Activation of Chloroplast Fructose-1, 6-Bisphosphatase of Pisum sativum by Mole Mass Change

Separation of extracts, obtained from isolated intact P. sativum chloroplasts, by fast protein liquid chromatography (FPLC) on superose 6, reveals a 1,400 kDa-FBPase II form at pH 6.0 and a 380 kDa form at pH 7.5. Addition of F1,6P₂, Mg⁺⁺ and ATP cause dissociation of the large form into the smaller one, which leads to an approximate 4-fold increase in activity. Reversibility of the mole mass change could be shown for the influence of pH and of fructose-1, 6-bisphosphate on purified enzyme samples, separated from crude leaf extracts. Compared to thelarge enzyme form, the small form has higher activity and is specific for the substrate fructose-1, 6-bisphosphate, while the large form is not. Activation of FBPase II in the light and inactivation in the dark is discussed on the basis of different oligomeric forms of the enzyme caused by changes in the concentration of intermediates and effectors in the chloroplast stroma. The conclusion is drawn that oligomerization of key enzymes might provide an effective mechanism for enzyme activation/inactivation in vivo.