Avidin

Avidin is a tetrameric biotin-binding protein produced in the oviducts of birds, reptiles and amphibians and deposited in the whites of their eggs. Dimeric members of the avidin family are also found in some bacteria. In chicken egg white, avidin makes up approximately 0.05% of total protein (approximately 180 μg per egg). The tetrameric protein contains four identical subunits (homotetramer), each of which can bind to biotin (Vitamin B7, vitamin H) with a high degree of affinity and specificity. The dissociation constant of the avidin-biotin complex is measured to be KD ≈ 10−15 M, making it one of the strongest known non-covalent bonds. Avidin is a tetrameric biotin-binding protein produced in the oviducts of birds, reptiles and amphibians and deposited in the whites of their eggs. Dimeric members of the avidin family are also found in some bacteria. In chicken egg white, avidin makes up approximately 0.05% of total protein (approximately 180 μg per egg). The tetrameric protein contains four identical subunits (homotetramer), each of which can bind to biotin (Vitamin B7, vitamin H) with a high degree of affinity and specificity. The dissociation constant of the avidin-biotin complex is measured to be KD ≈ 10−15 M, making it one of the strongest known non-covalent bonds. In its tetrameric form, avidin is estimated to be 66–69 kDa in size. 10% of the molecular weight is contributed by carbohydrate, composed of four to five mannose and three N-acetylglucosamine residues. The carbohydrate moieties of avidin contain at least three unique oligosaccharide structural types that are similar in structure and composition. Functional avidin is found only in raw egg, as the biotin affinity of the protein is destroyed by cooking. The natural function of avidin in eggs is not known, although it has been postulated to be made in the oviduct as a bacterial growth inhibitor, by binding biotin helpful for bacterial growth. As evidence for this, streptavidin, a loosely related protein with equal biotin affinity and a very similar binding site, is made by certain strains of Streptomyces bacteria, and is thought to serve to inhibit the growth of competing bacteria, in the manner of an antibiotic. A non-glycosylated form of avidin has been isolated from commercially prepared product; however, it is not conclusive as to whether the non-glycosylated form occurs naturally or is a product of the manufacturing process. Avidin was discovered by Esmond Emerson Snell (1914–2003). This discovery began with the observation that chicks on a diet of raw egg white were deficient in biotin, despite availability of the vitamin in their diet. It was concluded that a component of the egg-white was sequestering biotin which Snell verified in vitro using a yeast assay. Snell later isolated the component of egg white responsible for biotin binding, and, in collaboration with Paul Gyorgy, confirmed that the isolated egg protein was the cause of biotin deficiency or “egg white injury”. At the time the protein had been tentatively named avidalbumin (literally, hungry albumin) by researchers at the University of Texas. The name of the protein was later revised to 'avidin' based on its affinity for biotin (avid + biotin). Research in the 1970s helped establish the avidin-biotin system as a powerful tool in biological sciences. Aware of the strength and specificity of the avidin-biotin complex, researchers began to exploit avidin and streptavidin as probes and affinity matrices in numerous research projects. Soon after, researchers Bayer and Wilchek developed new methods and reagents to biotinylate antibodies and other biomolecules, allowing the transfer of the avidin-biotin system to a range of biotechnological applications. Today, avidin is used in a variety of applications ranging from research and diagnostics to medical devices and pharmaceuticals. Avidin's affinity for biotin is exploited in wide-ranging biochemical assays, including western blot, ELISA, ELISPOT and pull-down assays. In some cases the use of biotinylated antibodies has allowed the replacement of radioiodine labeled antibodies in radioimmunoassay systems, to give an assay system which is not radioactive. Avidin immobilized onto solid supports is also used as purification media to capture biotin-labelled protein or nucleic acid molecules. For example, cell surface proteins can be specifically labelled with membrane impermeable biotin reagent, then specifically captured using an avidin-based support. As a basically charged glycoprotein, avidin exhibits non-specific binding in some applications. Neutravidin, a deglycosylated avidin with modified arginines, exhibits a more neutral isoelectric point (pI) and is available as an alternative to native avidin, whenever problems of non-specific binding arise. Deglycosylated, neutral forms of avidin are available through Sigma-Aldrich (Extravidin), Thermo Scientific (NeutrAvidin), Invitrogen (NeutrAvidin), and e-Proteins (NeutraLite).