Streptavidin, a protein of 55,000 Daltons, is derived from Streptomyces avidinii and can bind 4 moles of biotin per mole of protein. The dissociation constant for biotin is approximately 10-15 M. The streptavidin-biotin complex is stable over a wide range of pH and temperatures. Streptavidin lacks carbohydrate residues present in the avidin molecule. This tends to reduce non-specific interactions with surface molecules and, therefore, streptavidin is preferred over avidin in many immunologic assays. Streptavidin can be covalently conjugated to fluorescent dyes and then used as a developer where the primary reagent was biotinylated.
