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{{short description|Fragment}}
[[File:ScFv-rotation.gif|300px|thumb|Rotating scFv fragment with highlighted
[[File:Single chain variable fragment.svg|thumb|The two possible structures of a single-chain variable fragment, with the antigen binding sites including the [[N-terminus|N-termini]] on the left and the [[C-terminus|C-termini]] on the right. The linker peptides are shown as arrows.]]
A '''single-chain variable fragment''' ('''scFv''') is not actually a [[Antibody fragment|fragment]] of an antibody, but instead is a [[fusion protein]] of the variable regions of the [[Immunoglobulin heavy chain|heavy]] (V<sub>H</sub>) and [[Immunoglobulin light chain|light chains]] (V<sub>L</sub>) of [[immunoglobulins]], connected with a short linker [[peptide]] of ten to about 25 [[amino acid]]s.<ref>{{cite journal | last1 = Huston | first1 = J. S. | last2 = Levinson | first2 = D. | last3 = Mudgett-Hunter | first3 = M. | last4 = Tai | first4 = M. S. | last5 = Novotný | first5 = J. | last6 = Margolies | first6 = M. N. | last7 = Crea | first7 = R. | year = 1988 | title = Protein engineering of antibody binding sites: recovery of specific activity in an anti-digoxin single-chain Fv analogue produced in Escherichia coli
This protein retains the specificity of the original immunoglobulin, despite removal of the constant regions and the introduction of the linker.<ref name="Peterson" /> The image to the right shows how this modification usually leaves the specificity unaltered.
These molecules were created to facilitate [[phage display]], where it is highly convenient to express the [[Fragment antigen-binding|antigen-binding ___domain]] as a single peptide. As an alternative, scFv can be created directly from [[subcloning|subcloned]] heavy and light chains derived from a [[hybridoma]]. ScFvs have many uses, e.g., [[flow cytometry]], [[immunohistochemistry]], and as antigen-binding domains of [[artificial T cell receptors]] (chimeric antigen receptor).
Unlike [[monoclonal antibodies]], which are often produced in mammalian cell cultures, scFvs are more often produced in bacteria cell cultures such as ''[[E. coli]]''.<ref name="Peterson">{{cite journal|title=Monoclonal Antibody Form and Function: Manufacturing the Right Antibodies for Treating Drug Abuse|
==Purification==
Single-chain variable fragments lack the constant [[Fc region]] found in complete antibody molecules, and, thus, the common binding sites (e.g., [[protein G]]) cannot be used to purify antibodies. These fragments can often be purified or immobilized using [[protein L]], since protein L interacts with the variable region of kappa light chains. More commonly, scientists incorporate a six histidine tag on the c-terminus of the scFv molecule and purify them using immobilized metal affinity chromatography (IMAC).
==Bivalent and trivalent scFvs==
[[File:Polyvalent single-chain variable fragments.svg|thumb|300px|Structure of divalent (top) and trivalent (bottom) scFvs, tandem (left) and di-/trimerisation format (right)]]
''Divalent'' (or ''bivalent'') single-chain variable fragments (di-scFvs, bi-scFvs) can be engineered by linking two scFvs. This can be done by producing a single peptide chain with two V<sub>H</sub> and two V<sub>L</sub> regions, yielding ''tandem scFvs''.<ref>{{cite journal|title=Development of tumor targeting anti-MUC-1 multimer: effects of di-scFv unpaired cysteine ___location on PEGylation and tumor binding|first5=SJ|last5=Denardo|first4=GL|last4=Denardo|first3=XB|last3=Shi|first2=A|last2=Natarajan |
All of these formats can be composed from variable fragments with specificity for two different antigens, in which case they are types of [[bispecific antibodies]].<ref>{{cite journal|pmid=11388794|year=2001|last1=Dincq|first1=S|last2=Bosman|first2=F|last3=Buyse|first3=MA|last4=Degrieck|first4=R|last5=Celis|first5=L|last6=De Boer|first6=M|last7=Van Doorsselaere|first7=V|last8=Sablon|first8=E|title=Expression and purification of monospecific and bispecific recombinant antibody fragments derived from antibodies that block the CD80/CD86-CD28 costimulatory pathway|volume=22|issue=1|pages=11–24|doi=10.1006/prep.2001.1417|journal=Protein Expression and Purification}}</ref><ref>{{cite
==Examples==
* [[Pexelizumab]], a scFv binding to [[Complement component 5|component 5]] of the [[complement system]] and designed to reduce side effects of [[cardiac surgery]]<ref>{{cite journal|pmid=15331798|year=2004|last1=Mathew|first1=JP|last2=Shernan|first2=SK|last3=White|first3=WD|last4=Fitch|first4=JC|last5=Chen|first5=JC|last6=Bell|first6=L|last7=Newman|first7=MF|title=Preliminary report of the effects of complement suppression with pexelizumab on neurocognitive decline after coronary artery bypass graft surgery|volume=35|issue=10|pages=2335–9|doi=10.1161/01.STR.0000141938.00524.83|journal=Stroke: A Journal of Cerebral Circulation|doi-access=free}}</ref>
* C6.5, a diabody targeting [[HER2/neu]]<ref name="Adams" /> found in some [[breast cancer]]s
* [[Brolucizumab]], a scFV binding to VEGF-A and used to treat wet age-related macular degeneration
* '''G6A''', a scFV binding to human Hsp70.1 and can be used to target this protein as a potential marker in vast cancers.<ref name="Vostakolaei 14711–14724"/>
==References==
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{{DEFAULTSORT:Single-Chain Variable Fragment}}
[[Category:Monoclonal antibodies|.]]
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