1,6-bisphosphate

Esterification. The hydroxyl groups of sugars can react with organic and inorganic acids just as other alcohols do. Both natural and synthetic carbohydrate esters are important in various apphcations (1,13). Phosphate monoesters of sugars are important in metabohc reactions. An example is the enzyme-catalyzed, reversible aldol addition between dibydroxyacetone phosphate [57-04-51 and D-ylyceraldehyde 3-phosphate [591-57-1 / to form D-fmctose 1,6-bisphosphate [488-69-7],... 

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.

Pyruvate, Citrate, Succinate, Glyceraldehyde-3-phosphate, Fructose-1,6-bisphosphate, 3-PhosphogIyceric acid... 

Fructose-6-phosphate + ATP. fructose-1,6-bisphosphate + ADP + Fructose-1,6-bisphosphate dihydroxyacetoiie-P + glyceraldehyde-3-P Dihydroxyacetoiie-P . glyceraldehyde-3-P ... 

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. 

If fructose-1,6-bisphosphatase and phosphofructokinase acted simultaneously, they would constitute a substrate cycle in which fructose-1,6-bisphosphate and fructose-6-phosphate became interconverted with net consumption of ATP ... 

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. 

The structure of human muscle fructose-1,6-bisphosphate aldolase, as determined by X-ray crystallography and downloaded from the Protein Data Bank. (PDB ID 1ALD Gamblin, S. J., Davies, G. J., Grimes, J. M., Jackson, R. M., Littlechild, J. A., Watson, H. C. Activity and specificity of human aldolases. J. Mol. Biol. v219, pp. 573-576, 1991.)... 

One of the steps in the biological pathway for carbohydrate metabolism is the conversion of fructose 1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Propose a mechanism for the transformation. 

Fructose 6-phosphate is phosphorylated by reaction with ATP to yield fructose 1,6-bisphosphate. 

Fructose 1,6-bisphosphate undergoes ring opening and is cleaved by a retro-aldol reaction into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (DHAP). 

Step 3 of Figure 29.7 Phosphorylation Fructose 6-phosphate is converted in step 3 to fructose 1,6-bisphosphate (FBP) by a phosphofmctokinase-catalyzed reaction with ATP (recall that the prefix bis- means two). The mechanism is similar to that in step 1, with Mg2+ ion again required as cofactor. Interestingly, the product of step 2 is the tv anomer of fructose 6-phosphate, but it is the (3 anomer that is phos-phorylated in step 3, implying that the two anomers equilibrate rapidly through the open-chain form. The result of step 3 is a molecule ready to be split into the two three-carbon intermediates that will ultimately become two molecules of pyruvate. 

Step 4 of Figure 29.7 Cleavage Fructose 1,6-bisphosphate is cleaved in step 4 into two 3-carbon pieces, dihydroxyacetone phosphate (DHAP) and glyceraldehvde 3-phosphate (GAP). The bond between C3 and C4 of fructose 1,6-bisphosphate... 

Figure 29.9 Mechanism of step 4 in Figure 29.7, the cleavage of fructose 1,6-bisphosphate to yield glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate join together in an aldol reaction to give fructose 1.6-bisphosphate. 

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