Low-temperature unfolding of a mutant of phage T4 lysozyme. 1. Equilibrium studies

The mutant protein I3C-C97/C54T of phage T4 lysozyme is free of sulfhydryl groups and has a genetically engineered disulfide bridge between positions 3 and 97 (Perry & Wetzel, 1986). This protein has a maximum stability at 12 degrees C in 3 M guanidinium chloride and undergoes reversible high- and low-temperature melting at 28 and -3 degrees C, respectively, in this medium. The free energy of stabilization of the protein has been studied over a range of temperature that includes both melting transitions. The stability curve fits a constant delta Cp model over the entire range, permitting an unusually complete determination of the thermodynamic parameters of the protein and demonstrating that the low-temperature unfolded form of the protein may be interpreted as an extrapolation with constant delta Cp of the high-temperature unfolded form. The free energy of unfolding is a linear function of guanidinium concentration within experimental error which permits a rough estimate of the stability of the protein at low temperatures and of the differential interaction of the unfolded protein with guanidinium chloride. These equilibrium studies provide a basis for the interpretation of the kinetic studies reported in the following paper.