Calorimetric Observation of a GroEL-Protein Binding Reaction with Little Contribution of Hydrophobic Interaction

Binding of Escherichia colichaperonin, GroEL, to substrate proteins with non-native structure, reduced α-lactalbumin (rLA) and denatured pepsin, were analyzed by isothermal titration calorimetry at various temperatures in the presence of salt (0.2 m KCl). Both proteins bound to GroEL with 1:1 stoichiometry and micromolar affinity at all temperatures tested. However, thermodynamic properties of their binding to GroEL are remarkably different from each other. While heat capacity changes (ΔCp) of rLA-GroEL binding showed large negative values, −4.19 kJ mol−1 K−1, that of denatured pepsin-GroEL binding was only −0.2 kJ mol−1K−1. These values strongly indicate that the hydrophobic interaction is a major force of rLA-GroEL binding but not so for denatured pepsin-GroEL binding. When salt was omitted from the solution, the affinity and ΔCp of the rLA-GroEL binding reaction were not significantly changed whereas denatured pepsin lost affinity to GroEL. Thus, in the non-native protein-GroEL binding reaction, thermodynamic properties, as well as the effect of salt, differ from protein to protein and hydrophobic interaction may not always be a major driving force.