheral edema, weight ...
, notably
Alzheimer's disease
Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...
.
History
![1gwe antipar betaSheet both](https://upload.wikimedia.org/wikipedia/commons/9/9b/1gwe_antipar_betaSheet_both.png)
The first β-sheet structure was proposed by
William Astbury
William Thomas Astbury Fellow of the Royal Society, FRS (25 February 1898 – 4 June 1961) was an English physicist and molecular biology, molecular biologist who made pioneering X-ray crystallography, X-ray diffraction studies of biomolecule, b ...
in the 1930s. He proposed the idea of hydrogen bonding between the
peptide bond
In organic chemistry, a peptide bond is an amide type of covalent chemical bond linking two consecutive alpha-amino acids from C1 (carbon number one) of one alpha-amino acid and N2 (nitrogen number two) of another, along a peptide or protein cha ...
s of parallel or antiparallel extended β-strands. However, Astbury did not have the necessary data on the bond geometry of the amino acids in order to build accurate models, especially since he did not then know that the
peptide bond
In organic chemistry, a peptide bond is an amide type of covalent chemical bond linking two consecutive alpha-amino acids from C1 (carbon number one) of one alpha-amino acid and N2 (nitrogen number two) of another, along a peptide or protein cha ...
was planar. A refined version was proposed by
Linus Pauling
Linus Carl Pauling (; February 28, 1901August 19, 1994) was an American chemist, biochemist, chemical engineer, peace activist, author, and educator. He published more than 1,200 papers and books, of which about 850 dealt with scientific top ...
and
Robert Corey
Robert Brainard Corey (August 19, 1897 – April 23, 1971) was an American biochemist, mostly known for his role in discovery of the α-helix and the β-sheet with Linus Pauling. Also working with Pauling was Herman Branson. Their discoveries we ...
in 1951. Their model incorporated the planarity of the peptide bond which they previously explained as resulting from keto-enol
tautomerization
Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert.
The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydr ...
.
Structure and orientation
Geometry
The majority of β-strands are arranged adjacent to other strands and form an extensive
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 ...
network with their neighbors in which the
N−H groups in the backbone of one strand establish
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 with the
C=O groups in the backbone of the adjacent strands. In the fully extended β-strand, successive side chains point straight up and straight down in an alternating pattern. Adjacent β-strands in a β-sheet are aligned so that their C
α atoms are adjacent and their side chains point in the same direction. The "pleated" appearance of β-strands arises from tetrahedral chemical bonding at the C
α atom; for example, if a side chain points straight up, then the bonds to the C′ must point slightly downwards, since its bond angle is approximately 109.5°. The pleating causes the distance between C and C to be approximately , rather than the expected from two fully extended ''
trans
Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of".
Used alone, trans may refer to:
Arts, entertainment, and media
* Trans (festival), a former festival in Belfast, Northern Ireland, United Kingdom
* ''Trans'' (film ...
''
peptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A ...
s. The "sideways" distance between adjacent C
α atoms in
hydrogen-bonded β-strands is roughly .
![Ramachandran plot general 100K](https://upload.wikimedia.org/wikipedia/commons/9/90/Ramachandran_plot_general_100K.jpg)
However, β-strands are rarely perfectly extended; rather, they exhibit a twist. The energetically preferred
dihedral angle
A dihedral angle is the angle between two intersecting planes or half-planes. In chemistry, it is the clockwise angle between half-planes through two sets of three atoms, having two atoms in common. In solid geometry, it is defined as the uni ...
s near (''φ'', ''ψ'') = (–135°, 135°) (broadly, the upper left region of the
Ramachandran plot
In biochemistry, a Ramachandran plot (also known as a Rama plot, a Ramachandran diagram or a ,ψplot), originally developed in 1963 by G. N. Ramachandran, C. Ramakrishnan, and V. Sasisekharan, is a way to visualize energetically allowed region ...
