Affinity chromatography
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2008-06-22T19:09:37Z
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[[Image:Colortest.jpg|thumb|160px|A chemist using column chromatographic apparatus in the mid-1950s]]
'''Affinity chromatography'''<ref>Pascal Bailon, George K. Ehrlich, Wen-Jian Fung and Wolfgang Berthold, An Overview of Affinity Chromatography, Humana Press, 2000. ISBN 978-0-89603-694-9, ISBN 978-1-60327-261-2. </ref> is a [[chromatography|chromatographic]] method of separating [[biochemistry|biochemical]] mixtures, based on a highly specific biologic interaction such as that between [[antigen]] and [[antibody]], [[enzyme]] and [[Substrate (biochemistry)|substrate]], or [[Receptor (biochemistry)|receptor]] and [[Ligand (biochemistry)|ligand]]. Affinity chromatography combines the size fractionation capability of [[gel permeation chromatography]] with the ability to design a [[Chromatography#Chromatography theory|stationary phase]] that reversibly binds to a known subset of molecules. The method was invented in 1968<ref>{{cite journal | author= Cuatrecasas P, Wilchek M, and Anfinsen CB | title= Selective enzyme purification by affinity chromatography | journal= Proc. Nat. Acad. Sci. USA | volume=61| year=1968| pages=636–43 | doi= 10.1073/pnas.61.2.636}}</ref> by Pedro Cuatrecasas and [[Meir Wilchek]] for which the [[Wolf Prize in Medicine]] in 1987 was awarded. Due to its interdisciplinary nature, affinity chromatography has been the means by which many scientists from different disciplines have been introduced to the exciting fields of modern biology<ref>{{cite journal | author= Meir Wilchek and Talia Miron| title= Thirty years of affinity chromatography | journal= Reactive and Functional Polymers|volume=41| year=1999| pages=263–268| doi= 10.1016/S1381-5148(99)00042-5 }}</ref>
==Uses==
[[Image:Affinity1.jpg|220px|right|The general principle of affinity chromatography]]
Affinity chromatography can be used to:
* Purify and concentrate a molecule from a mixture into a buffering solution
* Reduce the amount of a molecule in a mixture
* Discern what biological compounds bind to a particular molecule, such as drugs
==Principle==
Usually the starting point is an undefined heterogeneous group of molecules in solution, such as a cell lysate, growth medium or blood [[blood plasma|serum]]. The molecule of interest will have a well known and defined property which can be exploited during the affinity purification process. The process itself can be thought of as an entrapment, with the target molecule becoming trapped on a solid or stationary phase or medium. The other molecules in solution will not become trapped as they do not possess this property. The solid medium can then be removed from the mixture, washed and the target molecule released from the entrapment in a process known as elution.
==Batch and column setups==
[[Image:Affinity-column.png|right|thumb|Column chromatography|170px]][[Image:batch.jpg|170px|right|thumb|Batch chromatography]]
Binding to the solid phase may be achieved by [[column chromatography]], whereby the solid medium is packed onto a [[chromatography]] column, the initial mixture run through the column to allow binding, a wash buffer run through the column and the elution buffer subsequently applied to the column and collected. These steps are usually done at ambient pressure (as opposed to [[HPLC]] or [[Fast protein liquid chromatography|FPLC]]).
Alternatively binding may be achieved using a batch treatment, by adding the initial mixture to the solid phase in a vessel, mixing, separating the solid phase (by [[centrifugation]] for example), removing the liquid phase, washing, re-centrifuging, adding the elution buffer, re-centrifuging and removing the eluate.
Sometimes a hybrid method is employed, the binding is done by the batch method, then the solid phase with the target molecule bound is packed onto a column and washing and elution are done on the column.
A third method, [[expanded bed adsorption]], which combines the advantages of the two methods mentioned above, has also been developed. The solid phase particles are placed in a column where liquid phase is pumped in from the bottom and exits at the top. The gravity of the particles ensure that the solid phase does not exit the column with the liquid phase.
==Specific uses==
Affinity chromatography can be used in a number of applications, including [[nucleic acid]] purification,[[protein]] purification from cell free extracts,and [[antibody]] purification from blood [[blood plasma|serum]].
===Antibody affinity===
<b>also known as Immunoaffinity Chromatography<b><br>
Another use for the procedure is the affinity purification of antibodies from blood serum. If serum is known to contain antibodies against a specific [[antigen]] (for example if the serum comes from an organism [[immunize]]d against the antigen concerned) then it can be used for the affinity purification of that antigen. For example if an organism is immunised against a GST-fusion protein it will produce antibodies against the fusion-protein, and possibly antibodies against the GST tag as well. The protein can then be covalently coupled to a solid support such as [[agarose]].
For thoroughness the GST protein and the GST-fusion protein can each be coupled separately. The serum is initially allowed to bind to the GST affinity matrix. This will remove antibodies against the GST part of the fusion protein. The serum is then separated from the solid support and allowed to bind to the GST-fusion protein matrix. This allows any antibodies that recognize the antigen to be captured on the solid support. Elution of the antibodies of interest is most often achieved using a low [[pH]] buffer such as glycine pH 2.8. The eluate is collected into a neutral [[tris]] or phosphate buffer, to neutralize the low pH elution buffer and halt any degradation of the antibody's activity. This is a nice example as affinity purification is used to purify the initial GST-fusion protein, to remove the undesirable anti-GST antibodies from the serum and to purify the target antibody.
=== Immobilized metal ion affinity chromatography ===
Immobilized metal ion affinity chromatography (IMAC) is based on the specific [[coordinate covalent bond|coordinate covalent binding]] of [[amino acid]]s to metals, particularly histidine. This technique works by allowing proteins with an affinity for metal ions to be retained in a column containing immobilized metal ions, such as [[cobalt]], [[nickel]], [[copper]], [[zinc]], or [[iron]] ions. Many naturally occurring proteins do not have an affinity for metal ions, therefore [[recombinant DNA technology|recombinant DNA techniques]] can be used to introduce this property into a protein of interest. Methods used to elute the protein of interest include changing the pH, or adding a competitive molecule, such as [[imidazole]].
===Recombinant proteins===
Possibly the most common use of affinity chromatography is for the purification of recombinant proteins. Proteins with a known affinity are [[protein tag|tagged]] in order to aid their purification. The protein may have been genetically modified so as to allow it to be selected for affinity binding, this is known as a [[fusion protein]]. Tags include [[His-tag]]s and [[GST-tag|GST]] (glutathione-S-transferase) tags. His<sub>6</sub>-tags have an affinity for [[nickel]] or [[cobalt]] [[ion]]s which are [[Coordinate covalent bond|coordinated]] with a chelator for the purposes of solid medium entrapment. For elution, an excess amount of a compound able to act as a metal ion [[ligand]], such as [[imidazole]], is used. GST has an affinity for [[glutathione]] - commercially available immobilized as glutathione [[agarose]]. For elution, excess glutathione is used to displace the tagged protein.
==See also==
*[[Chromatography]]
*[[Ligand]]
*[[Antibody]]
*[[Protein tag]]
*[[Polyhistidine-tag]]
==References==
<references/>
[[Category:Chromatography]]
[[bg:Афинитетна хроматография]]
[[cs:Afinitní chromatografie]]
[[de:Affinitätschromatografie]]
[[ja:アフィニティークロマトグラフィー]]
[[ru:Аффинная хроматография]]
[[sv:Affinitetskromatografi]]
[[uk:Адсорбційна хроматографія]]
[[zh:亲和色谱]]