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| The '''β sheet''' (also '''β-pleated sheet''') is the second form of regular [[secondary structure]] in [[protein]]s. It is less common than the [[alpha helix]]. Beta sheets consist of '''beta strands''' connected laterally by at least two or three backbone [[hydrogen bond]]s, forming a generally twisted, pleated sheet. A [[beta strand]] (also '''β strand''') is a stretch of [[peptide|polypeptide]] chain typically 3 to 10 [[amino acid]]s long with backbone in an almost fully extended conformation. The higher-level association of β sheets has been implicated in formation of the protein aggregates and fibrils observed in many human diseases, notably the [[amyloidosis|amyloidoses]] such as [[Alzheimer's disease]].
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| [[Image:1gwe antipar betaSheet both.png|thumb|right|500px| An example of a 4-stranded [[Antiparallel (biochemistry)|antiparallel]] β sheet fragment from a crystal structure of the enzyme [[catalase]] (PDB file 1GWE at 0.88Å resolution). a) Front view, showing the antiparallel hydrogen bonds (dotted) between peptide NH and CO groups on adjacent strands. Arrows indicate chain direction, and electron density contours outline the non-H atoms. O atoms are red balls, N atoms are blue, and H atoms are omitted for simplicity; sidechains are shown only out to the first sidechain C atom (green). b) Edge-on view of the central two β strands in a, showing the righthanded ''twist'' and the ''pleat'' of Cαs and sidechains that alternately stick out in opposite directions from the sheet.]]
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| ==History==
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| The first β sheet structure was proposed by [[William Astbury]] in the 1930s. He proposed the idea of hydrogen bonding between the [[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]] was planar. A refined version was proposed by [[Linus Pauling]] and [[Robert Corey]] in 1951.
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| ==Structure and orientation==
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| {| style="float:right;"
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| | [[Image:Beta sheet bonding antiparallel-color.svg|right|thumb|Illustration of the [[hydrogen bonding]] patterns, represented by dotted lines, in an '''antiparallel''' beta sheet. [[Oxygen]] atoms are colored <span style="color:red;">'''red'''</span> and [[nitrogen]] atoms colored <span style="color:blue;">'''blue'''</span>.]] || [[Image:Beta sheet bonding parallel-color.svg|thumb|right|thumb|Illustration of the [[hydrogen bonding]] patterns, represented by dotted lines, in a '''parallel''' beta sheet. [[Oxygen]] atoms are colored <span style="color:red;">'''red'''</span> and [[nitrogen]] atoms colored <span style="color:blue;">'''blue'''</span>.]]
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| ===Geometry===
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| The majority of β strands are arranged adjacent to other strands and form an extensive [[hydrogen bond]] network with their neighbors in which the [[amine|N-H]] groups in the backbone of one strand establish [[hydrogen bond]]s with the [[carbonyl|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 C<sup>α</sup> 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<sup>α</sup> atom; for example, if a side chain points straight up, then the bond to the <math>\mathrm{C^{\prime}}</math> must point slightly downwards, since its bond angle is approximately 109.5°. The pleating causes the distance between <math>\mathrm{C^{\alpha}}_{i}</math> and <math>\mathrm{C^{\alpha}_{i+2}}</math> to be approximately 6 [[ångström|Å]], rather than the 7.6 Å (2 × 3.8 Å) expected from two fully extended [[Cis-trans isomerism|trans]] [[peptide bond|peptide]] [[virtual bond]]s. The "sideways" distance between adjacent C<sup>α</sup> atoms in [[hydrogen bond|hydrogen-bonded]] β strands is roughly 5 Å
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| [[Image:Ramachandran plot general 100K.jpg|thumb|left|200px| Ramachandran (φ,ψ) plot of about 100,000 high-resolution datapoints, showing the broad, favorable region around the conformation typical for β-sheet residues.]]
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| However, β strands are rarely perfectly extended; rather, they exhibit a twist due to the [[chirality (chemistry)|chirality]] of their component amino acids. The energetically preferred [[dihedral angle]]s near (φ, ψ) = (–135°, 135°) (broadly, the upper left region of the [[Ramachandran plot]]) diverge significantly from the fully extended conformation (φ, ψ) = (–180°, 180°).<ref>{{cite book |title=Biochemistry |last=Voet |first=Donald |authorlink= |coauthors=Voet, Judith G. |year=2004 |edition=3rd |publisher=Wiley |location=Hoboken, NJ |isbn=0-471-19350-X |page= |pages=227–231 |url= }}</ref> The twist is often associated with alternating fluctuations in the [[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]].
