Ribulose-5-phosphate metabolism

D-Ribulose Formed in metabolic processes. Ribulose phosphate is an intermediate in pentose phosphate pathway. ... 

Fig. 5. A simplified metabolic scheme of ethanol formation from glucose and xylose. Enzyme abbreviations GPDH Glucose 6-phosphate 1-dehydrogenase, PGDH Phosphogluconate dehydrogenase, PGI Glucose 6-phosphate-isomerase, RKI Ribose 5-phosphate isomerase, RPE Ribulose phosphate 3-epimerase, TAL Transaldolase, TKL Transketolase, XDH Xylitol dehydrogenase, XK-. Xylulokinase, XR Xylose reductase...

The phosphate derivatives of D-xylulose and D-ribulose occur as intermediates in the pentose phosphate metabolic pathway (see p. 202). 

Pentose Phosphate Metabolism. The interconversion of ribose-5-phosphate and ribulose-5-phosphate is catalyzed by phosphoribo-isomerase which Axelrod et al. (20) have shown to occur in pea and spinach leaves. Axelrod and Jang (22) have purified this enzyme 380-fold from alfalfa leaves. The further transformation of ribulose 5-phosphate to xylulose-5-phosphate is catalyzed by phosphoketo-pentose epimerase, which occurs in green leaves (J. Hurwitz and B. L. Horecker, unpublished observation). Transketolase, which reversibly transforms ribose-5-phosphate and xylulose-5-phosphate to sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate, and... 

M12. McNair Scott, D. B., and Cohen, S. S., The oxidative pathway of carbohydrate metabolism in Escherichia coli. 5. Isolation and identification of ribulose-5-phosphate produced from 6-phosphogluconate by the dehydrogenases of Escherichia coli. Biochem. J. 65, 686-689 (1957). 

The oxidative segment of the PPP converts glucose 6-phosphate to ribulose 5-phosphate. One CO2 and two NADPH+H" are formed in the process. Depending on the metabolic state, the much more complex regenerative part of the pathway (see B) can convert some of the pentose phosphates back to hexose phosphates, or it can pass them on to glycolysis for breakdown. In most cells, less than 10% of glucose 6-phosphate is degraded via the pentose phosphate 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... 

NADPH is required for many biosynthetic sequences. It is generated in different kinds of cells by a variety of reactions, including an NADP+-linked oxidation of malate to pyruvate and C02 and transfer of hydride ion from NADH to NADP+ in a mitochondrial reaction that is driven by metabolic energy. However, in many cases, including in the mammalian liver, a major part of the NADPH requirement is met by oxidation of glucose-6-phosphate to ribulose-5-phosphate and C02. The four electrons that are released by the oxidation are transferred to two molecules of NADP+. 

At this point the metabolic function of the sequence, when it is serving to supply electrons for biosynthesis, is fulfilled. Two molecules of NADPH are generated for each molecule of glucose-6-phosphate oxidized. It is only necessary to convert ribulose-5-phosphate, the end product of the... 

So in summary, three glucose-6-phosphate (3.1) molecules (3 x C6) are oxidized to three ribulose-5-phosphate (3.13) residues (3 x C5) and three molecules of C02 (3 x Ci) under generation of six molecules of NADPH. The three ribulose-5-phosphate residues are then converted to one glyceraldehyde-3-phosphate (3.14) molecule (lx C3) and two fructose-6-phosphate (3.2) molecules (2 x C6). Fructose-6-phosphate can be converted to glucose-6-phosphate and reenter the oxidative part of the pentose phosphate pathway. Fructose-6-phosphate and glyceraldehydes can also serve as intermediates in glycolysis (Section 5.1), which offers the cell considerable flexibility in terms of its metabolic flux. 

Enzymes of KDO Synthesis and Metabolism and Their Inhibition. The KDO pathway can be thought of as a minor branched pathway in carbohydrate metabolism initiating with the key intermediate in the hexose-monophosphate shunt, D-ribulose-5-phosphate. As shown in Figure 2 the biosynthesis and utilization are known to involve at least five sequential reactions ... 

