Streptavidin

Streptavidin is a biotin-binding protein found in the culture broth of the bacterium Streptomyces avidinii. Streptavidin can bind to four moles of biotin per mole of protein,and it shows extremely high affinity, approximately 10,000-15,000 Da. Streptavidin lacks the carbohydrate side chains present on avidin and has an isoelectric point of 6.5 to avidin's 10 far closer to that at which most useful biological interactions occur. As a result, Streptavidin frequently exhibits much lower non-specific binding than avidin does.