Dependence of conformational stability on hydrophobicity of the amino acid residue in a series of variant proteins substituted at a unique position of tryptophan synthase alpha subunit.

To elucidate the role of individual amino acid residues in stabilizing the conformation of a protein, we have constructed a series of variant alpha subunits of tryptophan synthase from Escherichia coli substituted by each of 20 amino acids at position 49, which is buried in the interior of the protein. The stabilities were quantitatively examined except for the mutant protein substituted by arginine, which was not obtained in enough quantity. The Gibbs energy of unfolding in water and the activation Gibbs energy of unfolding in 3 M guanidine hydrochloride for each protein were compared at pH 7.0 and pH 9.0. The Gibbs energy of unfolding in water at pH 7.0 varied from 0.72 to 1.92 times that of the wild-type protein by the substitutions, but the activation Gibbs energy of unfolding in 3 M guanidine hydrochloride varied only from 0.95 to 1.03 times that of the wild-type protein. Moreover, the stability of the protein substituted at this position, which is buried in the interior of the molecule, tended to increase linearly with increasing hydrophobicity of the substituted residue, unless the volume of the substituted residue was over a certain limit.