The Regulation of Cytoplasmic Fructose 1,6-bisphosphatase in Relation to the Control of Carbon Flow to Sucrose

Abstract

Summary The activities of sucrose phosphate synthetase (EC 2.4.1.14) and cytoplasmic fructose 1,6-bisphosphatase (EC 3.1.3.11) are constant in light and dark treated spinach leaf protoplasts. During steady-state photosynthesis the rate of sucrose synthesis was proportional to the rate of CO2 fixation. Sucrose biosynthesis is unchanged in protoplasts which have been prepared in the presence of 1.0 mM concentrations of either NaNO33, NaNO2 or NHCI. Cytoplasmic fructose 1,6-bisphosphatase requires free magnesium for activity, magnesium-EDTA cannot be utilised. The enzyme has a high affinity for free manganese which will replace magnesium as the co-factor for activity. The maximum velocity attained in the presence of manganese is only two-thirds of that obtained with magnesium. Calcium is a potent inhibitor of the cytoplasmic fructose 1,6-bisphosphatase. It is a competitive inhibitor with respect to magnesium and a non-competitive inhibitor with respect to fructose 1,6-bisphosphate. Concentrations of fructose 1,6-bisphosphate above 100 μM are inhibitory to enzyme activity. This substrate inhibition is independent of pH and magnesium concentration.