) diverge significantly from the fully extended conformation (''φ'', ''ψ'') = (–180°, 180°). The twist is often associated with alternating fluctuations in the
dihedral angle
A dihedral angle is the angle between two intersecting planes or half-planes. In chemistry, it is the clockwise angle between half-planes through two sets of three atoms, having two atoms in common. In solid geometry, it is defined as the uni ...
s to prevent the individual β-strands in a larger sheet from splaying apart. A good example of a strongly twisted β-hairpin can be seen in the protein
BPTI
The drug aprotinin (Trasylol, previously Bayer and now Nordic Group pharmaceuticals), is a small protein bovine pancreatic trypsin inhibitor (BPTI), or basic trypsin inhibitor of bovine pancreas, which is an antifibrinolytic molecule that inhibit ...
.
The side chains point outwards from the folds of the pleats, roughly perpendicularly to the plane of the sheet; successive amino acid residues point outwards on alternating faces of the sheet.
Hydrogen bonding patterns
Because peptide chains have a directionality conferred by their
N-terminus
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
and
C-terminus
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
, β-strands too can be said to be directional. They are usually represented in protein topology diagrams by an arrow pointing toward the C-terminus. Adjacent β-strands can form
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 in antiparallel, parallel, or mixed arrangements.
In an antiparallel arrangement, the successive β-strands alternate directions so that the N-terminus of one strand is adjacent to the C-terminus of the next. This is the arrangement that produces the strongest inter-strand stability because it allows the inter-strand hydrogen bonds between carbonyls and amines to be planar, which is their preferred orientation. The peptide backbone dihedral angles (''φ'', ''ψ'') are about (–140°, 135°) in antiparallel sheets. In this case, if two atoms C and C are adjacent in two
hydrogen-bonded β-strands, then they form two mutual backbone hydrogen bonds to each other's flanking
peptide groups; this is known as a close pair of hydrogen bonds.
In a parallel arrangement, all of the N-termini of successive strands are oriented in the same direction; this orientation may be slightly less stable because it introduces nonplanarity in the inter-strand hydrogen bonding pattern. The dihedral angles (''φ'', ''ψ'') are about (–120°, 115°) in parallel sheets. It is rare to find less than five interacting parallel strands in a motif, suggesting that a smaller number of strands may be unstable, however it is also fundamentally more difficult for parallel β-sheets to form because strands with N and C termini aligned necessarily must be very distant in sequence . There is also evidence that parallel β-sheet may be more stable since small amyloidogenic sequences appear to generally aggregate into β-sheet fibrils composed of primarily parallel β-sheet strands, where one would expect anti-parallel fibrils if anti-parallel were more stable.
In parallel β-sheet structure, if two atoms C and C are adjacent in two
hydrogen-bonded β-strands, then they do ''not'' hydrogen bond to each other; rather, one residue forms hydrogen bonds to the residues that flank the other (but not vice versa). For example, residue ''i'' may form hydrogen bonds to residues ''j'' − 1 and ''j'' + 1; this is known as a wide pair of hydrogen bonds. By contrast, residue ''j'' may hydrogen-bond to different residues altogether, or to none at all.
The hydrogen bond arrangement in parallel beta sheet resembles that in an
amide ring
Amide Rings are small motifs in proteins and polypeptides. They consist of 9-atom or 11-atom rings formed by two CO...HN hydrogen bonds between a side chain amide group and the main chain atoms of a short polypeptide. They are observed with gluta ...
motif with 11 atoms.
Finally, an individual strand may exhibit a mixed bonding pattern, with a parallel strand on one side and an antiparallel strand on the other. Such arrangements are less common than a random distribution of orientations would suggest, suggesting that this pattern is less stable than the anti-parallel arrangement, however bioinformatic analysis always struggles with extracting structural thermodynamics since there are always numerous other structural features present in whole proteins. Also proteins are inherently constrained by folding kinetics as well as folding thermodynamics, so one must always be careful in concluding stability from bioinformatic analysis.
The
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 ...
ing of β-strands need not be perfect, but can exhibit localized disruptions known as
β-bulges.
The hydrogen bonds lie roughly in the plane of the sheet, with the
peptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A ...
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
groups pointing in alternating directions with successive residues; for comparison, successive carbonyls point in the ''same'' direction in the
alpha helix
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues e ...
.
Amino acid propensities
Large aromatic residues (
tyrosine
-Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...
,
phenylalanine
Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amino a ...
,
tryptophan
Tryptophan (symbol Trp or W)
is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α- carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...
