Streptavidin

CAS NO.: 9013-20-1
Molecular Formula: C14H17BrClNO2S
Molecular weight: 378.709
Appearance: White powder
Assay: 14 (Streptavidin units/mg protein)
Solubility: Soluble in water

Streptavidin is a 52.8 kDa protein purified from the bacterium Streptomyces avidinii. Streptavidin homo-tetramers have an extraordinarily high affinity for biotin (also known as vitamin B7 or vitamin H). With a dissociation constant (Kd) on the order of ≈10−14 mol/L, the binding of biotin to streptavidin is one of the strongest non-covalent interactions known in nature. Streptavidin is used extensively in molecular biology and bionanotechnology due to the streptavidin-biotin complex's resistance to organic solvents, denaturants (e.g. guanidinium chloride), detergents (e.g. SDS, Triton), proteolytic enzymes, and extremes of temperature and pH.

Function and Application
Among the most common uses are the purification or detection of various biomolecules. The strong streptavidin-biotin bond can be used to attach various biomolecules to one another or onto a solid support. Harsh conditions are needed to break the streptavidin-biotin interaction, which often denatures the protein of interest being purified. However, it has been shown that a short incubation in water above 70 °C will reversibly break the interaction (at least for biotinylated DNA) without denaturing streptavidin, allowing re-use of the streptavidin solid support. A further application of streptavidin is for purification and detection of proteins genetically modified with the Strep-tag peptide.
Streptavidin is widely used in Western blotting and immunoassays conjugated to some reporter molecule, such as horseradish peroxidase. Streptavidin has also been used in the developing field of Nanobiotechnology, the use of biological molecules such as proteins or lipids to create nanoscale devices/structures. In this context streptavidin can be used as a building block to link biotinylated DNA molecules to create single walled carbon nanotube scaffolds or even complex DNA polyhedra. The tetrameric streptavidin has also been used as a hub around which other proteins may be arranged, either by an affinity tag such as Strep-tag or AviTag or by genetic fusion to SpyTag. Fusion to SpyTag allowed generation of assemblies with 8 or 20 streptavidin subunits. As well as a molecular force probe for atomic force microscopy studies, novel materials such as 3D crystalline lattices have also been created.
Streptavidin has a mildly acidic isoelectric point (pI) of ~5, but a recombinant form of streptavidin with a near-neutral pI is also commercially available.