Inhibition by Metabolic Intermediates of Pyrophosphate : Fructose 6-Phosphate Phosphotransferase from Germinating Castor Bean Endosperm

The aim of this work was to study the effects of metabolic intermediates on pyrophosphate: fructose 6-phosphate phosphotransferase (PFP) from endosperm of germinating castor bean (Ricinus communis). Glucose 1-phosphate, glucose 6-phosphate, UDPglucose, glucose, fructose, sucrose and pyruvate, each at 1 mM, and malate, succinate and glycolate, each at up to 5mM, had no significant effect on enzyme activity. PFP was inhibited by 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate, citrate and all nucleotides tested. Inhibition by MgATP, 3-phosphoglycerate, phosphoenolpyruvate and citrate was due primarily to their effect on the affinity of PFP for fructose 2,6-bisphosphate, a potent activator of the enzyme. These inhibitors each increased the concentration of fructose 2,6-bisphosphate required for half maximum activation of PFP up to 30-fold, from 6 to 180 nanomolar. The possible roles of these compounds in the control of PFP are discussed.