) and β-branched amino acids (
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 ...
,
valine
Valine (symbol Val or V) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α- carboxylic acid group (which is in the deprotonat ...
,
isoleucine
Isoleucine (symbol Ile or I) is an α-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), an α-carboxylic acid group (which is in the deprot ...
) are favored to be found in β-strands in the ''middle'' of β-sheets. Different types of residues (such as
proline
Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the prot ...
) are likely to be found in the ''edge'' strands in β-sheets, presumably to avoid the "edge-to-edge" association between proteins that might lead to aggregation and
amyloid
Amyloids are aggregates of proteins characterised by a Fibril, fibrillar morphology of 7–13 Nanometer, nm in diameter, a beta sheet (β-sheet) Secondary structure of proteins, secondary structure (known as cross-β) and ability to be Staining, ...
formation.
Common structural motifs
β-hairpin motif
A very simple
structural motif
In a polymer, chain-like biological molecule, such as a protein or nucleic acid, a structural motif is a common Biomolecular structure#Tertiary structure, three-dimensional structure that appears in a variety of different, evolutionarily unrel ...
involving β-sheets is the
β-hairpin, in which two antiparallel strands are linked by a short loop of two to five residues, of which one is frequently a
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
or a
proline
Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the prot ...
, both of which can assume the dihedral-angle conformations required for a tight
turn or a
β-bulge loop. Individual strands can also be linked in more elaborate ways with longer loops that may contain
α-helices
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...
.
Greek key motif
The Greek key motif consists of four adjacent antiparallel strands and their linking loops. It consists of three antiparallel strands connected by hairpins, while the fourth is adjacent to the first and linked to the third by a longer loop. This type of structure forms easily during the
protein folding
Protein folding is the physical process by which a protein chain is translated to its native three-dimensional structure, typically a "folded" conformation by which the protein becomes biologically functional. Via an expeditious and reproduci ...
process.
It was named after a pattern common to Greek ornamental artwork (see
meander
A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank ( cut bank) and deposits sediments on an inner, convex bank ...
).
β-α-β motif
Due to the chirality of their component amino acids, all strands exhibit right-handed twist evident in most higher-order β-sheet structures. In particular, the linking loop between two parallel strands almost always has a right-handed crossover chirality, which is strongly favored by the inherent twist of the sheet. This linking loop frequently contains a helical region, in which case it is called a
β-α-β motif. A closely related motif called a β-α-β-α motif forms the basic component of the most commonly observed protein
tertiary structure
Protein tertiary structure is the three dimensional shape of a protein. The tertiary structure will have a single polypeptide chain "backbone" with one or more protein secondary structures, the protein domains. Amino acid side chains may int ...
, the
TIM barrel
The TIM barrel (triose-phosphate isomerase), also known as an alpha/beta barrel, is a conserved protein fold consisting of eight alpha helices (α-helices) and eight parallel beta strands (β-strands) that alternate along the peptide backbone. ...
.
β-meander motif
A simple
supersecondary protein topology composed of two or more consecutive antiparallel β-strands linked together by
hairpin
A hairpin or hair pin is a long device used to hold a person's hair in place. It may be used simply to secure long hair out of the way for convenience or as part of an elaborate hairstyle or coiffure. The earliest evidence for dressing the hai ...
loops. This motif is common in β-sheets and can be found in several structural architectures including
β-barrels and
β-propellers.
The vast majority of β-meander regions in proteins are found packed against other motifs or sections of the polypeptide chain, forming portions of the hydrophobic core that canonically drives formation of the folded structure. However, several notable exceptions include the Outer Surface Protein A (OspA) variants
and the Single Layer β-sheet Proteins (SLBPs) which contain single-layer β-sheets in the absence of a traditional hydrophobic core. These β-rich proteins feature an extended single-layer β-meander β-sheets that are primarily stabilized via inter-β-strand interactions and hydrophobic interactions present in the turn regions connecting individual strands.
Psi-loop motif
The psi-loop (Ψ-loop) motif consists of two antiparallel strands with one strand in between that is connected to both by hydrogen bonds. There are four possible strand topologies for single Ψ-loops.
