COLD DENATURATION-INDUCED CONFORMATIONAL-CHANGES IN PHOSPHOGLYCERATE KINASE FROM YEAST

The temperature-dependent conformational equilibrium of 3-phosphoglycerate kinase has been studied in the temperature range from 1 to 30-degrees-C by means of dynamic light scattering, small-angle X-ray scattering, differential scanning calorimetry, circular dichroism spectroscopy, and fluorescence spectroscopy. At 28-degrees-C and in the presence of 0.7 M guanidine hydrochloride (GuHCl), the radius of gyration (R(G)) and the Stokes radius (R(S)) are 2.44 and 3.09 nm, respectively. Decreasing the temperature effects unfolding of the molecule, a process that involves two stages. The two stages correspond to the successive unfolding of the N-terminal and C-terminal domains. The peak maxima of the excess heat capacity, determined from differential calorimetric scans, extrapolated to 0 scan rate, are positioned at 16.5-degrees-C for the N-terminal domain and at 6.3-degrees-C for the C-terminal domain. At 4.5-degrees-C, the radius of gyration and the Stokes radius increase to 7.8 and 4.8 nm, respectively. The persistence length and the length of the statistical chain segment of the unfolded polypeptide chain are 1.74 and 3.48 nm, corresponding to five and ten amino acids, respectively. At 1-degrees-C, the dimensions of the unfolded chain nearly agree with the predicted dimensions under theta conditions. Thus, the conformational changes upon cold denaturation can be described by a transition from a compactly folded molecule to a random coil. The conformation-dependent ratio rho = R(G)R(S)-1 increases from rho = 0.79 to rho = 1.63. The volume of the unfolded chain is 30 times larger than that of the folded chain in the native state. The unfolding/refolding processes of PGK in the GuHCl-containing solvent are governed by slow kinetics. Thermodynamic equilibrium is reached only some hours after the temperature is changed.