Abstract
Summary Both the cytosolic and plastid PFK isoenzymes of cucumber are inhibited by sulphate at pH 7.2 but not at pH 8.0. Both the purine and pyrimidine triphosphates can be used as the phosphoryl donor to phosphorylate fructose 6-phosphate. Both isoenzymes exhibit lower Km-values for the pyrimidine triphosphates than purine triphosphates, while the maximum catalytic activity is higher with the purine triphosphates. The cytosolic PFK isoenzyme is not sensitive to GTP inhibition in contrast to the plastid isoenzyme where GTP is as inhibitory as ATP. Phosphoenol pyruvate and ADP are the strongest inhibitors of the plastid and cytosolic isoenzymes, respectively. Depending on the pH citrate, 2-phosphoglycerate, 3-phosphoglycerate and A TP also inhibit both isoenzymes. In most cases the inhibition caused by these compounds can be alleviated by the addition of phosphate. Fructose-2,6-bisphosphate has no effect on the activity of either isoenzymes in the presence and absence of the mentioned effectors.
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