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The beta sheet, (β-sheet) (also β-pleated sheet) is a common
motif Motif may refer to: General concepts * Motif (chess composition), an element of a move in the consideration of its purpose * Motif (folkloristics), a recurring element that creates recognizable patterns in folklore and folk-art traditions * Motif ...
of the regular
protein secondary structure Protein secondary structure is the three dimensional conformational isomerism, form of ''local segments'' of proteins. The two most common secondary structural elements are alpha helix, alpha helices and beta sheets, though beta turns and omega loo ...
. 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), ''Backbone'' (1923 film), 1923 lost silent film starring Alfred Lunt * Backbone (1975 film), ''Backbone'' (1975 film), 1975 Yugoslavi ...
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
s, forming a generally twisted, pleated sheet. A β-strand is a stretch of
polypeptide Peptides (from Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Greek language spoken ...
chain typically 3 to 10
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s long with backbone in an extended
conformation Conformation generally means structural arrangement and may refer to: * Conformational isomerism, a form of stereoisomerism in chemistry ** Carbohydrate conformation ** Cyclohexane conformation ** Protein conformation ** Conformation activity relat ...
. The supramolecular association of β-sheets has been implicated in the formation of the
fibrils Fibrils (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman ...
and protein aggregates observed in
amyloidosis Amyloidosis is a group of diseases in which abnormal protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one ...

amyloidosis
, notably
Alzheimer's disease Alzheimer's disease (AD), also referred to simply as Alzheimer's, is a neurodegenerative disease A neurodegenerative disease is caused by the progressive loss of structure or function of neuron A neuron or nerve cell is an membrane p ...
.


History

The first β-sheet structure was proposed by
William Astbury William Thomas Astbury FRS (25 February 1898 – 4 June 1961) was an English physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, ...
in the 1930s. He proposed the idea of hydrogen bonding between the
peptide bond In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, pro ...

peptide bond
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 Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, pro ...

peptide bond
was planar. A refined version was proposed by
Linus Pauling Linus Carl Pauling (; February 28, 1901 – August 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 t ...

Linus Pauling
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 Linus Carl Pauling (; February 28, 1901 – August 19, 1994) was a ...
in 1951. Their model incorporated the planarity of the peptide bond which they previously explained as resulting from keto-enol tautomerization.


Structure and orientation


Geometry

The majority of β-strands are arranged adjacent to other strands and form an extensive
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
network with their neighbors in which the
N−H
N−H
groups in the backbone of one strand establish
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
s with the
C=O
C=O
groups in the backbone of the adjacent strands. In the fully extended β-strand, successive side chains point straight up, then straight down, then straight up, etc. Adjacent β-strands in a β-sheet are aligned so that their Cα 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 Cα 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) trans was an annual, non-commercial arts festival held in Belfast, Northern Irel ...
''
peptide Peptides (from Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Greek language spoken ...

peptide
s. The "sideways" distance between adjacent Cα atoms in
hydrogen-bonded
hydrogen-bonded
β-strands is roughly . 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 Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, ele ...

dihedral angle
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 plot), originally developed in 1963 by Gopalasamudram Narayana Ramachandran, G. N. Ramachandran, C. Ramakrishnan, and V. Sasisekharan, is a way to ...
) 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 Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, ele ...

dihedral angle
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 Bayer Aktiengesellschaft, AG (; ) is a German multinational corporation, multinational pharmaceutical company, pharmaceutical and life sciences company and one of the largest pharmaceutical comp ...
. 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 Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to hav ...
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 Amino acids are organic compound In , organic compounds are generally any s that contain ...
, 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 A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
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
hydrogen-bonded
β-strands, then they form two mutual backbone hydrogen bonds to each other's flanking
peptide groups
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
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 Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Ke ...
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 A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
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 (from Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Greek language spoken ...

peptide
carbonyl In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, pro ...

carbonyl
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 Motif may refer to: General concepts * Motif (chess composition), an element of a move in the consideration of its purpose * Motif (folkloristics), a recurring element that creates recognizable patte ...

alpha helix
.


Amino acid propensities

Large aromatic residues (
tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemi ...

tyrosine
,
phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organ ...

phenylalanine
,
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 non-polar Aromatic hydrocarbon, aromatic amino ...

tryptophan
) 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 Amine, α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated ...

threonine
,
valine Valine (symbol Val or V) is an α-amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are k ...

valine
,
isoleucine Isoleucine (symbol Ile or I) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bon ...

isoleucine
) 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 Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

proline
) 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 β-sheet Secondary structure of proteins, secondary structure (known as cross-β) and ability to be Histological stain, st ...
formation.


Common structural motifs

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 three-dimensional structure that appears in a variety of different, evolutionarily unrelated molecules. A structural motif does not ...
involving β-sheets is the
β-hairpin
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 Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

glycine
or a
proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

proline
, both of which can assume the dihedral-angle conformations required for a tight
turn Turn may refer to: Arts and entertainment Dance and sports * Turn (dance and gymnastics), rotation of the body * Turn (swimming), reversing direction at the end of a pool * Turn (professional wrestling), a transition between face and heel * 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 structural motif, motif in the Protein secondary structure, secondary structure of proteins and is a Screw thread#Handedness, right hand-helix conformation in which every backbone amino, N−H group hydrogen b ...

