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Avidin is a tetrameric biotin-binding protein produced in the of , and and deposited in the Egg white of their eggs. Protein dimer 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 1800 μg per egg). The tetrameric protein contains four identical Protein subunit (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 in raw egg, but depending on the amount of heat it is exposed to during cooking, the quantity of molecules available for binding biotin can change. 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 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's affinity for biotin is exploited in wide-ranging biochemical assays, including western blot, ELISA, ELISPOT and Pull-down assay 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 biotinylation 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.
Given the strength of the avidin-biotin bond, dissociation of the avidin-biotin complex requires extreme conditions that cause protein denaturation. The non-reversible nature of the avidin-biotin complex can limit avidin's application in affinity chromatography applications where release of the captured ligand is desirable. Researchers have created an avidin with reversible binding characteristics through nitration or iodination of the binding site tyrosine. The modified avidin exhibits strong biotin binding characteristics at pH 4 and releases biotin at a pH of 10 or higher. A monomeric form of avidin with a reduced affinity for biotin is also employed in many commercially available affinity resins. The monomeric avidin is created by treatment of immobilized native avidin with urea or guanidine HCl (6–8 M), giving it a lower dissociation KD ≈ 10−7M. This allows elution from the avidin matrix to occur under milder, non-denaturing conditions, using low concentrations of biotin or low pH conditions. For a single high affinity biotin binding site without crosslinking, a monovalent version of avidin's distant relative, streptavidin, may be used.
A 1992 study found that thermal inactivation of the biotin binding activity of avidin was described by D121 °C = 25 min and Z-value = 33 °C. This study disagreed with prior assumptions "that the binding site of avidin is destroyed on heat denaturation".
The biotin-binding properties of avidin were exploited during the development of idrabiotaparinux, a long-acting low molecular weight heparin used in the treatment of venous thrombosis. Due to the long-acting nature of idraparinux, concerns were made about the clinical management of bleeding complications. By adding a biotin moiety to the idraparinux molecule, idrabiotaparinux was formed; its anticoagulant activity in the setting of a bleeding event can be reversed through an intravenous infusion of avidin.
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