In vivo measurements of lipopolysaccharide synthesis in E. coli B have indicated that two nanomoles of KDO must be synthesized per minute per mg of protein in order to meet the cellular requirement for LPS synthesis under the normal conditions of growth on glucose-minimal medium (27). We have measured the specific activities of the enzymes involved in KDO synthesis in crude extracts of E. coli B including those enzymes responsible for the synthesis of D-ribulose-5-phosphate, the precursor of D-arabinose-5-phosphate. D-Ribulose-5-phosphate is a key intermediate in carbohydrate metabolism as shown in Figure 2, since it is the direct precursor of both D-ribose-5-phosphate and D-arabinose-5-phosphate... 

Answer The reductive pentose phosphate pathway regenerates ribulose 1,5-bisphosphate from triose phosphates produced during photosynthesis, in a series of reactions involving sugars of three, four, five, six, and seven carbons and the enzymes transaldolase and transketo-lase. The oxidative pentose phosphate pathway plays a different metabolic role it provides NADPH for reductive biosynthesis and pentose phosphates for nucleotide synthesis. 

A third ketopentulose ester, xlulose 5-phosphate (Xlu 5-P) Fig- 2, was isolated as a product of Rib 5-P metabolism by Ashwell and Hickman (34). It was also shown by Srere et al. (35) that Xlu 5-P, rather than the earlier assigned Ru 5-P, was a definitive substrate of TK. Ribulose 5-phosphate-3 -epimerase (Fig. 2) catalyzed the formation of Xlu 5-P and imparted the transconflguration to the hydroxyls at carbons 3 and 4, which is a necessary stereochemical condition for substrate reactivity with TK. The 3 -epimerase was purified from a bacterial source... 

Interpreting the pathway as a means of oxidizing glucose to ribulose 5-phosphate and recycling the ribulose 5-phosphate back to glucose-6-phosphate is too narrow. The pathway performs a variety of functions, the least important of which seems to be that of an alternative pathway for glucose metabolism. Production of ribose 5-phosphate for nucleotide synthesis by de novo and... 

An enzymic counterpart of these complex base-catalysed rearrangements of sugars may be the reaction catalysed by 4-phospho-3,4-dihydroxy-2-butanone synthetase. The enzyme catalyses the formation of the eponymous intermediate in secondary metabolism from ribulose 5-phosphate. Labelling studies indicated that C1-C3 of the substrate became C1-C3 of the product, that H3 of the substrate derived from solvent and that C4 was lost as formate. X-ray crystal structures of the native enzyme and a partly active mutant in complex with the substrate are available. The active site of the enzyme from Met ha-nococcus jannaschii contains two metals, which can be any divalent cations of the approximate radius of Mg " or Mn ", the two usually observed. Their disposition is very reminiscent of those in the hydride transfer aldose-ketose isomerases, but also to ribulose-5-phosphate carboxylase, which works by an enolisation mechanism, so the enolisation route suggested by Steinbacher et al. is repeated in Figure 6.14, as is the Bilik-type alkyl group shift, for which an equivalent reverse aldol-aldol mechanism cannot be written. 

Another group of sugar epimerases, which uses a metal cofactor instead of NADH/NAD+, takes an entirely different approach to epimerization. L-ribulose 5-phosphate 4-epimerase, which is involved in the bacterial metabolism of arabinose, performs a retro-aldol cleavage of a C-C bond to yield a metal-stabilized enolate of dihydroxyacetone and glycoaldehyde phosphate, similar to the reaction catalyzed by class II aldolases [77-79]. The glycoaldehyde phosphate is thought to rotate, such that addition of the enolate generates the isomeric product. 

The net result of the metabolism of 3 moles of ribulose 5-phosphate in the pentose phosphate pathway is the formation of 2 moles of fructose 6-phosphate and 1 mole of glyceraldehyde 3-phosphate, which then continue through the glycolytic pathway with the production of NADH, ATP, and pyruvate. Because the pentose phosphate pathway begins with glucose 6-phosphate, and feeds back into the... 

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