Entropic Stabilization of the Tryptophan Synthase α-Subunit from a Hyperthermophile, Pyrococcus furiosus

The structure of the tryptophan synthase α-subunit from Pyrococcus furiosus was determined by x-ray analysis at 2.0-Å resolution, and its stability was examined by differential scanning calorimetry. Although the structure of the tryptophan synthase α₂β₂ complex fromSalmonella typhimurium has been already determined, this is the first report of the structure of the α-subunit alone. The α-subunit from P. furiosus (Pf-α-subunit) lacked 12 and 6 residues at the N and C termini, respectively, and one residue each in two loop regions as compared with that from S. typhimurium (St-α-subunit), resulting in the absence of an N-terminal helix and the shortening of a C-terminal helix. The structure of the Pf-α-subunit was essentially similar to that of the St-α-subunit in the α₂β₂ complex. The differences between both structures were discussed in connection with the higher stability of the Pf-α-subunit and the complex formation of the α- and β-subunits. Calorimetric results indicated that thePf-α-subunit has extremely high thermostability and that its higher stability is caused by an entropic effect. On the basis of structural information of both proteins, we analyzed the contributions of each stabilization factor and could conclude that hydrophobic interactions in the protein interior do not contribute to the higher stability of the Pf-α-subunit. Rather, the increase in ion pairs, decrease in cavity volume, and entropic effects due to shortening of the polypeptide chain play important roles in extremely high stability in Pf-α-subunit.