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| The side chains point outwards from the folds of the pleats, roughly perpendicularly to the plane of the sheet; successive residues point outwards on alternating faces of the sheet.
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| ===Hydrogen bonding patterns===
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| Because peptide chains have a directionality conferred by their [[N-terminal end|N-terminus]] and [[C-terminal end|C-terminus]], 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]]s in antiparallel, parallel, or mixed arrangements.
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| 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 <math>\mathrm{C^{\alpha}_{i}}</math> and <math>\mathrm{C^{\alpha}_{j}}</math> are adjacent in two [[hydrogen bond|hydrogen-bonded]] β strands, then they form two mutual backbone hydrogen bonds to each other's flanking [[peptide bond|peptide groups]]; this is known as a '''close pair''' of hydrogen bonds.
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| 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 was more stable.
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| In parallel β-sheet structure, if two atoms <math>\mathrm{C^{\alpha}_{i}}</math> and <math>\mathrm{C^{\alpha}_{j}}</math> are adjacent in two [[hydrogen bond|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 <math>i</math> may form hydrogen bonds to residues <math>j-1</math> and <math>j+1</math>; this is known as a ''wide pair'' of hydrogen bonds. By contrast, residue <math>j</math> may hydrogen-bond to different residues altogether, or to none at all.
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| 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.
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| The [[hydrogen bond]]ing of β strands need not be perfect, but can exhibit localized disruptions known as [[beta bulge]]s.
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| The hydrogen bonds lie roughly in the plane of the sheet, with the [[peptide bond|peptide]] [[carbonyl]] groups pointing in alternating directions with successive residues; for comparison, successive carbonyls point in the ''same'' direction in the [[alpha helix]].
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| ===Amino acid propensities===
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| Large aromatic residues (Tyr, Phe and Trp) and β-branched amino acids (Thr, Val, Ile) are favored to be found in β strands in the ''middle'' of β sheets. Interestingly, different types of residues (such as Pro) 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]] formation.<ref>{{cite journal |author= Richardson, J.S., and Richardson, D.C. |year= 2002 |title=Natural β-Sheet Proteins Use Negative Design to Avoid Edge-to-Edge Aggregation |journal=Proc. Nat. Acad. Sci. USA |volume=99 |pages=2754–2759 |doi=10.1073/pnas.052706099 |pmid= 11880627 |issue= 5 |pmc= 122420 }}</ref> It is also suggested that the dipole moments in parallel β-sheets, whose direction is from C-terminal (partially negative) to N-terminal (partially positive) may influence the propensity of certain residues (like Lys and Arg) for the ''caps'' of this structure.<ref>{{cite journal |last= FarzadFard |first= Fahim |authorlink= |coauthors=''et al.'' |year=2008 |month= |title=β-Sheet capping: Signals that initiate and terminate β-sheet formation |journal=Journal of Structural Biology |volume=161 |issue=1 |pages=101–110 |doi=10.1016/j.jsb.2007.09.024 |pmid= 18006332 |url= |accessdate= |quote= }}</ref>
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| ==Common structural motifs==
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| [[Image:Beta hairpin.png|thumb|right|130px|Representation of a [[beta hairpin]]]]
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| [[Image:Anthrax toxin protein key motif.svg|left|thumb|200px|Greek-key motif in protein structure.]]
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| A very simple [[structural motif]] involving β sheets is the [[beta 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]] or a [[proline]], both of which can assume the unusual dihedral-angle conformations required for a tight [[turn (biochemistry)|turn]]. However, individual strands can also be linked in more elaborate ways with long loops that may contain [[alpha helix|alpha helices]] or even entire protein domains.
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| ===Greek key motif===
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| 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]] process.<ref>[http://swissmodel.expasy.org/course/text/chapter4.htm Tertiary Protein Structure and Folds: section 4.3.2.1]. From [http://swissmodel.expasy.org/course/ Principles of Protein Structure, Comparative Protein Modelling, and Visualisation]</ref><ref name="pmid8506258">{{cite journal | author = Hutchinson EG, Thornton JM | title = The Greek key motif: extraction, classification and analysis | journal = Protein Eng. | volume = 6 | issue = 3 | pages = 233–45 |date=April 1993 | pmid = 8506258 | doi = 10.1093/protein/6.3.233 | url = | issn = }}</ref> It was named after a pattern common to Greek ornamental artwork (see [[meander (art)]]).
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| ===The β-α-β motif===
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| Due to the chirality of their component amino acids, all strands exhibit a "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 [[beta-alpha-beta|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]], the [[TIM barrel]].
