Structural determinants responsible for the thermostability of Sso7d and its single point mutants

Sso7d is a small protein especially attractive as a model for characterizing by NMR, the structural determinants responsible for thermal stability. With this aim, structural parameters of the Sso7d protein have been compared with those of single point mutants, in particular with K12L, more stable (T-m around 100 degrees C) and F31A, less stable (T-m 61 degrees C) than the wild-type. In addition, a mutant lacking eight residues of the C-terminal helix (T-m 50 degrees C) has been studied to investigate the role of the helix in structure stabilization. Melting temperatures, Delta Delta G's of unfolding, cavities, electrostatic interactions, solvent accessible areas, hydrophobic cores, and aromatic cluster geometries have been analyzed in order to gain insights into the key structural factors determining the thermostability differences. Our data suggest that absence of cavities and a larger salt bridge network represent the major determinants contributing to the enhanced stability of K12L relative to the Sso7d protein and its mutant F31A.