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. There are also dimeric members of the avidin family found in some bacteria. In chicken egg white, avidin makes up approximately 0.05% of total protein (approximately 180 ug 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 KD ≈ 10 M, making it one of the strongest known non-covalent bonds.