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| [[Image:beta-meander1.png|left|200px|thumb|<div style="text-align:center;">'''Beta-meander motif'''<br/>Portion of outer surface Protein A of [[Borrelia burgdorferi]] complexed with a murine [[monoclonal antibodies|monoclonal antibody]].<br/></div>]]
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| ===β-meander motif===
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| A simple [[structural motif|supersecondary]] protein topology composed of 2 or more consecutive antiparallel β-strands linked together by [[beta hairpin|hairpin]] loops.<ref>[http://scop.mrc-lmb.cam.ac.uk/scop/data/scop.b.c.bbf.html SCOP: Fold: WW domain-like<!-- Bot generated title -->]</ref><ref>[http://www.cryst.bbk.ac.uk/PPS2/course/section9/sss/super2.html PPS '96 - Super Secondary Structure<!-- Bot generated title -->]</ref> This motif is common in β-sheets and can be found in several structural architectures including [[beta barrel|β-barrels]] and [[beta propeller|β-propellers]].
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| [[Image:5CPAgood.png|right|thumb|200px|<div style="text-align:center;">'''Psi-loop motif'''<br/>Portion of [[Carboxypeptidase A]].<br/></div>]]
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| ===Psi-loop motif===
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| The psi-loop, Ψ-loop, motif consists of two antiparallel strands with one strand in between that is connected to both by hydrogen bonds.<ref>{{cite journal |last=Hutchinson |first=E. |authorlink= |coauthors=Thornton, J. |year=1996 |month= |title=PROMOTIF—A program to identify and analyze structural motifs in proteins |journal=Protein Science |volume=5 |issue=2 |pages=212–220 |pmid=8745398 |url= |accessdate= |quote= |pmc=2143354 |doi=10.1002/pro.5560050204 }}</ref> There are four possible strand topologies for single Ψ-loops as cited by Hutchinson ''et al.'' (1990). 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 acid protease|aspartic protease]] family.<ref name="pmid2281084">{{cite journal | author = Hutchinson EG, Thornton JM | title = HERA--a program to draw schematic diagrams of protein secondary structures | journal = Proteins | volume = 8 | issue = 3 | pages = 203–12 | year = 1990 | pmid = 2281084 | doi = 10.1002/prot.340080303 | url = | issn = }}</ref> | |
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| ==Structural architectures of proteins with beta-sheets==
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| Beta-sheets are present in [[All-β proteins|all-β]], [[Α+β proteins|α+β]] and [[Α/β proteins|α/β]] domains according to [http://scop.mrc-lmb.cam.ac.uk/scop/ Structural Classification of Proteins] and in many [[peptide]]s or small proteins with poorly defined overall architecture. [[All-β proteins|All-β domains]] may form [[beta barrel|β barrels]], [[beta sandwich|β sandwiches]], [[beta prism|β prisms]], [[Beta-propeller domain|β propellers]], and [[Beta helix|β-helices]].
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| ==Structural topology==
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| The '''topology''' of a β sheet describes the order of [[hydrogen bond|hydrogen-bonded]] β strands along the backbone. For example, the [[flavodoxin fold]] 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]] of a β sheet can be described roughly by giving the number of strands, their topology, and whether their [[hydrogen bond]]s are parallel or antiparallel.
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| β sheets can be ''open'', meaning that they have two edge strands (as in the [[flavodoxin fold]] or the [[immunoglobulin fold]]) or they can be ''closed [[beta-barrel|beta barrel]]s'' (such as the [[TIM barrel]]). [[beta-barrel|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]]) or form horseshoe shapes (as in the [[ribonuclease inhibitor]]). Open β sheets can assemble face-to-face (such as the [[beta-propeller domain]] or [[immunoglobulin fold]]) or edge-to-edge, forming one big β sheet.
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| ==Dynamic features==
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| Beta-sheets in proteins may carry out [[low-frequency collective motion in proteins and DNA|low-frequency]] accordion-like motion as observed by the [[Raman spectroscopy]]<ref name="pmid7115900">{{cite journal | author = Painter PC, Mosher LE, Rhoads C | title = Low-frequency modes in the Raman spectra of proteins | journal = Biopolymers | volume = 21 | issue = 7 | pages = 1469–72 |date=July 1982 | pmid = 7115900 | doi = 10.1002/bip.360210715 | url = | issn = }}</ref> and analyzed with the quasi-continuum model.<ref name="pmid4052563">{{cite journal | author = Chou KC | title = Low-frequency motions in protein molecules. Beta-sheet and beta-barrel | journal = Biophys. J. | volume = 48 | issue = 2 | pages = 289–97 |date=August 1985 | pmid = 4052563 | pmc = 1329320 | doi = 10.1016/S0006-3495(85)83782-6 | url = | issn = }}</ref>
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| ==Parallel β helices==
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| [[Image:1QRE L3betaHelix.jpg|thumb|right|200px| End-view ribbon schematic of a 3-sided, lefthanded β helix protein (PDB file 1QRE).]]
