Energetic roles of hydrogen bonds at the ureido oxygen binding pocket in the streptavidin-biotin complex

The high-affinity streptavidin-biotin complex is characterized by an extensive hydrogen-bonding network. A study of hydrogen-bonding energetics at the ureido oxygen of biotin has been conducted with site-directed mutations at Asn 23, Ser 27, and Tyr 43. A new competitive biotin binding assay was developed to provide direct equilibrium measurements of the alterations in Kd. S27A, Y43F, Y43A, N23A, and N23E mutants display DeltaDeltaG degrees at 37 degrees C relative to wild-type streptavidin of 2.9, 1.2, 2.6, 3.5, and 2.6 kcal/mol, respectively. The equilibrium-binding enthalpies for all of the mutants were measured by isothermal titration calorimetry, and the Y43A and N23A mutants display large decreases in the equilibrium binding enthalpy at 25 degrees C of 8.9 and 6.9 kcal/mol, respectively. The S27A and N23E mutants displayed small decreases in binding enthalpy of 1.6 and 0.9 kcal/mol relative to wild-type, while the Y43F mutant displayed a -2.6 kcal/mol increase in the binding enthalpy at 25 degrees C. At 37 degrees C, the Y43A and N23A mutants display decreases of 7.8 and 7.9 kcal/mol, respectively, while the S27A, N23E, and Y43F mutants displayed decreases of 4.9, 3.7, and 1.2 kcal/mol relative to wild-type. Kinetic analyses were also conducted to probe the contributions of the hydrogen bonds to the activation barrier. Wild-type streptavidin at 37 degrees C displays a koff of (4.1 +/- 0.3) x 10(-5) s-1, and the conservative Y43F, S27A, and N23A mutants displayed increases in koff to (20 +/- 1) x 10(-5) s-1, (660 +/- 40) x 10(-5) s-1, and (1030 +/- 220) x 10(-)5 s-1, respectively. The Y43A and N23E mutants displayed 93-fold and 188-fold increases in koff, respectively. Activation energies and enthalpies for each of the mutants were determined by transition-state analysis of the dissociation rate temperature dependence. All of the mutants except Y43F display large reductions in the activation enthalpy. The Y43F mutant has a more positive activation enthalpy, and thus a more favorable activation entropy that underlies the overall reduction in the activation barrier. For the most conservative mutant at each ureido oxygen hydrogen-bonding position, bound-state alterations account for most of the energetic changes in a single transition-state model, suggesting that the ureido oxygen hydrogen-bonding interactions are broken in the dissociation transition state.