REGULATION OF FRUCTOSE 1,6-BISPHOSPHATASE ACTIVITY OF CHLORELLA BY MOLE MASS CHANGE

Fast protein liquid chromatography on Superose 6 of partially purified FBPase II from Chlorella reveals a 1350 kDa-form at pH 6.0 and a 67 k Da-form at pH 8.5. Treatment of the large enzyme form with 5mM concent rations of Mg2+, F1,6P(2), DTT or ATP leads to dissociation into small er ones of 215-470 kDa. Aggregation/dissoziation is a reversible proce ss, as has been shown for the effect of F1,6P(2) and of pH, by rechrom atography. The change in mole mass results in alterations of the activ ity and df the kinetic properties of the enzyme forms, obtained. Disso ciation results in a 4-6 fold increase in activity, as can be shown fo r F1,6P(2)-treated samples. Halfsaturation constants, as well as the d egree of cooperativity of the 67- and the 1350-kDa form, are different for substrate affinity, activation by Mg2+ and DTT, and for inhibitio n by ATP. Both enzyme forms hydrolyse fructose 1,6 bisphosphate and se duheptulose 1,7 bisphosphate better than other phosphorylated compound s. The ratio of F1,6P(2)- to SDP-cleavage is 100:58 for the small enzy me form and 100:84 for the large one. Activation of FBPase II in the l ight and inactivation in the dark is discussed on the basis of differe nt oligomeric forms of the enzyme, generated by changes in the concent ration of intermediates and effecters in the chloroplast stroma, leadi ng to dissociation or aggregation. The conclusion is drawn that oligom erization of key enzymes, resulting in enzyme forms with different act ivities and different kinetic properties, might provide an effective m echanism for enzyme regulation in vivo.