Fructose-2,6-bisphosphate liver

Winder, W. W., Yang, H. T., and Arogyasami,). (19fWt). Liver fructose 2,6-bisphosphate in rats running different treadmill speeds. Am. ]. Physiol. 255, R3S-R41. 

Naerum, L. Norskov-Lauridsen, P. B. Rasmussen, L. Thim, F. C. Wiberg, and K. Lundgren, Characterization of the allosteric binding pocket of human liver fructose 1,6-bisphosphate by protein crystallography and inhibitor activity studies, Protein Sci. 6 971 (1997). 

The same protein kinase that phosphorylates glycogen phosphorylase and glycogen synthase does not phosphorylate the enzymes of pseudocycle II. Rather an enzyme gets phos-phorylated that catalyzes the synthesis of a potent allosteric effector of the two relevant enzymes, phosphofructokinase and fructose bisphosphate phosphatase. In the liver the un-phosphorylated form this enzyme synthesizes fructose-2,6-bisphosphate. Phosphorylation converts it into a degradative enzyme for the same compound. Fructose-2,6-bisphosphate is an activator of phosphofructokinase and an inhibitor of fructose bisphosphate phosphatase. As a result the net effect of glucagon on pseudocycle II is to stimulate fructose bisphosphate phosphatase while inhibiting phosphofructokinase (see table 12.2 and fig. 12.30). 

Fructose-2,6-bisphosphate is a potent activator of the liver phosphofructokinase (PFK-1) and a potent inhibitor of liver fructose-1,6-bisphosphate phosphatase (FBPase-1). Fructose-2,6-bisphosphate is the product of a second phosphofructokinase (PFK-2) and is hydrolyzed to fructose-6-phosphate by FBPase-2. The activities of PKF-2 and FBPase-2 reside on a single, bifunctional protein in liver. The bifunctional protein is under glucagon control imposed via cAMP. 

Pyruvate kinase possesses allosteric sites for numerous effectors. It is activated by AMP and fructose-1,6-bisphosphate and inhibited by ATP, acetyl-CoA, and alanine. (Note that alanine is the a-amino acid counterpart of the a-keto acid, pyruvate.) Furthermore, liver pyruvate kinase is regulated by covalent modification. Flormones such as glucagon activate a cAMP-dependent protein kinase, which transfers a phosphoryl group from ATP to the enzyme. The phos-phorylated form of pyruvate kinase is more strongly inhibited by ATP and alanine and has a higher for PEP, so that, in the presence of physiological levels of PEP, the enzyme is inactive. Then PEP is used as a substrate for glucose synthesis in the pathway (to be described in Chapter 23), instead... 

The hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate (Eigure 23.7), like all phosphate ester hydrolyses, is a thermodynamically favorable (exergonic) reaction under standard-state conditions (AG° = —16.7 kj/mol). Under physiological conditions in the liver, the reaction is also exergonic (AG = —8.6 kJ/mol). Fructose-1,6-bisphosphatase is an allosterically regulated enzyme. Citrate stimulates bisphosphatase activity, hut fructose-2,6-bisphosphate is a potent allosteric inhibitor. / MP also inhibits the bisphosphatase the inhibition by / MP is enhanced by fructose-2,6-bisphosphate. 

Fructose 2,6-Bisphosphate Plays a Unique Role in the Regulation of Glycolysis Gluconeogenesis in Liver... 

The most potent positive allosteric effector of phospho-ffuctokinase-1 and inhibitor of fructose-1,6-bisphos-phatase in liver is fructose 2,6-bisphosphate. It relieves inhibition of phosphofructokinase-1 by ATP and increases affinity for fructose 6-phosphate. It inhibits fructose-1,6-bisphosphatase by increasing the for fructose 1,6-bisphosphate. Its concentration is under both substrate (allosteric) and hormonal control (covalent modification) (Figure 19-3). 

Figure 19-3. Control of glycolysis and gluconeoge-nesis in the liver by fructose 2,6-bisphosphate and the bifunctional enzyme PFK-2/F-2,6-Pase (6-phospho-fructo-2-kinase/fructose-2,6-bisphosphatase). (PFK-1, phosphofructokinase-1 [6-phosphofructo-1 -kinase] ...

