Enthalpic Barriers to the Hydrophobic Binding of Oligosaccharides to Phage P22 Tailspike Protein

The structural thermodynamics of the recognition of complex carbohydrates by proteins are not well understood. The recognition of O-antigen polysaccharide by phage P22 tailspike protein is a highly suitable model for advancing knowledge in this field. The binding to octa- and dodecasaccharides derived from Salmonella enteritidis O-antigen was studied by isothermal titration calorimetry and stopped-flow spectrofluorimetry. At room temperature, the binding reaction is enthalpically driven with an unfavorable change in entropy. A large change of -1.8 +/- 0.2 kJ mol(-1) K(-1) in heat capacity suggests that the hydrophobic effect and water reorganization contribute substantially to complex formation. As expected from the large heat-capacity change, we found enthalpy-entropy compensation. The calorimetrically measured binding enthalpies were identical within error to van't Hoff enthalpies determined from fluorescence titrations. Binding kinetics were determined at temperatures ranging from 10 to 30 degrees C. The second-order association rate constant varied from 1 x 10(5) M(-1) s(-1) for dodecasaccharide at 10 degrees C to 7 x 10(5) M(-1) s(-1) for octasaccharide at 30 degrees C. The first-order dissociation rate constants ranged from 0.2 to 3.8 s(-1). The Arrhenius activation energies were close to 50 and 100 kJ mol(-1) for the association and dissociation reactions, respectively, indicating mainly enthalpic barriers. Despite the fact that this system is quite complex due to the flexibility of the saccharide, both the thermodynamic and kinetic data are compatible with a simple one-step binding model.