Streptavidin is a tetrameric protein (4 x 13 kDa), which binds very tightly to the small molecule Biotin. The binding constant for this interaction is very high (Ka ~ 1013 M-1) and turns the Streptavidin/Biotin system into a basis of many biochemical applications. Streptavidin-coated surfaces are characterised by very low nonspecific bindings.
