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Avidin is a tetrameric or dimeric biotin-binding protein produced in the oviducts of birds, reptiles and amphibians and deposited in the whites of their eggs. In chicken egg white, avidin makes up approximately 0.05% of total protein (approximately 1.8 mg 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 avidin is measured to be ''K''D ≈ 10−15 M, making it one of the strongest known non-covalent bonds. In its tetrameric form, avidin is estimated to be between 66–69 kDa in size. 10% of the molecular weight is attributed to carbohydrate content 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 avidity 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 the bacteria need. 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. ==Discovery of avidin== Avidin was first discovered by Esmond Emerson Snell (1914–2003). The route to 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 the involved researchers at the University of Texas.〔 The name of the protein was later revised to "avidin" based on its affinity for biotin (avid + biotin). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「avidin」の詳細全文を読む スポンサード リンク
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