Pentose phosphate pathway, oxidative phase

List the two phases of the pentose phosphate pathway (oxidative generation of NADPH and nonoxidative interconversion of sugars). List the biochemical pathways that require NADPH from the pentose phosphate pathway. 

Figure 6-3. The pentose phosphate pathway. In the oxidative phase of the pentose phosphate pathway, NADP is reduced to NADPH H, with feedback regulation by NADPH at the step catalyzed by glucose 6-phosphate dehydrogenase. In the nonoxidative phase, multiple sugar interconversions catalyzed by three different enzymes occur.

The first phase of the pentose phosphate pathway consists of two oxidations that convert glucose 6-phosphate to ribulose 5-phosphate... 

The pentose phosphate pathway can be divided in two phases an oxidative phase during which glucose-6-phosphate is converted to ribulose-5-phosphate, and a non-oxidative phase constituting of a series of reversible reactions in which two pentose-phosphate residues are converted to a series of sugar-phosphate molecules of differing lengths (Figure 3-2). 

D-Glucose may enter pentose phosphate pathway in Schizosaccharomyces pombe via the oxidative phase of the pentose phosphate pathway ... 

The pentose phosphate pathway meets the need of all organisms for a source of NADPH to use in reductive biosynthesis (Table 20.2). This pathway consists of two phases the oxidative generation of NADPH and the nonoxidative interconversion of sugars (Figure 20.19). In the oxidative phase, NADPH is generated when glucose 6-phosphate is oxidized to ribose 5-phosphate. This five-carbon sugar and its derivatives are components of RNA and DNA, as well as ATP, NADH, FAD, and coenzyme A. 

Figure 20.19. Pentose Phosphate Pathway. The pathway consists of (1) an oxidative phase that generates NADPH and (2) a nonoxidative phase that interconverts phosphorylated sugars.

Figure 20.20. Oxidative Phase of the Pentose Phosphate Pathway. Glucose 6-phosphate is oxidized to 6-phosphoglucono-6-lactone to generate one molecule of NADPH. The lactone product is hydrolyzed to 6-phosphogluconate, which is oxidatively decarboxylated to ribulose 5-phosphate with the generation of a second molecule ofNADPH.

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... 

Mode 4. Both NADPH and ATP are required. Alternatively, ribose 5-phosphate formed by the oxidative phase of the pentose phosphate pathway can be converted into pyruvate. Fructose 6-phosphate and glyceraldehyde 3-phosphate derived from ribose 5-phosphate enter the glycolytic pathway rather than reverting to glucose 6-phosphate. In this mode, ATP and NADPH are concomitantly generated, and five of the six carbons of glucose 6-phosphate emerge in pyruvate. 

Figure 20.20 Oxidative phase of the pentose phosphate pathway.

Mode 3. Much more NADPII than ribose 5-phosphate is required. For example, adipose tissue requires a high level of NADPH for the synthesis of fatty acids (Table 20.4). In this case, glucose 6-phosphate is completely oxidi i to CO>. Three groups of reactions are active in this situation. First, the oxidative phase of the pentose phosphate pathway forms two molecules ot NADPH and one molecule of ribose S-phosphate. Then, ribose 5-phosphate... 

Tracing glucose. Glucose labeled wuth at C-6 is added to a solution containing the enzymes and cofactors of the oxidative phase ol the pentose phosphate pathway. W hat is the fate of the radioactive label ... 

The pentose phosphate pathway is an alternative metabolic pathway for glucose oxidation in which no ATP is generated. Its principal products are NADPH, a reducing agent required in several anabolic processes, and ribose-5-phosphate, a structural component of nucleotides and nucleic acids. The pentose phosphate pathway occurs in the cytoplasm in two phases oxidative and nonoxidative. In the oxidative phase of the pathway, the conversion of glucose-6-phosphate to ribu-lose-5-phosphate is accompanied by the production of two molecules of NADPH. 

The pentose phosphate pathway, in which glucose-6-phos-phate is oxidized, occurs in two phases. In the oxidative phase, two molecules of NADPH are produced as glucose-6-phosphate is converted to ribulose-5-phosphate. In the nonoxidative phase, ribose-5-phosphate and other sugars are synthesized. If cells need more NADPH than ribose-5-phos-phate, a component of nucleotides and the nucleic acids, then metabolites of the nonoxidative phase are converted into glycolytic intermediates. 

Later we shall see which solution has been adopted by living organisms for the equivalent biochemical problem, but before we can do this we need to see how the problem as I have presented it relates to the pentose phosphate pathway. This consists of both oxidative and nonoxidative phases, but we shall consider only the nonoxidative phase, which involves exchanging the carbon atoms of sugars so as to transform six pentose molecules into five hexose molecules, that is, to convert 6C5 into 5C6. The exchanges are brought about by enzymes that transfer a certain number of carbon atoms from one sugar to another. The mechanisms available to the cell are the transfer of two carbon... 

To generate ribose 5-phosphate from the oxidative pathway, the ribulose 5-phos-phate formed from the action of the two oxidative steps is isomerized to prodnce ribose 5-phosphate (a ketose-to-aldose conversion, similar to fmctose 6-phos-phate being isomerized to glncose 6-phosphate see section III.B.l below). The ribose 5-phosphate can then enter the pathway for nucleotide synthesis, if needed, or can be converted to glycolytic intermediates, as described below for the nonox-idative phase of the pentose phosphate pathway. The pathway through which the ribose 5-phosphate travels is determined by the needs of the cell at the time of its synthesis. 

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