Calorimetric study of tryptophan synthase alpha-subunit and two mutant proteins

Heat-denaturation of tryptophan synthase alpha-subunit from E. coli and two mutant proteins (Glu 49 leads to Gln or Ser; called Gln 49 or Ser 49, respectively) has been studied by the scanning microcalorimetric method at various pH, in an attempt to elucidate the role of individual amino acid residues in the conformational stability of a protein. The partial specific heat capacity in the native state at 20 degrees, Cp20, has been found to be (0.43 +/- 0.02) cal . k-1 . g-1, the unfolding heat capacity change, delta dCp, (0.10 +/- 0.01) cal . K-1 . g-1, and the unfolding enthalpy value extrapolated to 110 degrees, delta dh110, (9.3 +/- 0.5) cal . g-1 for the three proteins. The value of Cp20 was larger than those found for "fully compact protein" and that of delta dh110 was smaller. Unfolding Gibbs energy, delta dG at 25 degrees for Wild-type, Gln 49, and Ser 49 were 5.8, 8.4, and 7.1 kcal . mol-1 at pH 9.3, respectively. Unfolding enthalpy, delta dH, of the three proteins seemed to be the same and equal to (23.2 +/- 1.2) kcal . mol-1 at 25 degrees. As a consequence of the same value of delta dH and the different value in delta dG, substantial differences in unfolding entropy, delta dS, were found for the three proteins. The values of delta dG for the three proteins at 25 degrees coincided with those from equilibrium methods of denaturation by guanidine hydrochloride.