The binding of biotin to avidin and strepavidin
Biotin is a vitamin with a molecular weight of 244 daltons.
Avidin is a protein with a molecular weight of about 68,000 daltons. It consists of four identical subunit chains each 128 amino acids long. Avidin is found predominately in the egg white of birds, amphibia, and reptiles.
Biotin is bound by avidin with fierce tenacity: it is the strongest non-covalent attachment known, with an association constant of 1015 M-1. This binding reaction has been studied intensively for many decades, and there is a rich literature. The great strength of this binding suggests that it might be a good model system for the study biological binding reactions in general. It has also formed the basis for many immunological methods for both research and clinical labs.
The protein strepavidin, produced by the bacterium Streptomyces avidinii, has a structure very similar to avidin, and also binds biotin tightly. It often exhibits lower nonspecific binding, and thus is frequently used in place of avidin.