Control of CO2 fixation regulation of stromal fructose-1,6-bisphosphatase in spinach by pH and Mg(2+) concentration.

The effect of pH and of Mg(2+) concentration on the light activated form of stromal fructose-1,6-bisphosphatase (FBPase) was studied using the enzyme rapidly extracted from illuminated spinach chloroplasts. The (fructose-1,6-bisphosphate(4-))(Mg(2+)) complex has been identified as the substrate of the enzyme. Therefore, changes of pH and Mg(2+) concentrations have an immediate effect on the activity of FBPase by shifting the pH and Mg(2+) dependent equilibrium concentration of the substrate. In addition, changes of pH and Mg(2+) concentration in the assay medium have a delayed effect on FBPase activity. A correlation of the activities observed using different pH and Mg(2+) concentrations indicates, that the effect is not a consequence of the pH and Mg(2+) concentration as such, but is caused by a shift in the equilibrium concentration of a hypothetical inhibitor fructose-1,6-bisphosphate(3-) (uncomplexed), resulting in a change of the activation state of the enzyme. The interplay between a rapid effect on the concentration of the substrate and a delayed effect on the activation state enables a rigid control of stromal FBPase by stromal Mg(2+) concentrations and pH. Fructose-1,6-bisphosphatase is allosterically inhibited by fructose-6-phosphate in a sigmoidal fashion, allowing a fine control of the enzyme by its product.