Fructose-2,6-bisphosphate regulation

See also Oxidative Phosphorylation (from Chapter 15), Regulation of Glycolysis, Fructose-2,6-Bisphosphate Regulation (from Chapter 16), Reactions/Energies of Glycolysis, Lactic Acid fermentation. Pyruvate Decarboxylase, Alcohol Dehydrogenase, Alcoholic Fermentation, Aerobic vs. Anaerobic Glycolysis... 

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. 

Clayton, C. (1986) Structure and regulated expression of genes encoding fructose bisphosphate aldolase in Trypanosoma brucei. EMBO J. 4 2997-3003. 

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.)...

As discussed in section 10.2.2, the formation of fructose bisphosphate, catalysed by phosphofructokinase, is an important step for the regulation of glucose metabolism. Once it has been formed, fructose bisphosphate is cleaved into two three-carbon compounds, which are interconvertible. The metabolism of these three-carbon sugars is linked to both the reduction of NAD" to NADH and direct (substrate-level) phosphorylation of ADP to ATP (section 3.3). The result is the formation of 2 mol of pyruvate from each mole of glucose. 

Since is positive, cycling will increase the sensitivity of the flux to regulator X provided that s j>r ,), that is, if the sensitivity of the subsequent reaction of the system to the concentration of the product, P, is greater than the sensitivity of the reverse reaction of the cycle to the concentration of P. This situation can be met in several ways (9). For example, in the fructose 6-phosphate/fructose bisphosphate cycle, fructose bisphosphatase (equivalent to C above) has aK for fructose 1,6-bis-phosphate of about 1 pM (or at least two orders of magnitude lower than many other glycolytic enzymes for their glycolytic substrates) so that it is probably saturated with fructose bisphosphate in vivo (equivalent to P in... 

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. 

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... 

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

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

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. 

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... 

Fructose 2,6-bisphosphate is not a metabolic intermediate but an allosteric regulator. It has two important roles it increases the activity of PFK-1 but decreases the activity of fructose 1,6-bisphosphatase (FBPase). Consequently an increase in the concentration of fructose 2,6-bisphosphate favours glycolysis but restricts gluconeogenesis. 

The activity of PFK-2, and hence the concentration of fructose 2,6-bisphosphate, is regulated in two ways ... 

Phosphofructokinase-1 is a regulatory enzyme (Chapter 6), one of the most complex known. It is the major point of regulation in glycolysis. The activity of PFK-1 is increased whenever the cell s ATP supply is depleted or when the ATP breakdown products, ADP and AMP (particularly the latter), are in excess. The enzyme is inhibited whenever the cell has ample ATP and is well supplied by other fuels such as fatty acids. In some organisms, fructose 2,6-bisphosphate (not to be confused with the PFK-1 reaction product, fructose 1,6-bisphosphate) is a potent allosteric activator of PFK-1. The regulation of this step in glycolysis is discussed in greater detail in Chapter 15. 

The most significant allosteric regulator of PFK-1 is fructose 2,6-bisphosphate, which strongly activates the enzyme. We return to this role of fructose 2,6-bisphosphate later. 

---