Avidin

Avidin
core-streptavidin mutant d128a at pH 4.5
Identifiers
Symbol	Avidin
Pfam	PF01382
InterPro	IPR005468
PROSITE	PDOC00499
SCOP	1slf
SUPERFAMILY	1slf
Available protein structures: Pfam structures PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

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 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 helpful for bacterial growth. 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.