Editing Beta sheet (section)

==Common structural motifs==
[[File:Beta hairpin.png|thumb|right|upright=0.6|The [[beta hairpin|β-hairpin]] motif]]
[[File:Anthrax toxin protein key motif.svg|left|thumb|The Greek-key motif]]

=== β-hairpin motif ===
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 dihedral-angle conformations required for a tight [[turn (biochemistry)|turn]] or a [[beta bulge loop|β-bulge loop]]. Individual strands can also be linked in more elaborate ways with longer loops that may contain [[alpha helix|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]] 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 | vauthors = Hutchinson EG, Thornton JM | title = The Greek key motif: extraction, classification and analysis | journal = Protein Engineering | volume = 6 | issue = 3 | pages = 233–45 | date = April 1993 | pmid = 8506258 | doi = 10.1093/protein/6.3.233 }}</ref> It was named after a pattern common to Greek ornamental artwork (see [[meander (art)|meander]]).

===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.<ref>See sections II B and III C, D in {{cite book | vauthors = Richardson JS |year= 1981 |title=Anatomy and Taxonomy of Protein Structures |chapter= The Anatomy and Taxonomy of Protein Structure |journal=Advances in Protein Chemistry |volume=34 |pages=167–339|isbn=0-12-034234-0 |doi=10.1016/s0065-3233(08)60520-3|pmid= 7020376 }}</ref> 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]].

[[File:beta-meander1.png|left|thumb|300px| The β-meander motif from Outer surface protein A (OspA).<ref name=":0">{{cite journal | vauthors = Makabe K, McElheny D, Tereshko V, Hilyard A, Gawlak G, Yan S, Koide A, Koide S | display-authors = 6 | title = Atomic structures of peptide self-assembly mimics | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 47 | pages = 17753–8 | date = November 2006 | pmid = 17093048 | pmc = 1693819 | doi = 10.1073/pnas.0606690103 | bibcode = 2006PNAS..10317753M | doi-access = free }}</ref>  The image above shows a variant of OspA (OspA+3bh) that contains a central, extended β-meander β-sheet featuring three additional copies (in red) of the core OspA β-hairpin (in grey) that have been duplicated and reinserted into the parent OspA β-sheet.]]
[[File:5CPAgood.png|right|thumb|Psi-loop motif from [[Carboxypeptidase A]]]]

===β-meander motif===
A simple [[structural motif|supersecondary]] protein topology composed of two or more consecutive antiparallel β-strands linked together by [[beta hairpin|hairpin]] loops.<ref>{{Cite web |url=http://scop.mrc-lmb.cam.ac.uk/scop/data/scop.b.c.bbf.html |title=SCOP: Fold: WW domain-like<!-- Bot generated title --> |access-date=2007-06-01 |archive-url=https://web.archive.org/web/20120204065925/http://scop.mrc-lmb.cam.ac.uk/scop/data/scop.b.c.bbf.html |archive-date=2012-02-04 |url-status=dead }}</ref><ref>{{Cite web |url=http://www.cryst.bbk.ac.uk/PPS2/course/section9/sss/super2.html |title=PPS '96 – Super Secondary Structure<!-- Bot generated title --> |access-date=2007-05-31 |archive-date=2016-12-28 |archive-url=https://web.archive.org/web/20161228125148/http://www.cryst.bbk.ac.uk/PPS2/course/section9/sss/super2.html |url-status=dead }}</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]].

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.<ref>{{cite journal|vauthors=Biancalana M, Makabe K, Koide S|date=February 2010|title=Minimalist design of water-soluble cross-beta architecture|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=107|issue=8|pages=3469–74|doi=10.1073/pnas.0912654107|pmc=2840449|pmid=20133689|bibcode=2010PNAS..107.3469B|doi-access=free}}</ref>  However, several notable exceptions include the Outer Surface Protein A (OspA) variants<ref name=":0" /> and the Single Layer β-sheet Proteins (SLBPs)<ref>{{Cite journal|last1=Xu|first1=Qingping|last2=Biancalana|first2=Matthew|last3=Grant|first3=Joanna C.|last4=Chiu|first4=Hsiu-Ju|last5=Jaroszewski|first5=Lukasz|last6=Knuth|first6=Mark W.|last7=Lesley|first7=Scott A.|last8=Godzik|first8=Adam|last9=Elsliger|first9=Marc-André|last10=Deacon|first10=Ashley M.|last11=Wilson|first11=Ian A.|date=September 2019|title=Structures of single-layer β-sheet proteins evolved from β-hairpin repeats|journal=Protein Science |volume=28|issue=9|pages=1676–1689|doi=10.1002/pro.3683|issn=1469-896X|pmc=6699103|pmid=31306512}}</ref> 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.<ref>{{cite journal | vauthors = Hutchinson EG, Thornton JM | title = PROMOTIF--a program to identify and analyze structural motifs in proteins | journal = Protein Science | volume = 5 | issue = 2 | pages = 212–20 | date = February 1996 | pmid = 8745398 | pmc = 2143354 | doi = 10.1002/pro.5560050204 }}</ref> There are four possible strand topologies for single Ψ-loops.<ref name="pmid2281084">{{cite journal | vauthors = 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 | s2cid = 28921557 }}</ref> 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"/>