Avidin
Avidin (66kDa), found in raw egg white, is a highly cationic glycoprotein with an isoelectric point of about 10.5 and can associate stoichiometrically with biotin. Avidin's extinction coefficient is 15.5 (E(1%) at 280nm).
Avidin-biotin system has been established as a powerful tool in biological sciences and used in a large range of biochemical assays, including western blot, ELISA as well as ELISPOT to amplify signals.
However, because of its heterogeneous structure composed largely of mannose and N-acetylglucosamine positively charged residues, some studies showed that avidin can interact nonspecifically with negatively charged cell surfaces and nucleic acids. These interactions could increase the background in IHC and FACS.
Streptavidin
Streptavidin (52.8kDa), a nonglycosylated protein with a mildly acidic isoelectric point of 5, was originally isolated from Streptomyces avidinii. Streptavidin is known to exhibit less nonspecific binding than avidin. As a tetrameric protein, each streptavidin subunit binds one molecule of biotin. Streptavidin's extinction coefficient is 41326 M-1 cm-1 (at 280nm).
Streptavidin has one of the strongest non-covalent interactions with a dissociation constant of ~10-15 mol/L for biotin. In addition to its use in purification, thanks to the strong streptavidin-biotin bond, steptavidin is also used to attach biomolecules to solid supports.
Some studies showed streptavidin expressing a tripeptide sequence Arg-Tyr-Asp would mimic the Arg-Gly-Asp binding sequence of fibronectin, the specific recognition sequence binding integrins. Consequently, cross-reactivity and an increase of background can occur after the use of streptavidin.
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