Thioredoxin has been chosen as our system of investigation of the forces contributing to protein stability and for the design of stable proteins of biotechnological interest. Thioredoxin is a small (12kDa) heat-stable protein found in all living cells from Archaea to humans. The structural stability of thioredoxin from Bacillus acidocaldarius (BacTrx) investigated by CD, DSC and nanogravimetry has shown higher thermostability when compared to the well-characterised thioredoxin from Escherichia coil (E.coli Trx) [1]. In order to further investigate on protein thermostability we have designed some mutants of BacTrx and one mutant from E.coli Trx based on molecular dynamics studies [2]. We obtained two single mutants of BacTrx in which we have substituted Lys18, Arg82 for the corresponding Gly (K18G) and Glu (R82E) present in E.coli Trx, the double mutant (K18G/R82E) and the Asp102X where the four C-terminus aminoacids from Asp102 were deleted [3]. We have substituted Glu85 in E.coli Trx with the Arg (E85R) present in the corresponding position in BacTrx. The mutants obtained were characterised by CD spectroscopy, spectrofluorimetry, thermodynamic comparative studies and limited proteolysis, all data were supported by molecular dynamic analysis at 500 K in vacuo. The results obtained, as modelling predicted, show the importance of hydrogen bonds and ion pairs in determining the protein stability and are useful to evaluate the contribution of localised structural determinants of protein stability.

[1] Bartolucci S., Guagliardi A., Pedone E., de Pascale D., Cannio R., Camardella L., Rossi M., Nicastro G., de Chiara C., Facci P., Mascetti G., Nicolini C., Biochem. J., 328, 277-285, 1997.

[2] Pedone E., Bartolucci S., Rossi M., Saviano M., J. Biomol Struct Dyn, 16, 437-46, 1998.

[3] Pedone E., Cannio R., Saviano M., Rossi M., Bartolucci S., Biochem. J., 339, 309-317, 1999.