This motif is rare as the process resulting in its formation seems unlikely to occur during protein folding. The Ψ-loop was first identified in the
aspartic protease
Aspartic proteases are a catalytic type of protease enzymes that use an activated water molecule bound to one or more aspartate residues for catalysis of their peptide substrates. In general, they have two highly conserved aspartates in the active ...
family.
Structural architectures of proteins with β-sheets
β-sheets are present in
all-β,
α+β and
α/β domains, and in many
peptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A ...
s or small proteins with poorly defined overall architecture.
All-β domains may form
β-barrels,
β-sandwich
Beta-sandwich, β-sandwich domains consisting of 80 to 350 amino acids occur commonly in proteins. They are characterized by two opposing antiparallel beta sheets (β-sheets). The number of strands found in such domains may differ from one protein ...
es, β-prisms,
β-propellers, and
β-helices.
Structural topology
The topology of a β-sheet describes the order of
hydrogen-bonded β-strands along the backbone. For example, the
flavodoxin fold 300px, Ribbon diagram of CheY (a regulator of the chemotactic response in bacteria, PDB accession code 3CHY), which adopts the flavodoxin fold. Ribbon is colored from blue (N-terminus) to red (C-terminus).
The flavodoxin fold is a common α/β pr ...
has a five-stranded, parallel β-sheet with topology 21345; thus, the edge strands are β-strand 2 and β-strand 5 along the backbone. Spelled out explicitly, β-strand 2 is H-bonded to β-strand 1, which is H-bonded to β-strand 3, which is H-bonded to β-strand 4, which is H-bonded to β-strand 5, the other edge strand. In the same system, the Greek key motif described above has a 4123 topology. The
secondary structure
Protein secondary structure is the three dimensional conformational isomerism, form of ''local segments'' of proteins. The two most common Protein structure#Secondary structure, secondary structural elements are alpha helix, alpha helices and beta ...
of a β-sheet can be described roughly by giving the number of strands, their topology, and whether their
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 are parallel or antiparallel.
β-sheets can be ''open'', meaning that they have two edge strands (as in the
flavodoxin fold 300px, Ribbon diagram of CheY (a regulator of the chemotactic response in bacteria, PDB accession code 3CHY), which adopts the flavodoxin fold. Ribbon is colored from blue (N-terminus) to red (C-terminus).
The flavodoxin fold is a common α/β pr ...
or the
immunoglobulin fold) or they can be ''closed
β-barrels'' (such as the
TIM barrel
The TIM barrel (triose-phosphate isomerase), also known as an alpha/beta barrel, is a conserved protein fold consisting of eight alpha helices (α-helices) and eight parallel beta strands (β-strands) that alternate along the peptide backbone. ...
).
β-Barrels are often described by their ''stagger'' or ''shear''. Some open β-sheets are very curved and fold over on themselves (as in the
SH3 domain
The SRC Homology 3 Domain (or SH3 domain) is a small protein domain of about 60 amino acid residues. Initially, SH3 was described as a conserved sequence in the viral adaptor protein v-Crk. This domain is also present in the molecules of phos ...
) or form horseshoe shapes (as in the
ribonuclease inhibitor
Ribonuclease inhibitor (RI) is a large (~450 residues, ~49 kDa), acidic (pI ~4.7), leucine-rich repeat protein that forms extremely tight complexes with certain ribonucleases. It is a major cellular protein, comprising ~0.1% of all cellular prot ...
). Open β-sheets can assemble face-to-face (such as the
β-propeller domain or
immunoglobulin fold) or edge-to-edge, forming one big β-sheet.
Dynamic features
β-pleated sheet structures are made from extended β-strand polypeptide chains, with strands linked to their neighbours by
hydrogen bonds
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 ...
. Due to this extended backbone conformation, β-sheets resist
stretching
Stretching is a form of physical exercise in which a specific muscle or tendon (or muscle group) is deliberately flexed or stretched in order to improve the muscle's felt elasticity and achieve comfortable muscle tone. The result is a feeling ...
. β-sheets in proteins may carry out
low-frequency accordion-like motion as observed by the
Raman spectroscopy
Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...
and analyzed with the quasi-continuum model.
