A Mechanism of Regulation of Hepatic Fru 2,6-P2Concentration upon Refeeding: Involvement of Xylulose 5-P and Cyclic-AMP
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Dephosphorylation
Phosphofructokinase 2
Fructose 2,6-bisphosphate
Fructolysis
Fructose 1,6-bisphosphatase
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Phosphofructokinase 2
Fructose 2,6-bisphosphate
Fructose 1,6-bisphosphatase
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Fructolysis
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Fructose 2,6-bisphosphate
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Phosphofructokinase 2
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Activities catalyzing the synthesis and degradation of fructose 2,6-bisphosphate—6-phosphofructo-2-kinase (ATP:D-fructose-6-phosphate-2-phosphotransferase, EC 2.7.1.105) and fructose-2,6-bisphosphatase (D-fructose-2,6-bisphosphate 2-phosphohydrolase, EC 3.1.3.46)—were isolated from spinach leaves by an improved procedure and separated on the basis of both charge and molecular weight. The separated activities showed no detectable cross-contamination, indicating, in contrast to all previous data, that they are not present on a single bifunctional protein of the classical type in liver. The fructose-2,6-bisphosphatase—a newly discovered phosphatase enzyme—differed from previous mixed preparations by showing greater specificity but lower affinity for fructose 2,6-bisphosphate, greater sensitivity to inhibition by inorganic phosphate, and in being sensitive to inhibition by Mg 2+ . The 6-phosphofructo-2-kinase was found to be inhibited by low levels of inorganic pyrophosphate and, in addition, to be regulated by the metabolites described previously. Similar results were obtained with preparations from lettuce leaves. The results support the view that, through individual regulation of the activities catalyzing its synthesis and breakdown, cytosolic metabolites are key factors in controlling the fructose 2,6-bisphosphate content of leaves.
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Fructose 2,6-bisphosphate has been discovered as a potent stimulator of liver phosphofructokinase. It is also an inhibitor of fructose 1,6-biphosphatase and a stimulator of PP i : fructose 6-phosphate phosphotransferase from higher plants. It is formed from fructose 6-phosphate and ATP by a 6-phosphofructo 2-kinase and hydrolysed by a fructose 2,6-bisphosphatase. These two enzymes have very similar physicochemical properties and could not be separated from each other. They are substrates for cyclic-AMP-dependent protein kinase, which inactivates the first enzyme and activates the second.
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Purified chicken liver 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase was phosphorylated either from fructose 2,6-bis[2-32P]phosphate or fructose 2-phosphoro[35S]thioate 6-phosphate. The turnover of the thiophosphorylated enzyme intermediate as well as the overall phosphatase reaction was four times faster than with authentic fructose 2,6-bisphosphate. Fructose 2-phosphorothioate 6-phosphate was 10-100-fold less potent than authentic fructose 2,6-bisphosphate in stimulating 6-phosphofructo-1-kinase and pyrophosphate:fructose 6-phosphate phosphotransferase, but about 10 times more potent in inhibiting fructose 1,6-bisphosphatase. The analogue was twice as effective as authentic fructose 2,6-bisphosphate in stimulating pyruvate kinase from trypanosomes.
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Historical Perspective and Discovery of Fructose-2,6-Bisphosphate. Effects of Fructose-2,6-Bisphosphate on 6-Phosphofructo 1-Kinase and Fructose-1,6-Bisphosphatase. The Sites of Interaction of Fructose-2,6-Bisphosphate and Fructose-1,6-Bisphosphate with Their Target Enzymes: 6-Phosphofructo-1-Kinase and Fructose-1,6-Bisphosphatase. Analogs of Fructose-2,6-Bisphosphate as Activators of 6-Phosphofructo-1-Kinase and Pyrophosphate-Dependent Phosphofructokinase and as Inhibitors of Fructose-1,6-Bisphosphatase. Role of Fructose-2,6-Bisphosphate in the Regulation of Hepatic Carbohydrate Metabolism. Liver 6-Phosphofructo-2-Kinase/Fructose-2,-6-Bisphosphatase. Tertiary Structure Modeling of 6-Phosphofructo-2-Kinase/Fructose-2,6-Bisphosphatase: Structural, Funtional, and Evolutionary Design of Bifunctional Enzyme. Fructose-2,6-Bisphosphate in Extra Hepatic Tissues. The Role of Fructose-2,6-Bisphosphate in Plant Tissues. On The Nature of Fructose-2,6-P2 Metabolizing Enzymes in Plants. Role of Fructose-2,6-P2 in Yeast. Fructose-2,6-Bisphosphate in Primitive Systems. The Importance of Being Bifunctional (with Apologies to Oscar Wilde). Reflections on Future Research on Fructose-2,6-Bisphosphate Metabolism. c. 192 pp., 7x10, 1989, ISBN 0-8493-4795-5.
