Pentose phosphate pathway nonoxidative

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.

FIGURE 14-22 Nonoxidative reactions of the pentose phosphate pathway, (a) These reactions convert pentose phosphates to hexose phosphates, allowing the oxidative reactions (see Fig. 14-21) to continue. The enzymes transketolase and transaldolase are specific to this pathway the other enzymes also serve in the glycolytic or gluconeogenic pathways, (b) A schematic diagram showing the pathway... 

The five E. coli genes inserted in Z. mobilis allowed the entry of arabinose into the nonoxidative phase of the pentose phosphate pathway (Fig. 14-22), where it was converted to glucose 6-phosphate and fermented to ethanol. 

Berthon HA, Kuchel PW, and Nixon PE (1992) High control coefficient of transketolase in the nonoxidative pentose phosphate pathway ofhuman erythrocytes NMR, antibody, and computer simulation studies. Biochemistry 31,12792-8. 

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.

The second stage of the pentose phosphate pathway is the nonoxidative, reversible metabolism of five-carbon phosphosugars into phosphorylated three-carbon and six-carbon glycolytic intermediates. Thus, the nonoxidative branch can either introduce riboses into glycolysis for catabolism or generate riboses from glycolytic intermediates for biosyntheses. 

Nonoxidative phase of the pentose phosphate pathway. Numbers in parentheses show the distribution of carbon atoms between the various branches of each reaction. TK = transketolase GAP = glyceraldehyde 3-phosphate DHAP = dihydroxyacetone phosphate P = phosphate. 

Glycolysis. Ribose 5-phosphate can also be produced from intermediates of glycolysis (Figure 27-6). The enzymes involved are those of the nonoxidative phase of the pentose phosphate pathway, that occur in many tissues. 

Which one of the following compounds is common to both the oxidative branch and the nonoxidative branch of the pentose phosphate pathway ... 

The answer is e. (Murray, pp 219-229. Scrivei pp 1521-1552. Sack, pp 121-138. Wilson, pp 287-317.) The pentose phosphate pathway generates reducing power in the form of NADPH in the oxidative branch of the pathway and synthesizes five-carbon sugars in the nonoxidative branch of the pathway. The pentose phosphate pathway also carries out the interconversion of three-, four-, five-, six-, and seven-carbon sugars in the nonoxidative reactions. The final sugar product of the oxidative branch of the pathway is ribulose-5-phosphate. The first step of the nonoxidative branch of the pathway is the conversion of ribulose-5-phosphate to ribose-5-phosphate or xylulose-5-phosphate in the presence of the enzymes phosphopentose isomerase and phosphopentose epimerase, respectively. Thus ribulose-5-phosphate is a key intermediate that is common to both the oxidative and nonoxidative branches of the pentose phosphate pathway. 

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. 

If the cell requires more NADPH than ribose molecules, it can channel the products of the nonoxidative phase of the pentose phosphate pathway into glycolysis. As this overview of the two pathways illustrates, excess ribose-5-phos-phate can be converted into the glycolytic intermediates fructose-6-phosphate and glyceraldehyde-3-phosphate. 

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. 

In the first step of this pathway, glucose-6-phosphate + NADP+ is converted to 6-phosphogluconate + NADPH + H+ and, in the second step, 6-phosogluconate + NADP+ is converted to ribulose-5-phosphate + NADPH + H+. These reactions are considered to be the oxidative arm of the pentose-phosphate pathway and are critical for producing much of the NADPH used in biosynthetic pathways. The remainder of the cycle (i.e., the nonoxidative portion) consists of converting 5-carbon phosphorylated sugars to 3-, 4-, 5-, and 7-carbon intermediates, finally achieving the resynthesis of hexose-6-phosphate and triose phosphate. Some of... 

Fig. 4.2 The pentose phosphate pathway. The squares under each chemical name indicate the number of carhon atoms that each molecule contains glucose 6-phosphate, for example, is a hexose, with six carhon atoms, rihose 5-phosphate is a pentose, with five, and so on. The names shown in gray at the left of the diagram belong to molecules in the oxidative part of the pathway, which is not considered in the analysis in the text. The remainder, the nonoxidative part, can be interpreted as a scheme to allow the hexoses and pentoses to be converted into one another...

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