In liver, cAMP activates gluconeogenesis, but in muscle, it activates glycolysis. Let s do liver first, and the muscle answer will just be the opposite. So, we want to activate gluconeogenesis in liver in response to increased phosphorylation (increased levels of cAMP). Phosphorylation of our enzyme (PFK-2) must have an effect that is consistent with the activation of gluconeogenesis. If gluconeogenesis is on and glycolysis is off, the level of fructose 2,6-bisphosphate (an activator of glycolysis) must fall. If fructose 2,6-bisphosphate is to fall, the PFK-2 that synthesizes it must be made inactive. So, in liver, phosphorylation of PFK-2 must inactivate the enzyme. 

There s also a fructose 2,6-bisphosphatase that hydrolyzes fructose 2,6-bisphosphate see if you can figure out what happens to the phosphatase activity in liver and muscle when the enzyme is phosphorylated. As a check to your answer, you might recall the PFK-2 and fructose 2,6-bisphosphatase are one and the same protein. Phosphorylation-dephosphorylation actually shifts the activity of this single protein between the kinase and the phosphatase. So the answer you get should be opposite to the one we got earlier. 

Feed-forward control is more likely to be focused on a reaction occurring at or near the end of a pathway. Compounds produced early in the pathway act to enhance the activity of the control enzyme and so prevent a back log of accumulated intermediates just before the control point. An example of feed-forward control is the action of glucose-6-phosphate, fructose-1,6-bisphosphate (F-l,6bisP) and phosphoenol pyruvate (PEP), all of which activate the enzyme pyruvate kinase in glycolysis in the liver. 

The first phosphatase step is very important FBPase converts fructose,1-6-bisphos-phate into fructose-6-phosphate under allosteric control of several factors but during fasting, glucagon-induced regulation is crucial. One effect of glucagon stimulation of liver cells is to reduce the concentration of fructose-2,6-bisphosphate, an isomer that activates PFK-1 and is itself synthesized by PFK-2 when fructose-6-phosphate concentration rises... 

When fructose 2,6-bisphosphate binds to its allosteric site on PFK-1, it increases that enzyme s affinity for its substrate, fructose 6-phosphate, and reduces its affinity for the allosteric inhibitors ATP and citrate. At the physiological concentrations of its substrates ATP and fructose 6-phosphate and of its other positive and negative effectors (ATP, AMP, citrate), PFK-1 is virtually inactive in the absence of fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate activates PFK-1 and stimulates glycolysis in liver and, at the same time, inhibits FBPase-1, thereby slowing gluconeogenesis. 

PFK-1 is allosterically inhibited by ATP and citrate. In most mammalian tissues, including liver, PFK-1 is allosterically activated by fructose 2,6-bisphosphate. 

Effect of elevated insulin concentration on the intracellular concentration of fructose 2,6-bisphosphate in liver. PFK-2 = phosphofructokinase-2-, FBP-2 = Fructose bisphospate phosphatase-2. 

Feed-forward regulation In liver, pyruvate kinase is activated by fructose 1,6-bisphosphate, the product of the phosphofructo-kinase reaction. This feed-forward (instead of the more usual feedback) regulation has the effect of linking the two kinase activities increased phosphofructokinase activity results in elevated levels of fructose 1,6-bisphosphate, which activates pyruvate kinase. 

Allosteric changes usually involve rate-determining reactions. For example, glycolysis in the liver is stimulated following a meal by an increase in fructose 2,6-bisphosphate, an allosteric activator of phosphofructokinase (see p. 98). Gluconeogenesis is inhibited by fructose 2,6-bisphosphate, an inhibitor of fructose 1,6-bisphos-phatase (see p. 118). 

Correct answer = B. The increased insulin and decreased glucagon levels characteristic of the fed state promote the synthesis of fructose 2,6-bisphosphate. Most covalently modified enzymes are in the dephosphorylated state and are active. Acetyl CoA is not elevated in the fed state. The transport of glucose in the liver is not insulin sensitive. Synthesis of glucokinase is enhanced in the fed state. 

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