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Fructose 2,6-bisphosphate
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An enzyme activity that catalyzes the hydrolysis of phosphate from the C-2 position of fructose 2,6-bisphosphate has been detected in rat liver cytoplasm. The S0.5 for fructose 2,6-bisphosphate was about 15 microM and the enzyme was inhibited by fructose 6-phosphate (Ki 40 microM) and activated by Pi (KA 1 mM). Fructose 2,6-bisphosphatase activity was purified to homogeneity by specific elution from phosphocellulose with fructose by specific elution from phosphocellulose with fructose 6-phosphate and had an apparent molecular weight of about 100,000, 6-phosphofructo 2-kinase activity copurified with fructose 2,6-bisphosphatase activity at each step of the purification scheme. Incubation of the purified protein with [gamma-32P]ATP and the catalytic subunit of the cAMP-dependent protein kinase resulted in the incorporation of 1 mol of 32P/mol of enzyme subunit (Mr = 50,000). Concomitant with this phosphorylation was an activation of the fructose 2,6-bisphosphatase and an inhibition of the 6-phosphofructo 2-kinase activity. Glucagon addition to isolated hepatocytes also resulted in an inhibition of 6-phosphofructo 2-kinase and activation of fructose 2,6-bisphosphatase measured in cell extracts, suggesting that the hormone regulates the level of fructose 2,6-bisphosphate by affecting both synthesis and degradation of the compound. These findings suggest that this enzyme has both phosphohydrolase and phosphotransferase activities i.e. that it is bifunctional, and that both activities can be regulated by cAMP-dependent phosphorylation.
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Isolated rat hepatocytes convert 2,5-anhydromannitol to 2,5-anhydromannitol-1-P and 2,5-anhydromannitol-1,6-P2. Cellular concentrations of the monophosphate and bisphosphate are proportional to the concentration of 2,5-anhydromannitol and are decreased by gluconeogenic substrates but not by glucose. Rat liver phosphofructokinase-1 phosphorylates 2,5-anhydromannitol-1-P; the rate is less than that for fructose-6-P but is stimulated by fructose-2,6-P2. At 1 mM fructose-6-P, bisphosphate compounds activate rat liver phosphofructokinase-1 in the following order of effectiveness: fructose-2,6-P2 much greater than 2,5-anhydromannitol-1,6-P2 greater than fructose-1,6-P2 greater than 2,5-anhydroglucitol-1,6-P2. High concentrations of fructose-1,6-P2 or 2,5-anhydromannitol-1,6-P2 inhibit phosphofructokinase-1. Rat liver fructose 1,6-bisphosphatase is inhibited competitively by 2,5-anhydromannitol-1,6-P2 and noncompetitively by 2,5-anhydroglucitol-1,6-P2. The AMP inhibition of fructose 1,6-bisphosphatase is potentiated by 2,5-anhydroglucitol-1,6-P2 but not by 2,5-anhydromannitol-1,6-P2. Rat liver pyruvate kinase is stimulated by micromolar concentrations of 2,5-anhydromannitol-1,6-P2; the maximal activation is the same as for fructose-1,6-P2. 2,5-Anhydroglucitol-1,6-P2 is a weak activator. 2,5-Anhydromannitol-1-P stimulates pyruvate kinase more effectively than fructose-1-P. Effects of glucagon on pyruvate kinase are not altered by prior treatment of hepatocytes with 2,5-anhydromannitol. Pyruvate kinase from glucagon-treated hepatocytes has the same activity as the control pyruvate kinase at saturating concentrations of 2,5-anhydromannitol-1,6-P2 but has a decreased affinity for 2,5-anhydromannitol-1,6-P2 and is not stimulated by 2,5-anhydromannitol-1-P. The inhibition of gluconeogenesis and enhancement of glycolysis from gluconeogenic precursors in hepatocytes treated with 2,5-anhydromannitol can be explained by an inhibition of fructose 1,6-bisphosphatase, an activation of pyruvate kinase, and an abolition of the influence of phosphorylation on pyruvate kinase.
Fructose 2,6-bisphosphate
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