Energetics of denaturation and m values of staphylococcal nuclease mutants

In a continuation of an earlier study [Carra, J., Anderson, E., & Privalov, P. (1994) Biochemistry 33, 10842-10850], we used differential scanning calorimetry to measure the enthalpy and heat capacity changes of denaturation for 11 mutant forms of staphylococcal nuclease, including the triple mutant [V66L+G88V+G79S]. Several mutant proteins with m- characteristics of guanidinium chloride denaturation were found to denature via a three-state mechanism with increasing temperature. Enthalpy changes for the transitions from the native to intermediate and from the intermediate to denatured states were determined. In the case of the triple mutant, the enthalpy of the second endothermic transition is greater than that of the first. Observation of this second transition provides an explanation for the previously reported large changes in the delta H denaturation for the triple mutant versus wild-type nuclease. The sequence specificity of structure in the intermediate state is discussed with relevance to m values of guanidinium chloride denaturation. The enthalpic level of the intermediate state depends upon the amino acid sequence, suggesting that stabilizing mutations can increase the extent or cohesion of structure present in the intermediate.