Avidin Protein

Avidin is composed of four identical subunits, each capable of binding to biotin (Vitamin B7 or Vitamin H) with a high degree of affinity and specificity . The dissociation constant of the avidin-biotin complex is measured to be approximately 10^-15 M, making it one of the strongest known non-covalent bonds . The tetrameric form of avidin is estimated to be 66–69 kDa in size, with about 10% of its molecular weight contributed by carbohydrate moieties .

Avidin was first isolated from raw chicken egg white by Esmond Emerson Snell . The discovery began with the observation that chicks on a diet of raw egg white were deficient in biotin, despite the availability of the vitamin in their diet . Snell concluded that a component of the egg white was sequestering biotin, which he verified in vitro using a yeast assay . He later isolated the component responsible for biotin binding and confirmed that it was the cause of biotin deficiency or "egg white injury" .

The natural function of avidin in eggs is not entirely understood. However, it has been postulated that avidin is produced in the oviduct as a bacterial growth inhibitor by binding biotin, which is essential for bacterial growth . Streptavidin, a related protein with equal biotin affinity, is produced by certain strains of Streptomyces bacteria and is thought to inhibit the growth of competing bacteria, acting similarly to an antibiotic .

The high affinity of avidin for biotin has been exploited in various scientific applications since the mid-1970s . Avidin and its bacterial counterpart, streptavidin, have become standard reagents for diverse detection schemes, including histochemical applications, immunoassays, and DNA hybridization procedures . These techniques often involve applying a biotinylated probe to a sample and then detecting the bound probe with a labeled avidin or streptavidin .