Noncooperativity of biotin binding to tetrameric streptavidin.

Streptavidin tetramers have been separated according to their biotin content by anion exchange chromatography. Biotin-free and biotin-saturated streptavidin were coincubated. Streptavidin at intermediate ligation levels, i.e., with one, two, or three molecules of bound biotin, accumulates over time. A steady state distribution of ligation levels is reached after 2 days. When biotin was allowed to redistribute starting from homogeneous populations containing two molecules of biotin per tetramer, a similar steady state distribution of ligation levels was observed, thereby demonstrating an equilibrium distribution. Quantification of this equilibrium indicates that biotin binds to streptavidin with no cooperativity.