Differential scanning calorimetric study of the thermal unfolding of mutant forms of phage T4 lysozyme.

In two recent papers, we reported the effects of several point mutations on the thermodynamics of the thermal unfolding of the lysozyme of phage T4 as determined by differential scanning calorimetry. The mutants studied were R96H [Kitamura, S., & Sturtevant, J.M. (1989) Biochemistry 28, 3788-3792] and T157 replaced by A, E, I, L, N, R, and V [Connelly, P., Ghosaini, L., Hu, C.-Q., Kitamura, S., Tanaka, A., & Sturtevant, J.M. (1991) Biochemistry 30, 1887-1891]. Here we report the results of a similar study of the single mutations A82P, A93P, and G113A and the double mutation C54T:C97A. The three single mutants all show small apparent stabilization at pH 2.5 and 46.2 degrees C (the denaturational temperature of the wild-type protein), amounting to -0.5 +/- 0.4 kcal mol-1 in free energy, whereas the double mutant shows a weak apparent destabilization, +0.8 +/- 0.4 kcal mol-1. As in all our previous studies of mutant proteins, the enthalpy changes produced by these mutations are in general of much larger magnitude than the corresponding free energy changes and frequently of opposite sign.