α-helices
.


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 Physical changes are changes affecting the form of a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass ...

protein folding
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 sediment Sediment is a naturally occurring material that is broken down by processes of weath ...
).


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 Biomolecular structure is the intricate folded, three-dimensional shape that is formed by a molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an elect ...

tertiary structure
, the
TIM barrel The TIM barrel, also known as an alpha/beta barrel, is a conserved protein foldA protein superfamily is the largest grouping (clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a gro ...
.


β-meander motif

A simple supersecondary protein topology composed of 2 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 A hairstyle, hairdo, haircut or coiffure refer ...

hairpin
loops. This motif is common in β-sheets and can be found in several structural architectures including β-barrels and β-propellers.


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 A protease (also called a peptidase or proteinase) is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecul ...
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 (from Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Greek language spoken ...
s or small proteins with poorly defined overall architecture. All-β domains may form β-barrels, β-sandwiches, β-prisms, β-propellers, and β-helices.


Structural topology

The topology of a β-sheet describes the order of β-strands along the backbone. For example, the
flavodoxin foldIMage:3chy_flavodoxin_fold.png, left, 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 ...
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 Biomolecular structure is the intricate folded, three-dimensional shape that is formed by a molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an elect ...

secondary structure
of a β-sheet can be described roughly by giving the number of strands, their topology, and whether their
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
s are parallel or antiparallel. β-sheets can be ''open'', meaning that they have two edge strands (as in the
flavodoxin foldIMage:3chy_flavodoxin_fold.png, left, 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 ...
or the
immunoglobulin fold The immunoglobulin domain is a type of protein domain that consists of a 2-layer sandwich of 7-9 antiparallel beta sheet, β-strands arranged in two beta sheet, β-sheets with a Beta sheet#Greek key motif, Greek key topology, consisting of about 125 ...
) or they can be ''closed β-barrels'' (such as the
TIM barrel The TIM barrel, also known as an alpha/beta barrel, is a conserved protein foldA protein superfamily is the largest grouping (clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a gro ...
). 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 A protein domain is a region of the protein's Peptide, polypeptide chain that is self-stabilizing and that folds independently from the rest. Each domain forms a compact pr ...
) 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 prote ...
). Open β-sheets can assemble face-to-face (such as the β-propeller domain or
immunoglobulin fold The immunoglobulin domain is a type of protein domain that consists of a 2-layer sandwich of 7-9 antiparallel beta sheet, β-strands arranged in two beta sheet, β-sheets with a Beta sheet#Greek key motif, Greek key topology, consisting of about 125 ...
) 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 A hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of ...
. Due to this extended backbone conformation, β-sheets resist
stretching Stretching is a form of physical exercise Exercise is any bodily activity that enhances or maintains physical fitness Physical fitness is a state of health Health is a state of physical, mental and social well-being Well-bein ...

stretching
. 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 Sir Chandrasekhara Venkata Raman (; 7 November 188821 November 1970) was an Indian physicist known mainly for his work in the field of light scattering. With his student K. S ...
and analyzed with the quasi-continuum model.


Parallel β-helices

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 (geometry), prism; it is a polyhedron made of a triangle, triangular base, a Translation (geometry), translated copy, and 3 faces joining corresponding sides. A right triangular prism has rec ...

triangular prism
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. Righthanded β-helices, typified by the pectate lyase enzyme shown at left or
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 gene, ...
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 A protease (also called a peptidase or proteinase) is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The m ...
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 plaqueImage:Cerebral amyloid angiopathy -2a- amyloid beta - high mag.jpg, Amyloid beta immunostaining showing amyloid plaques (brown). Amyloid plaques (also known as neuritic plaques, Aβ plaques or senile plaques) are extracellular deposits of the amyloi ...
) 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), also referred to simply as Alzheimer's, is a neurodegenerative disease A neurodegenerative disease is caused by the progressive loss of structure or function of neuron A neuron or nerve cell is an membrane p ...
. 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 The collagen triple helix or type-2 helix is the primary secondary structure Biomolecular structure is the intricate folded, three-dimensional shape that is formed by a molecule A scanning tunneling microscopy image of pentacene molecule ...
* Foldamers *
Folding (chemistry) thumbnail, 200px, Crystal structure of a foldamer reported by Lehn and coworkers in Helv. Chim. Acta., 2003, 86, 1598–1624. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compoun ...
*
Tertiary structure Biomolecular structure is the intricate folded, three-dimensional shape that is formed by a molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an elect ...

Tertiary structure
*
α-helix The alpha helix (α-helix) is a common structural motif, motif in the Protein secondary structure, secondary structure of proteins and is a Screw thread#Handedness, right hand-helix conformation in which every backbone amino, N−H group hydrogen b ...

α-helix
*
Structural motif In a polymer, chain-like biological molecule, such as a protein or nucleic acid, a structural motif is a common three-dimensional structure that appears in a variety of different, evolutionarily unrelated molecules. A structural motif does not ...


References


Further reading

* *


External links


Anatomy & Taxonomy of Protein Structures -surveyNetSurfP - Secondary Structure and Surface Accessibility predictor
{{DEFAULTSORT:Beta Sheet Protein structural motifs