Transformations of Fructose 6-Phosphate

The PPC allows the generation of NADPH reduction equivalents required for cell anabolism, and ribose 5-phosphate molecules for the synthesis of nucleic acids. Alternatively, ribose 5-phosphate can also be generated or transformed into fructose 6-phosphate or glyceraldehyde 3-phosphate, providing metabolic flexibility to the cell, in order to balance the fluxes through these pathways. The flux through the PPC is related to the nucleic acid requirements for DNA duplication or RNA transcription, and could probably be controlled by the cell cycle (Wagner, 1997). 

Rose and O Connell (1961) postulated that in muscle only a small fraction of tritium from glucose-2- H 6-phosphate appears in position 1 of fructose 6-phosphate while the most important fraction is recovered in H2O. Similar results were obtained with liver. In adipose tissue, only one-half of the hydrogen from position 2 of G-6-P is exchanged with protons via the phosphohexose-isomerase reaetion (Katz and Rognstad, 1969). The H from glucose-2- H will be recovered into fatty acids mainly after its transformation into fructose 6-phosphate-1,3- H via the oxidative pathway. 

Although we have described metabolic transformations in plant cells in terms of individual pathways, these pathways interconnect so completely that we should instead consider pools of metabolic intermediates shared among these pathways and connected by readily reversible reactions (Fig. 20-37). One such metabolite pool includes the hexose phosphates glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate a second includes the 5-phosphates of the pentoses ri-bose, ribulose, and xylulose a third includes the triose phosphates dihydroxyacetone phosphate and glycer-aldehyde 3-phosphate. Metabolite fluxes through these... 

In a very imaginative piece of research Frost and coworkers have developed a plasmid-based method for synthesizing aromatic amino acids, by incorporating the genes that code for the enzymes that perform the series of conversions from D-fructose-6-phosphate to D-erythrose-4-phosphate to 3-deoxy-D-arabinoheptulosonic acid-7-phos-phate (DAHP) near each other on a plasmid that can be transformed in E. coli. The enzymes are the thiamin diphosphate-dependent enzyme transketolase in the nonoxida-tive pentose shunt and DAHP synthase. The DAHP is then converted to the cyclic dehydroquinate, a precursor to all aromatic amino acids L-Tyr, L-Phe and L-Trp165,166 (equation 27). 

In the following stage, fructose-6-phosphate is phosphorylated again by the action of phosphofructokinase to form fructose-1,6-diphosphate. This reaction also consumes ATP. Later, the enzyme aldolase cleaves to fructose-6-phosphate. As a result of this reaction two triose phosphates are formed dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. This reaction produces a much greater proportion of dihydroxyacetone phosphate (96%), which is rapidly transformed into glycer-aldehyde-3-phosphateby triose phosphate isomerase (Heinisch and Rodicio 1996). 

Thiamine pyrophosphate is also an important cofactor for the transketolase reactions in the pentose phosphate pathway of carbohydrate metabolism (Fignre 15-3). These reactions are important in the reversible transformation of pentoses into the glycolytic intermediates fructose 6-phosphate and glyc-eraldehyde 3-phosphate. Again, it is the reactive carbon on the thiazole ring of TPP that reacts with a ketose phosphate (xylnlose 5-phosphate) to canse the release of an aldose phosphate with two fewer carbons (glyceraldehyde 3-phosphate). The TPP-bonnd glycoaldehyde unit is then transferred to a different aldose phosphate (ribose 5-phosphate or erythrose 4-phosphate) to produce a ketose phosphate that has two carbons more (sedoheptulose 7-phosphate or fructose 6-phosphate). 

P 11.91%, O 55.35%. Present in animal tissues as an equilibrium mixture with glucose-6-phosphate. The glucose-6-phosphate may be reversibly transformed into Iructose-6-phosphate by the enzyme phosphohexose isomerase. Prepn by hydrolysis of 1,6-fructose diphosphate with dil acid Neuberg, Biochem. Z. 88, 432 (1917) Ger, pat. 334,280 (Bayer) Chem. Zenfr. 1921, II, 961 Frdl. 13, 948. Role jn metabolic regulation and heat generation E. A. News-... 

Of significance are D-glucosamine and D-mannosamine (Fig. 29). d-G1ucos-amine-6-phosphate is formed from fructose-6-phosphate by the pathway given in Fig. 32. It is subsequently transformed to iV-acetylglucosamine-6-phosphate and iV-acetylmuramic acid. Synthesis of D-glucosamine-6-phosphate includes an intramolecular oxidoreduction at positions 1 and 2 transforming the ketose to an aldose derivative. 

One pathway for the metabolism of D-glucose 6-phosphate is its enzyme-catalyzed conversion to D-fructose 6-phosphate. Show that this transformation can be accomplished as two enzyme-catalyzed keto-enol tautomerisms. 

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