Streptavidin Protein, AP. Streptavidin is biotin-binding protein that was originally isolated from Streptomyces avidinii. In contrast to avidin, streptavidin has no carbohydrate and has a mildly acidic pI of 5. Thermo Scientific Streptavidin products use a recombinant form of streptavidin having a mass of 53,000 daltons and a near-neutral pI. Streptavidin is a tetrameric protein, with each subunit binding one molecule of biotin with affinity similar to that of avidin. Guanidinium chloride will dissociate avidin and streptavidin into subunits, but streptavidin is more resistant to dissociation.