Entropic stabilization of the tryptophan synthase alpha-subunit from a hyperthermophile, Pyrococcus furiosus - X-ray analysis and calorimetry

The structure of the tryptophan synthase alpha -subunit from Pyrococcus fur iosus was determined by x-ray analysis at 2.0-Angstrom resolution, and its stability was examined by differential scanning calorimetry. Although the s tructure of the tryptophan synthase alpha (2)beta (2) complex from Salmonel la typhimurium has been already determined, this is the first report of the structure of the alpha -subunit alone, The alpha -subunit from P, furiosus (Pf-alpha -subunit) lacked 12 and 6 residues at the N and C termini, respe ctively, and one residue each in two loop regions as compared with that fro m S, typhimurium (St-alpha -subunit), resulting in the absence of an N-term inal helix and the shortening of a C-terminal helix, The structure of the P f-alpha -subunit was essentially similar to that of the St-alpha -subunit i n the alpha (2)beta (2) complex, The differences between both structures we re discussed in connection with the higher stability of the Pf-alpha -subun it and the complex formation of the alpha- and beta -subunits. Calorimetric results indicated that the Pf-alpha -subunit has extremely high thermostab ility and that its higher stability is caused by an entropic effect. On the basis of structural information of both proteins, we analyzed the contribu tions of each stabilization factor and could conclude that hydrophobic inte ractions in the protein interior do not contribute to the higher stability of the Pf-alpha -subunit. Rather, the increase in ion pairs, decrease in ca vity volume, and entropic effects due to shortening of the polypeptide chai n play important roles in extremely high stability in Pf-alpha -subunit.