Streptavidin (SA) is a ~ 60 kDa homologous tetramer protein isolated from the culture medium of Streptomyces avidinii but is now more often recombinantly produced. Like avidin, streptavidin behaves as an antibiotic and binds 4 moles of biotin per mole of protein with a high affinity virtually unmatched in nature (Kd 10-15 mol/L). Due to the lack of the glycoprotein portion in streptavidin and its near-neutral isoelectric point, it exhibits less nonspecific binding than avidin in detection.
