Streptavidinisaproteinthathassimilarbindingpropertiestoeggwhiteavidin.Itisisolatedfromstreptomycesavidinii.Streptavidinhasamolecularweightof60KDandhas4subunits.Eachsubunitcanbindonemoleculeofbiotin.Biotinisawater-solublevitamin.Streptavidinhasanextremelyhighbindingaffinity(Kd=10-15)forbiotin.Ithasprovenusefulinthedetectionofantigenscoupledwithbiotinylatedsecondaryantibodies.Thereareseveraladvantageswhenusingstreptavidinversusanavidincomplex(ABC).Incontrasttoavidin,streptavidinisnotglycosylatedandisthereforeunchargedatneutralpH(6.5versus10).Thislowersnonspecificbackgroundstaining.Streptavidinalsolackscarbohydratesidechainsthatmaybeanothercauseofnon-specificbackground.Anotherkeyadvantageofstreptavidinisthesignificantincreaseinsensitivity(probablyduetolesssterichindrance),thusfacilitatinganincreaseinoverallbindingcapacity.Finally,streptavidin-enzymeconjugatesaremuchmorestablethananABCcomplex.TheABCcomplexmustbefreshlymade30minutespriortouseandisstableonlyforafewdays.Incontrast,astreptavidin-conjugatecanbestoredforupto1-2years.Thereagentiscolor-codeandcomesinaready-to-useformat,thussavingtimeandpotentialmistakes.Finally,streptavidin-enzymeconjugatesaremuchmorestablethananABCcomplex.TheABCcomplexmustbefreshlymade30minutespriortouseandisstableonlyforafewdays.Incontrast,astreptavidin-conjugatecanbestoredforupto1-2years.Thereagentiscolor-codeandcomesinaready-to-useformat,thussavingtimeandpotentialmistakes.