Interaction of biotin with streptavidin. Thermostability and conformational changes upon binding

The effect of biotin binding on streptavidin (STV) structure and stability was studied using differential scanning calorimetry, Fourier transform infrared spectroscopy (FT-IR), and fluorescence spectroscopy. Biotin increases the midpoint temperature Tm, of thermally induced denaturation of STV from 75 degrees C in unliganded protein to 112 degrees C at full ligand saturation. The cooperativity of thermally induced unfolding of STV changes substantially in presence of biotin. Unliganded STV monomer has at least one domain that unfolds independently. The dimer bound to biotin undergoes a single coupled denaturation process. Simulations of thermograms of STV denaturation that take into account only the thermodynamic effects of the ligand with a Ka approximately 10(15) reproduce the behavior observed, but the estimated values of Tm are 15-20 degrees C lower than those experimentally determined. This increased stability is attributed to an enhanced cooperativity of the thermal unfolding of STV. The increment in the cooperativity is as consequence of a stronger intersubunit association and an increased structural order upon binding. FT-IR and fluorescence spectroscopy data reveal that unordered structure found in unliganded STV disappears under fully saturating conditions. The data provide a rationale for previous suggestions that biotin binding induces an increase in protein tightness (structural cooperativity) leading, in turn, to a higher thermostability.