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| A [[beta helix|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 [[beta helix]] article for further information.<br>
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| In lefthanded β helices, the strands themselves are quite straight and untwisted; the resulting helical surfaces are nearly flat, forming a regular [[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.<ref>{{cite journal |author=Liou, Y.C. et al.|title=Mimicry of ice structure by surface hydroxyls and water of a beta-helix antifreeze protein |journal=Nature |year=2000|volume=406|pages=322–324|doi= 10.1038/35018604 |pmid=10917536 |issue=6793 }}</ref> <br>
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| [[Image:Pectate lyase R3betaHelix.jpg|thumb|left|200px| End-view ribbon schematic of a 3-sided, righthanded β helix protein (PDB file 2PEC).]]
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| 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.<ref>{{cite book |title=Introduction to Protein Structure |last=Branden |first=Carl |authorlink= |coauthors=Tooze, John |year=1999 |publisher=Garland |location=New York |isbn=0-8153-2305-0 |pages=20–32 }}</ref> <br>
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| A two-sided β helix (right-handed) is found in some bacterial [[metalloprotease]]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.<ref>{{cite journal |author=Bauman, U. et al.| title=Three-dimensional structure of the alkaline protease of Pseudomonas aeruginosa: a two-domain protein with a calcium binding parallel beta roll motif| journal=EMBO Journal |year=1993|volume=12|pages=3357–3364 |pmid=8253063 |issue=9 |pmc=413609 }}</ref> This fold is called a beta-roll in the SCOP classification.
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| ==β sheets in pathology==
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| Some proteins that are disordered or helical as monomers, such as amyloid β (see [[amyloid plaque]]) 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 disease|Alzheimer's]]. Its structure has yet to be determined in full, but recent data suggest that it may resemble an unusual two-strand β helix.<ref name="pmid15944695">{{cite journal | author = Nelson R, Sawaya MR, Balbirnie M, Madsen AØ, Riekel C, Grothe R, Eisenberg D | title = Structure of the cross-beta spine of amyloid-like fibrils | journal = Nature | volume = 435 | issue = 7043 | pages = 773–8 |date=June 2005 | pmid = 15944695 | pmc = 1479801 | doi = 10.1038/nature03680 | url = | issn = }}</ref>
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| 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),<ref>SHUGUANG ZHANG, TODD HOLME, CURTIS LOCKSHIN, AND ALEXANDER RICH. PNAS April 15, 1993 vol. 90 no. 8 3334-3338</ref> which can be useful if the sheet is to form a boundary between polar/watery and nonpolar/greasy environments.
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| ==See also==
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| *[[Folding (chemistry)]]
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| *[[Tertiary structure]]
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| *[[Alpha helix|α helix]]
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| *[[Collagen helix]]
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| *[[Foldamers]]
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| ==References==
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| {{Reflist|2}}
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| ==Further reading==
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| *Cooper, J. [http://www.cryst.bbk.ac.uk/PPS2/course/section9/sss/super2.html "Super Secondary Structure - Part II"], May 31, 1996. Accessed May 25, 2007.
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| *Structural Classification of Proteins (SCOP) [http://scop.mrc-lmb.cam.ac.uk/scop/data/scop.b.c.bbf.html "Open-sided Beta-meander"], October 20, 2006. Accessed May 31, 2007.
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| ==External links==
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| *[http://www.cbs.dtu.dk/services/NetSurfP/ NetSurfP - Secondary Structure and Surface Accessibility predictor]
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| *[http://www2.ufp.pt/~pedros/anim/2frame_sheeten.htm Interactive model of an anti-parallel β sheet] (plug-in required)
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| *[http://www2.ufp.pt/~pedros/anim/2frame_psheeten.htm Interactive model of a parallel β sheet] (plug-in required)
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| *[http://www3.interscience.wiley.com:8100/legacy/college/voet/047119350X/animated_figures/html/8-16.html Animated details of β-pleated sheet]
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| *[http://www.pdb.org/pdb/explore/explore.do?structureId=2Y6Q Monooxygenase]
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| {{Protein secondary structure}}
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| {{DEFAULTSORT:Beta Sheet}}
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| [[Category:Protein structural motifs]]
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