STABILITY OF OXIDIZED ESCHERICHIA-COLI THIOREDOXIN AND ITS DEPENDENCEON PROTONATION OF THE ASPARTIC-ACID RESIDUE IN THE 26 POSITION

The effects of pH in the range 6.0-8.0 on the thermodynamics of the re versible thermal unfolding of Escherichia coli thioredoxin in the oxid ized state have been determined over a range of concentrations using d ifferential scanning calorimetry. The thermal denaturation indicated a n inverse temperature dependence on concentration. The data were shown to fit a model based on dimerization of both the native and denatured states of the protein. The degree of dimerization of both states was found to be pH dependent. The previously described importance of proto nation of the anomalously titrating aspartic acid 26 residue [Langsetm o, K., Fuchs, J., & Woodward, C. (1991) Biochemistry 30, 7603-7609] wa s apparently verified by the agreement between the experimentally dete rmined DELTADELTAG-degrees(d) and the calculated DELTADELTAG-degrees(H ) in the pH range 7.0-8.0.