Avidin-biotin systems dissociation constant

The original interest in avidin was because of the egg white injury that was subsequently shown to be avidin-induced biotin deficiency. Thereafter, avidin was used because of its high affinity for biotin (a dissociation constant of 10 mol per L), not only to induce experimental biotin deficiency, but also to bind to biotin in isolated enzymes and thus, by irreversible inhibition, demonstrate the coenzyme role of biotin. Because of the stability of the avidin-biotin complex, it has not been possible to use immobilized avidin as a means of purifying biotin enzymes - there seems to be no way in which the enzyme can be released from avidin binding. Because of its high affinity for biotin, avidin is used to provide an extremely sensitive system for linking reporter molecules in a variety of analytical systems. 

A similarly interesting non-covalent complex could be envisioned with the streptavidin or avidin/biotin complex because of its extremely low dissociation constant of 10 15 M. This system has been extensively exploited in immunological techniques, particularly to improve sensitivity and specificity of immunoassays... 

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