Thermodynamics of the interaction of an anionic detergent, sodium dodecyl sulfate (SDS), with cellulase from Trichoderma reesei has been studied by isothermal titration calorimetry and fluorescence titration. The binding of SDS to cellulase is driven by a favorable entropy increase with a less favorable enthalpy decrease, and shows strong enthalpy-entropy compensation and weak affinity. A larger negative heat capacity change of the binding, - 186 J·mol~(-1)·K~(-1), at all temperatures examined indicates that hydrophobic interaction is a major force for the binding. SDS quenches the intrinsic fluorescence of cellulase, and causes both a red shift in the maximum fluorescence emission wavelength of the protein and a partial loss in the enzymatic activity. These results indicate that the interaction of SDS with cellulase includes contributions of the binding and the partial unfolding of die protein induced by SDS.

Key words: CALORIMETRY, FLUORIMETRIC ANALYSIS, CELLULASE, SDS, FLUORESCENCE QUENCHER