Editing Affinity chromatography (section)

===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]]ged 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. [[Protein tag]]s include hexahistidine ([[histidine|His]]), [[glutathione]]-S-transferase (GST), [[maltose]] binding protein (MBP), and the Colicin E7 variant CL7 tag. [[Histidine]] tags have an affinity for [[nickel]], [[cobalt]], [[zinc]], [[copper]] and [[iron]] ions which have been immobilized by forming coordinate covalent bonds with a chelator incorporated in the stationary phase. 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 which is commercially available immobilized as glutathione agarose. During elution, excess glutathione is used to displace the tagged protein. CL7 has an affinity and specificity for Immunity Protein 7 (Im7) which is commercially available immobilized as Im7 agarose resin. For elution, an active and site-specific protease is applied to the Im7 resin to release the tag-free protein.<ref>{{Cite journal |last1=Vassylyeva |first1=Marina N. |last2=Klyuyev |first2=Sergiy |last3=Vassylyev |first3=Alexey D. |last4=Wesson |first4=Hunter |last5=Zhang |first5=Zhuo |last6=Renfrow |first6=Matthew B. |last7=Wang |first7=Hengbin |last8=Higgins |first8=N. Patrick |last9=Chow |first9=Louise T. |last10=Vassylyev |first10=Dmitry G. |date=2017-06-27 |title=Efficient, ultra-high-affinity chromatography in a one-step purification of complex proteins |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=114 |issue=26 |pages=E5138–E5147 |doi=10.1073/pnas.1704872114 |doi-access=free |issn=0027-8424 |pmc=5495267 |pmid=28607052|bibcode=2017PNAS..114E5138V }}</ref>