Parallel β-helices
![1QRE L3betaHelix](https://upload.wikimedia.org/wikipedia/commons/5/5e/1QRE_L3betaHelix.jpg)
A
β-helix is formed from repeating structural units consisting of two or three short β-strands linked by short loops. These units "stack" atop one another in a helical fashion so that successive repetitions of the same strand hydrogen-bond with each other in a parallel orientation. See the
β-helix article for further information.
In lefthanded β-helices, the strands themselves are quite straight and untwisted; the resulting helical surfaces are nearly flat, forming a regular
triangular prism
In geometry, a triangular prism is a three-sided prism; it is a polyhedron made of a triangular base, a translated copy, and 3 faces joining corresponding sides. A right triangular prism has rectangular sides, otherwise it is ''oblique''. A unif ...
shape, as shown for the 1QRE archaeal carbonic anhydrase at right. Other examples are the lipid A synthesis enzyme
LpxA and insect antifreeze proteins with a regular array of Thr sidechains on one face that mimic the structure of ice.
![Pectate lyase R3betaHelix](https://upload.wikimedia.org/wikipedia/commons/e/e8/Pectate_lyase_R3betaHelix.jpg)
Righthanded β-helices, typified by the
pectate lyase enzyme shown at left or
P22 phage tailspike protein, have a less regular cross-section, longer and indented on one of the sides; of the three linker loops, one is consistently just two residues long and the others are variable, often elaborated to form a binding or active site.
A two-sided β-helix (right-handed) is found in some bacterial
metalloprotease
A metalloproteinase, or metalloprotease, is any protease enzyme whose catalytic mechanism involves a metal. An example is ADAM12 which plays a significant role in the fusion of muscle cells during embryo development, in a process known as myogen ...
s; its two loops are each six residues long and bind stabilizing calcium ions to maintain the integrity of the structure, using the backbone and the Asp side chain oxygens of a GGXGXD sequence motif.
This fold is called a β-roll in the SCOP classification.
In pathology
Some proteins that are disordered or helical as monomers, such as amyloid β (see
amyloid plaque
Amyloid plaques (also known as neuritic plaques, amyloid beta plaques or senile plaques) are extracellular deposits of the amyloid beta (Aβ) protein mainly in the grey matter of the brain. Degenerative neuronal elements and an abundance of micr ...
) can form β-sheet-rich oligomeric structures associated with pathological states. The amyloid β protein's oligomeric form is implicated as a cause of
Alzheimer's
Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As t ...
. Its structure has yet to be determined in full, but recent data suggest that it may resemble an unusual two-strand β-helix.
The side chains from the amino acid residues found in a β-sheet structure may also be arranged such that many of the adjacent sidechains on one side of the sheet are hydrophobic, while many of those adjacent to each other on the alternate side of the sheet are polar or charged (hydrophilic),
which can be useful if the sheet is to form a boundary between polar/watery and nonpolar/greasy environments.
See also
*
Collagen helix
In molecular biology, the collagen triple helix or type-2 helix is the main secondary structure of various types of fibrous collagen, including type I collagen. In 1954, Ramachandran & Kartha (13, 14) advanced a structure for the collagen triple ...
*
Foldamers
In chemistry, a foldamer is a discrete chain molecule (oligomer) that folds into a conformationally ordered state in solution. They are artificial molecules that mimic the ability of proteins, nucleic acids, and polysaccharides to fold into w ...
*
Folding (chemistry)
In chemistry, folding is the process by which a molecule assumes its shape or conformation. The process can also be described as intramolecular self-assembly, a type of molecular self-assembly, where the molecule is directed to form a specifi ...
*
Tertiary structure
Protein tertiary structure is the three dimensional shape of a protein. The tertiary structure will have a single polypeptide chain "backbone" with one or more protein secondary structures, the protein domains. Amino acid side chains may int ...
*
α-helix
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues e ...
*
Structural motif
In a polymer, chain-like biological molecule, such as a protein or nucleic acid, a structural motif is a common Biomolecular structure#Tertiary structure, three-dimensional structure that appears in a variety of different, evolutionarily unrel ...
References
Further reading
*
*
External links
Anatomy & Taxonomy of Protein Structures -surveyNetSurfP - Secondary Structure and Surface Accessibility predictor
{{DEFAULTSORT:Beta Sheet
Protein structural motifs