Ribose-5-phosphate, formation oxidation

This pathway is variously known as the pentose phosphate, hexose monophosphate or phosphogluconate pathway, cycle or shunt. Although the pentose phosphate pathway achieves oxidation of glucose, this is not its function, as indicated by the distribution of the pathway in different tissues. Only one of the carbons is released as CO2, the key products are NADPH and ribose 5-phosphate, both of which are important for nucleotide phosphate formation and hence for synthesis of nucleic acids (Chapter 20). The... 

In 1932 Levene and Harris128 showed that the hydrolysis of xanthylic acid gave rise to the formation of a D-ribose phosphate which was not identical with the known D-ribose 5-phosphate. Since xanthylic acid is the monophosphate derivative of a ribofuranoside of xanthine it followed that the new phosphate was either D-ribose 2-phosphate or the 3-isomer (L). Shortly thereafter the same authors129 succeeded in reducing the new D-ribose phosphate with hydrogen in the presence of platinum oxide to a ribitol phosphoric acid (LI) which was completely... 

The pyrimidine ring is assembled first and then linked to ribose phosphate to form a pyrimidine nucleotide. PRPP is the donor of the ribose phosphate moiety. The synthesis of the pyrimidine ring starts with the formation of carbamoylaspartate from carbamoyl phosphate and aspartate, a reaction catalyzed by aspartate transcarbamoylase. Dehydration, cyclization, and oxidation yield orotate, which reacts with PRPP to give orotidylate. Decarboxylation of this pyrimidine nucleotide yields UMP. CTP is then formed by the amination of UTP. 

Mode 2. The needs for NADPH and ribose 5-phosphate are balanced. The predominant reaction under these conditions is the formation of two molecules of NADPH and one molecule of ribose 5-phosphate from one molecule of glucose 6-phosphate in the oxidative phase of the pentose phosphate pathway. The stoichiometry of mode 2 is... 

The pentose phosphate pathway also catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions. All these reactions occur in the cytosol, and in plants part of the pentose phosphate pathway also participates in the formation of hexoses from CO2 in photosynthesis. Thus, D-ribulose 5-phosphate can be directly converted into D-ribose 5-phosphate by phosphopentose isomerase, or to D-xylulose 5-phosphate by phosphopentose epimerase. D-Xylulose 5-phosphate can then be combined with D-ribose 5-phosphate to give rise to sedoheptulose 7-phosphate and glyceraldehyde-3-phosphate. This reaction is a transfer of a two-carbon unit catalyzed by transketolase. Both products of this reaction can be further converted into erythrose 4-phosphate and fructose 6-phosphate. The four-carbon sugar phosphate erythrose 4-phosphate can then enter into another transketolase-catalyzed reaction with the D-xylulose 5-phosphate to form glyceraldehyde 3-phosphate and fructose 6-phosphate, both of which can finally enter glycolysis. 

Only the nonoxidative branch of the pathway is significantly active when much more ribose 5-phosphate than NADPH needs to be synthesized. Under these conditions, fructose 6-phosphate and glyceraldehyde 3-phosphate (formed by the glycolytic pathway) are converted into ribose 5-phosphate without the formation of NADPH. Alternatively, ribose 5-phosphate formed by the oxidative branch can be converted into pyruvate through fructose 6-phosphate and glyceraldehyde 3-phosphate. In this mode, ATP and NADPH are generated, and five... 

A coenzyme formed by the chemical combination of nicotinamide, adenine, ribose and phosphate. Its formation requires the vitamin niacin. In the body it is employed as a hydrogen (electron) acceptor during the oxidation (breakdown) of foods to form energy. 

Phosphoribosyl-l-pyrophosphate (PRPP) may be considered a precursor in the de novo sjmthetic reactions of purines, since this ribose derivative was required for the formation of 5-phosphoribosylamine (PRA). PRA was the precursor of nitrogen 9, ribose, and phosphate of the completed purine nucleotide structure (Section II, B, 1). PRPP was also a key substance in the biosynthesis of pyrimidine nucleotides. This compound was formed from ribose 5-phosphate and ATP by a pyrophosphorylation of carbon 1 of ribose 5-phosphate (78-80). This was an unusual kinase reaction in that pyrophosphate was transferred rather than phosphate as was the case with most kinases. The ribose 5-phosphate required for the syntheas of PRPP probably originated from glucose, and was formed either by an oxidative pathway from glucose 6-phosphate via 6-pho hogluconate and ribulose 5-phosphate (81) or anaerobically from fructose 6-pho hate (88). The formation of PRPP is shown in Fig. 4. 

The study of these metabolic steps is quite active at the present time. As noted above, Dickens found that ribose-5-phosphate was fermented anaerobically to ethanol, a 2-carbon compound, inorganic phosphate, and CO2. Racker observed that extracts of E. colt converted ribose-5-phosphate to a triose phosphate, which could be analyzed in the presence of triose phosphate isomerase as dihydroxyacetone phosphate.Therefore, the products of the oxidative pathway eventually join the Embden-Meyerhof scheme at the triose phosphate stage, the major difference being the formation of 2 moles of triose phosphate in the latter pathway and only 1 mole via the phosphogluconate pathway. 

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