| || |
Streptavidin and avidin are tetrameric proteins with four specific, high-affinity binding sites for biotin. These biotin-binding proteins are most commonly used for the isolation and detection of biomolecules, including proteins and nucleic acids. Avidin is a highly glycosylated and positively charged 69 kDa protein that is isolated from the egg whites of birds, amphibians, and reptiles, whereas streptavidin is a nonglycosylated 53 kDa protein with a near-neutral isoelectric point, that is purified from bacteria. Streptavidin is typically used more often in biological systems than avidin because it displays much lower nonspecific binding, due to its nonglycosylation and its neutral isoelectric point.