Fructose 2,6-bisphosphate activity

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. 

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). 

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. 

Elevated concentration of fructose 2,6-bisphosphate activates PFK-1, which leads to an increased rate of glycolysis. 

In 1980, fructose 2,6-bisphosphate (F-2,6-BP) was identified as a potent activator of phosphofructokinase. Fructose 2,6-bisphosphate activates phosphofructokinase by increasing its affinity for fructose 6-phosphate and diminishing the... 

As with other biochemical pathways, hormones affect gluconeogenesis by altering the concentrations of allosteric effectors and the rate key enzymes are synthesized. As mentioned previously, glucagon depresses the synthesis of fructose-2,6-bisphosphate, activating the phosphatase function of PFK-2. The lowered concentration of fructose-2,6-bisphosphate reduces activation of PFK-1 and releases the inhibition of fructose-1,6-bisphosphatase. 

Indeed it appears that PPj-PFK is not a point of control for glycolysis in these anaerobic organisms and is not affected by concentrations of ATP/ADP/AMP which are key controllers of mammalian PFK. Nor is the main effector molecule of mammalian PFK, fructose-2,6-bisphosphate, active in this way in these anaerobic organisms. 

Fig. 22.14. Regulation of PFK-1 by AMP, ATP and fructose-2,6-bisP. A. AMP and fructose 2,6-bisphosphate activate PFK-1. B. ATP increases the rate of the reaction at low concentrations, but allosterically inhibits the enzyme at high concentrations.

Fig. 31.9. Enzymes involved in regulating the substrate cycles of glycolysis and gluconeogenesis. Heavy arrows indicate the three substrate cycles. F-2,6-P, fructose 2,6-bisphosphate +, activated by inhibited by circled t, inducible enzyme.

Fructose bisphosphate aldolase of animal muscle is a Class I aldolase, which forms a Schiff base or imme intermediate between the substrate (fructose-1,6-bisP or dihydroxyacetone-P) and a lysine amino group at the enzyme active site. The chemical evidence for this intermediate comes from studies with the aldolase and the reducing agent sodium borohydride, NaBH4. Incubation of fructose bisphosphate aldolase with dihydroxyacetone-P and NaBH4 inactivates the enzyme. Interestingly, no inactivation is observed if NaBH4 is added to the enzyme in the absence of substrate. 

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). 

In proteins with a symmetric structure, circular permutation can account for the shift of active-site residues over the course of evolution. A very good model of symmetric proteins are the (/Ja)8-barrel enzymes with their typical eightfold symmetry. Circular permutation is characterized by fusion of the N and C termini in a protein ancestor followed by cleavage of the backbone at an equivalent locus around the circular structure. Both fructose-bisphosphate aldolase class I and transaldolase belong to the aldolase superfamily of (a/J)8-symmetric barrel proteins both feature a catalytic lysine residue required to form the Schiff base intermediate with the substrate in the first step of the reaction (Chapter 9, Section 9.6.2). In most family members, the catalytic lysine residue is located on strand 6 of the barrel, but in transaldolase it is not only located on strand 4 but optimal sequence and structure alignment with aldolase class I necessitates rotation of the structure and thus circular permutation of the jS-barrel strands (Jia, 1996). 

Malcovati,M. Valentini, G. AMP- and fructose 1,6-bisphosphate-activated pyruvate kinases from Escherichia coli. Methods EnzymoL, 90, 170-179 (1982)... 

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. 

Fig. 2 Lactate dehydrogenase a) a ribbon representation of the tetramer of the B. stearothermophilus enzyme with each peptide chain depicted in a different color. The cofactor and oxamate inhibitor are colored according to atom type, as is fructose bisphosphate. which is an allosteric regulator of the enzyme, b) On the left is a detailed view of the enzyme active site as seen in the crystal structure. The ligand is highlighted in green and key amino acid residues are labeled. This is compared with the traditional two-dimensional representation of the enzyme mechanism on the right. Note that the residue numbers differ slightly from those of the muscle enzyme discussed in the test. (View this an i i color at www.dekker.com.)...

Fructose bisphosphate aldolase (isoenzyme B, M, 156,000) (EC 4.1.2.13). Fructosemia, fructosuria and hypoglucosemia after intake of fructose. Intracellular accumulation of fructose 1-phosphate, Hyperurate-mia. Hepatomegaly. Renal tubular dysfunction. Intraocular bleeding. Patients symptom-free and healthy if fructose avoided. Aldolase A (muscle and most other tissues) and aldolase C (brain and heart) present and fully active. 

Scheme 11.5. A cartoon representation of the catalyzed (fructose-bisphosphate aldolase, EC 4.1.2.13) aldol-type condensation between glyceraldehyde 3-phosphate and dihydroxyacetone monophosphate to produce the six-carbon ketosugar fructose-1,6-bisphosphate. An active site lysine Lys-NH2 [" H3NCH2CH2CH2CH2CH(NH3 )C02 ] apparently serves as the catalyst through addition at the carbonyl followed by proton tautomerization.

If alanine accumulates in muscle, it acts as an allosteric inhibitor of pyruvate kinase, so reducing the rate at which pyruvate is formed. This end-product inhibition of pyruvate kinase by alanine is over-ridden by high concentrations of fructose bisphosphate, which acts as a feed-forward activator of pyruvate kinase. ATP is an inhibitor of pyruvate kinase, and at high concentrations acts to inhibit the enzyme. More importantly, ATP acts as an allosteric inhibitor of phosphofructokinase (section 10.2.2.1). This means that, under conditions in which the supply of ATP (which can be regarded as the end-product of all energy-yielding metabolic pathways) is more than adequate to meet requirements, the metabolism of glucose is inhibited. 

Following fructose-bisphosphate aldolase activity, the second bypass reaction circumvents the... 

Figure 6.24 The function of the enzyme phosphofructokinase. (a) Phosphofructokinase is a key enzyme in the gycolytic pathway, the breakdown of glucose to pyruvate. One of the end products in this pathway, phosphoenolpyruvate, is an allosteric feedback inhibitor to this enzyme and ADP is an activator, (b) Phosphofructokinase catalyzes the phosphorylation by ATP of fructose-6-phosphate to give fructose-1,6-bisphosphate. (c) Phosphoglycolate, which has a structure similar to phosphoenolpyruvate, is also an inhibitor of the enzyme.

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