The subunit interactions of fumarase.

The subunit interactions of fumarase have been examined as a function of enzyme concentration, pH, and concentration of guanidine hydrochloride. Fumarase was found to dissociate, with loss of activity, either at low concentrations of the enzyme ( 1 M). At pH 9 a species of enzyme was produced which was fully associated, but whose hydrodynamic structure was different from that of the native enzyme. Fumarase dissociated in dilute acid or alkali or by guanidine hydrochloride could be reassociated with nearly complete recovery of activity. The reassociation of the acid-dissociated fumarase was observed to follow first order kinetics with respect to enzyme concentration. L-Malate, citrate, phosphate, and ATP all reduced the rate of dissociation and increased the extent of recombination of the subunits. As judged by optical rotatory dispersion measurements, little change in polypeptide chain conformation was noted between native fumarase and the enzyme dissociated between pH 5 and Il. The thiol groups of the dissociated enzyme, however, were found to be more exposed to solvent than the same groups located in the tetrameric molecule.