Fructose- 1,6-bisphosphate

It should be noted that Fru-6-P,2K and Fru-2,6-P2ase of animal tissues are regulated by phosphorylation via a cAMP-dependent protein kinase that, in turn, is regulated hormonally. So far, there is no evidence that plant Fru-6-P,2K and Fru- 

As discussed above, the Pj released in sucrose synthesis is recycled to the chlo-roplast, via the phosphate translocator in strict counter-exchange for triose phosphate [53]. 3-PGA can also be transported by this same carrier but, as also noted above, its export from the chloroplast in the light is restricted. 

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FIGURE 19.9 Fructose-2,6-bisphosphate activates phosphofructokinase, iucreasiug the affinity of the enzyme for fructose-6-phosphate and restoring the hyperbolic dependence of enzyme activity on substrate. 

FIGURE 19.10 Fructose-2,6-bisphosphate decreases the inhibition of phosphofructo-... 

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. 

As described in Chapter 19, Emile Van Schaftingen and Henri-Gery Hers demonstrated in 1980 that fructose-2,6-bisphosphate is a potent stimulator of phosphofructokinase. Cognizant of the reciprocal nature of regulation in glycolysis and gluconeogenesis. Van Schaftingen and Hers also considered the... 

FIGURE 23.12 Inhibition of fructose-1,6-bisphosphatase by fructose-2,6-bisphosphate in the (a) absence and (b) presence of 25 /xM AMP. In (a) and (b), enzyme activity is plotted against substrate (fructose-1,6-bisphosphate) concentration. Concentrations of fructose-2,6-bisphosphate (in fiM) are indicated above each curve, (c) The effect of AMP (0, 10, and 25 fiM) on the inhibition of fructose-1,6-bisphosphatase by fructose-2,6-bisphos-phate. Activity was measured in the presence of 10 /xM fructose-1,6-bisphosphate. 

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-bisphosphatase, a regulatory enzyme of gluconeogenesis (Chapter 19), catalyzes the hydrolytic release of the phosphate on carbon 2 of fructose 2,6-bisphosphate. Figure 7-8 illustrates the roles of seven active site residues. Catalysis involves a catalytic triad of one Glu and two His residues and a covalent phos-phohistidyl intermediate. 

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). 

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... 

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] ...

The primary hormonal signals serve as extracellular signals that are interpreted by a signal transduction apparatus and turned into signals within the cell—these second messengers such as cAMP and fructose 2,6-bisphosphate warn individual enzymes within the cell about what s happening outside. 

How does phosphorylation affect the activity of phosphofructo-2-kinase (PFK-2), the enzyme that synthesizes fructose 2,6-bisphosphate, a regulator of glycolysis There are two possible answers it either activates it or inactivates it. The simplest approach to the question is just to flip a coin. You should stand a 50 50 chance of getting it right. The next simplest way is to figure it out. 

Fructose 2,6-bisphosphate stimulates glycolysis by allosterically activating phosphofructo-1-kinase (PFK-1). First, decide what should... 

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. 

In muscle, phosphorylation of PFK-2 in response to increased cAMP activates the enzyme, the level of fructose 2,6-bisphosphate rises, and glycolysis is activated. 

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. 

A. R. Fernie, A. Roscher, R. G. Ratcliffe, and N. J. Kruger, Fructose 2,6 bisphosphate activates pyrophosphate fructose 6 phosphate 1 phosphotransferase and increases triose phosphate to hexose phosphate cycling in heterotrophic cells. Planta 212, 250 263 (2001). 

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... 

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