DENATURED STATES OF YEAST PHOSPHOGLYCERATE KINASE

Structures of proteins in unfolded states have important implications for the protein folding problem and for the translocation of polypepti de chains. Acid-denatured, cold-denatured, and 6 M guanidine hydrochlo ride (GuHCl) denatured yeast phosphoglycerate kinase (PGK) are ensembl es of flexible unfolded molecules with rapidly interconverting structu res of the individual polypeptide chains. They differ, however, in the ir physical properties, such as in coil size and in stiffness over a s hort distance along the chain. These properties of polypeptide chains can be described well by persistence statistics. A solution containing 0.7 M GuHCl at 4.5 degrees C is nearly a theta-solvent for PGK. By co ntrast, 6 M GuHCl is a good solvent for PGK. Acid-denatured PGK at low ionic strength has the most expanded and stiffest chains. The conform ation of heat-denatured PGK should be more compact than that of random walk chains at the theta-point, as can be inferred from measurements on other proteins. Investigations of heat-denatured PGK by scattering methods are unfeasible due to aggregation of the protein. The persiste nce length as a measure of chain stiffness varies between a 1.74 nm fo r cold-denatured PGK and a = 3.0 nm for acid-denatured PGK. The distri bution functions of the gyration radii were calculated from the X-ray scattering data for all unfolded states and compared with the radius o f gyration of the natively folded molecule.