Streptavidin 's a 52,800 dalton tetrameric protein purified from the bacterium Streptomyces avidinii. It finds wide use in molecular biology through its extraordinarily strong affinity for biotin (also known as vitamin H); the dissociation constant (Kd) of the biotin-streptavidin complex's on the order of ~10-14 mol/L, ranking among one of the strongest non-covalent interactions known in nature. Structure The crystal structure of streptavidin with biotin bound was first solved in 1989 by Hendric… (More on Streptavidin)