Streptavidin is a tetrameric protein composed of identical subunits. Each subunit specifically binds one biotin molecule with a KD of 4 x 10-14 M, making it one of the strongest non-covalent interactions. The preparation contains an N- and C-terminal shortened variant (core streptavidin), which shows improved properties concerning homogeneity, solubility, resistance towards proteolytic degradation, and accessibility of the biotin-binding pocket as compared to native streptavidin.
The streptavidin: biotin system is widely used for immobilization and detection of biotinylated molecules, like proteins and nucleic acids. Streptavidin is available as a dissolved or lyophilized version. The bulk amounts of reliable and high-quality lyophilized streptavidin are available at competitive prices to generate streptavidin-coated surfaces (microplates, SPR chips, beads) or for the detection of reagents conjugated with, for example, fluorescent dyes. It is important to note that streptavidin is not applicable for detecting, immobilizing, and purification of Strep-tag®II and Twin-Strep-tag® fusion proteins since the binding affinity for both tags is too low.