Gluconeogenesis sugar phosphates

ATP-proton motive force interconversion Electron transport Entner-Doudoroff Fermentation Glycolysis/gluconeogenesis Pentose phosphate pathway Pyruvate dehydrogenase Sugars TCA cycle Methanogenesis Polysaccharides Other... 

Now let us consider the further conversion of PEP and of the triose phosphates to glucose 1-phosphate, the key intermediate in biosynthesis of other sugars and polysaccharides. The conversion of PEP to glucose 1-P represents a reversal of part of the glycolysis sequence. It is convenient to discuss this along with gluconeogenesis, the reversal of the complete glycolysis sequence from lactic acid. This is an essential part of the Cori cycle (Section F) in our own bodies, and the same process may be used to convert pyruvate derived from deamination of alanine or serine (Chapter 24) into carbohydrates. 

In the non-oxidative portion of the Pentose Phosphate Pathway a series of sugar interconversions takes the RU-5-P to intermediates of other pathways Ribose-5-P for nucleotide biosynthesis, and F-6-P and Ga-3-P for glycolysis/ gluconeogenesis. All of these reactions are near equilibrium, with fluxes driven by supply and use of the three intermediates listed above. 

G6P is a phosphorylated form of glucose commonly found in cells. G6P is an intermediate in glycolysis, gluconeogenesis, the pentose phosphate pathway, the Calvin Cycle, glycogen biosynthesis, glycogen breakdown, and sugar interconversion. The latter three pathways indirectly involve G6P via the enzyme phosphoglucomutase. 

The other two reactions in which gluconeogenesis differs from glycolysis are ones in which a phosphate-ester bond to a sugar-hydroxyl group is hydrolyzed. Both reactions are catalyzed by phosphatases, and both reactions are exergonic. The first reaction is the hydrolysis of fructose-1,6- sphosphate to produce fructose-6-phosphate and phosphate ion AG" = -16.7 y moH = -4.0 kcal moH). 

The pathway borrows heavily from the nonoxidative branch of the pentose phosphate pathway and from gluconeogenesis. Without doubt, the pathways yield sugars as well as NADPH for reductive biosynthesis. Thus, only a few new enzymes would have to evolve through mutations to enable the complete Calvin cycle to function. 

Tissues which are more active in the synthesis of lipids than nucleotides require NADPH rather than ribose moieties. In such tissues, e.g. adipose tissue, the ribose 5-phosphate enters a series of sugar interconversion reactions which connect the pentose phosphate pathway with glycolysis and gluconeogenesis. These interconversion reactions constitute the non-oxidative phase of the pathway (Figure 11.14) and since oxidation is not involved, NADPH is not produced. Two enzymes catalyse the important reactions transketolase which contains thiamin diphosphate (Figure 12.3a) as its prosthetic group and transaldolase. Both enzymes function in the transfer of carbon units transketolase transfers two-carbon units and transaldolase transfers three-carbon units. The transfer always occurs from a ketose donor to an aldose acceptor. The interconversion sequence requires the oxidative phase to operate three times, i.e. three molecules of glucose 6-phosphate yield three molecules of ribulose 5-phosphate. 

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