Affinity purification of the avidin protein family, based on crystal structures of avidin-HABA complexes.

Since the importance of the high affinity between avidin and biotin, Kd  = 3 × 10-16  M, gained universal recognition, numerous chemical, biological and medical avidin-biotin based applications have been developed. However, in some cases the high affinity may be a disadvantage, as this interaction is irreversible under physiological conditions. The dye, 4'-hydroxyazobenzene-2-carboxylic acid (HABA), binds avidin, at the biotin binding site, as determined by X-ray, at a much lower affinity constant, Kd  = 6 × 10-6  M. We prepared a HABA affinity column (amber colored). Avidin bound to the column at a pH between 4 and 8.5, causing a change of color to red, and it could be eluted at mild conditions with buffers containing biotin, HABA, 1.5 M potassium chloride or a pH lower than 4.0 or higher than 8.5. Avidin eluted with HABA, created a red avidin-HABA complex, which was visualized. HABA free avidin was obtained by dialysis, which was followed by the loss of red coloration. The novel and easy to use HABA-affinity column was employed in our lab to prepare pure, fully glycosylated avidin from egg white. Most importantly, it may serve as an ideal tool for educational purposes, illuminating concepts of molecular recognition, reversible molecular binding, structure-based molecular design and solid phase chemical synthesis, as it is a reliable and visible reagent.