Avidin

Avidin is a tetrameric or dimeric biotin-binding protein produced in the oviducts of birds, reptiles and amphibians 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.