Evidence for an intermediate in the denaturation and assembly of phosphoglucose isomerase

The denaturation of dimeric rabbit muscle phosphoglucose isomerase in guanidine hydrochloride occurs in two discrete steps consisting of partial unfolding followed by subunit dissociation. In 3.5 to 4.5 m guanidine hydrochloride the enzyme forms a stable denaturation intermediate. Formation of this intermediate abolishes catalytic activity, shifts the protein fluorescence emission maximum from 332 to 345 nm, exposes all of the unavailable sulfhydryl groups, and decreases the s_20,w from 6.8 to 4.6 S. The intermediate dissociates into fully unfolded polypeptide chains with further increases in the concentration of the denaturant. The fluorescence maximum shifts to 352 nm and the s_20,w of the denatured monomer is 1.6 S. From the equilibrium constant for subunit association, 3 × 10⁴M^?1, in 4.7 m guanidine hydrochloride, the apparent free energy of association is estimated to be ?6 kcal mol^?1. Reconstitution of the enzyme protein takes place by the reversal of the steps observed upon denaturation. The denatured monomers refold and associate to reform the dimeric intermediate which then anneals to yield the intact enzyme molecule.