| || | 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.