Fructose-1,6-bisphosphate phosphatase

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. 

Fructose-bisphosphate Phosphatase Converts Fructose-1,6-bisphosphate to Fructose-6-phosphate... 

Fructose-1,6-bisphosphate is converted to fructose-6-phos-phate by hydrolysis of the phosphoryl ester bond at C-l in a reaction catalyzed by fructose bisphosphate phosphatase. The standard free energy change for this reaction is about —4 keal/mol, corresponding to an equilibrium constant of about 103. Thus, the two conversions (the phosphorylation of fructose-6-phosphate to form fructose 1,6-bisphosphate with ATP as the phosphate donor, and the hydrolysis of fructose-bisphosphate to form fructose-6-phosphate) are both thermodynamically favored under any conditions that are likely to exist in a living cell. These two reactions constitute a pseudocycle and, consistent with the principles enunciated in the previous chapter, the pathways have evolved so the number of ATP-to-ADP conversions is greater in one direction than in the other. 

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). 

Glucagon and epinephrine also regulate pseudocycle II so as to stimulate gluconeogenesis while inhibiting glycolysis. They do this through a chain of reactions that results in a lowering of the concentration of the allosteric effector fructose-2,6-bisphosphate. This effector stimulates phosphofructokinase while it inhibits fructose bisphosphate phosphatase. 

If fructose bisphosphate phosphatase and phosphofructokinase were to both operate at high rates in the cell, a great amount of ATP would be broken down to no effect. The two reactions would oppose each other ... 

Fructose bisphosphate phosphatase is regulated by the same allosteric effectors as is phosphofmctokinase, except in the opposite manner. For example, phosphatase is activated by fmctose-2,6-bispho-sphate, whereas phosphofmctokinase is inactivated by it. If there were no coordinate regulation of these steps, the net result would be the runaway consumption of ATP in a futile cycle. The regulatory mechanism doesn t completely shut down either reaction rather, it ensures that there is a greater flow of carbon in one direction or the other. The small amount of ATP that is consumed by the futile cycle is the cost associated with the regulation. 

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. 

An analogous use of ATP is found in photosynthetic reduction of carbon dioxide in which ATP phos-phorylates ribulose 5-P to ribulose bisphosphate and the phosphate groups are removed later by phosphatase action on fructose bisphosphate and sedoheptulose bisphosphate (Section J,2). Phosphatases involved in synthetic pathways usually have a high substrate specificity and are to be distinguished from nonspecific phosphatases which are essentially digestive enzymes (Chapter 12). 

The reactions enclosed within the shaded box of Fig. 17-14 do not give the whole story about the coupling mechanism. A phospho group was transferred from ATP in step a and to complete the hydrolysis it must be removed in some future step. This is indicated in a general way in Fig. 17-14 by the reaction steps d, e, and/. Step/represents the action of specific phosphatases that remove phospho groups from the seven-carbon sedoheptulose bisphosphate and from fructose bisphosphate. In either case the resulting ketose monophosphate reacts with an aldose (via transketolase, step g) to regenerate ribulose 5-phosphate, the C02 acceptor. The overall reductive pentose phosphate cycle (Fig. 17-14B) is easy to understand as a reversal of the oxidative pentose phosphate pathway in which the oxidative decarboxylation system of Eq. 17-12 is... 

For all of these reactions, the equilibrium is in the direction of glycolysis, because of the utilization of ATP in the reaction and the high ratio of ATP to ADP in the cell. The reactions of phosphofructokinase and hexokinase are reversed in gluconeogenesis by simple hydrolysis of fructose bisphosphate to fructose 6-phosphate plus phosphate (catalysed by fructose bisphosphatase) and of glucose 6-phosphate to glucose plus phosphate (catalysed by glucose 6-phosphatase). 

Van Schaftingen, E., and Hers, H.-G., 1981. Inhibidon of fructose-1,6-bis-phosphatase by fructose-2,6-bisphosphate. Proceedings of the National Academy of Sciences, USA 78 2861-2863. 

Fructose 2,6-bisphosphate is formed by phosphorylation of fructose 6-phosphate by phosphofructoki-nase-2. The same enzyme protein is also responsible for its breakdown, since it has fructose-2,6-hisphos-phatase activity. This hifrmctional enzyme is under the allosteric control of fructose 6-phosphate, which stimulates the kinase and inhibits the phosphatase. Hence, when glucose is abundant, the concentration of fructose 2,6-bisphosphate increases, stimulating glycolysis by activating phosphofructokinase-1 and inhibiting... 

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. 

In contrast to kinases, phosphatases catalyse the removal of phosphoryl groups, again, either from phosphorylated metabolites such as glucose-6-phosphate or fructose-1,6-bisphosphate... 

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... 

Insulin activates PFK-2 (via the tyrosine kinase receptor and activation of protein phosphatases), which converts a tiny amovmt of fructose 6-phosphate to fructose 2,6-bisphosphate (F2,6-BP). F2,6-BP activates PFK-1. Glucagon inhibits PFK-2 (via cAMP-dependent protein kinase A), lowering F2,6 BP and thereby inhibiting PPK-1. 

Glucagon or epinephrine decreases [fructose 2,6-bisphosphate]. The hormones do this by raising [cAMP] and bringing about phosphorylation of the bifunctional enzyme that makes and breaks down fructose 2,6-bisphosphate. Phosphorylation inactivates PFK-2 and activates FBPase-2, leading to breakdown of fructose 2,6-bisphosphate. Insulin increases [fructose 2,6-bisphosphate] by activating a phosphoprotein phosphatase that dephosphorylates (activates) PFK-2. 

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