What is biotin?
Biotin is a small molecule (~244 Da) produced by living organisms. Also known as Vitamn B7, biotin acts as a coenzyme, and is necessary for the synthesis of various physiologically relevant compounds.
What is avidin/streptavidin?
Avidin and streptavidin are both tetrameric proteins that possess an exceptionally high affinity for biotin. Avidin is naturally found in egg-whites, and in the oviducts of a variety of egg producing animals, while streptavidin is produced by Streptomyces avidinii. Curiously, while these two structures share little sequence similarity, they produce very similar higher-order structures. Both are thought to have evolved as a method of inhibiting bacterial growth (a number of bacterial species rely upon biotin for critical metabolic processes).
Why use biotin/streptavidin?
In molecular biology and biotechnology, biotin is widely utilized as conjugate for antibodies or other proteins. The relatively small size of biotin means that it can be safely conjugated to many complex, folded protein structures without the concern that stearic inhibition will interfere with production, folding, or physiological function of the host protein.
The interaction between biotin and streptavidin is among the strongest non-covalent chemical bonds ever identified, and biotin-streptavidin is often applied like “molecular velcro” to physically attach two different structures that would typically not interact.
For example, biotin conjugated antibodies are frequently used with a variety of streptavidin conjugated dyes or enzymes. By utilizing this approach, and separating the antibody from its detection conjugate, one develops an exceptionally powerful and versatile tool. Now a single antibody could potentially be used with a streptaividin-conjugated fluorescent dye for cell sorting and IF microscopy, a streptavidin-conjugated peroxidase enzyme for histochemistry or ELISA, and even strepatividin-conjugate gold particles for immune-electron microscopy.
Biotinylated recombinant proteins
Biotin-streptavidin is a core element in modern proteomic analysis. In fact, most ELISA kits make use of biotinylated antibodies, and streptavidin conjugated peroxidase enzymes.
One vital, but often underexplored avenue however is biotinylated recombinant proteins.
By conjugating an active, recombinant protein with biotin, one opens up a world of possibilities for investigating protein-protein interactions, or probing physiological function.