Pentose catabolic pathway

Engineering the Pentose Catabolic Pathways forD-Xylose and L-Arabinose... 

Engineering the Pentose Catabolic Pathway for Hemicellulase Overexpression... 

Vnother pathway of glucose catabolism, the pentose phosphate pathway, is the primary source of N/ E)PH, the reduced coenzyme essential to most reductive biosynthetic processes. For example, N/VDPH is crucial to the biosynthesis of... 

One of the steps in the pentose phosphate pathway for glucose catabolism is the reaction of sedoheptulose 7-phosphate with glyceraldehyde 3-pho phate in the presence of a transaldolase to yield erythrose 4-phosphate and fructose 6-phosphate. 

As a general rule, NAD+ is associated with catabolic reactions and it is somewhat unusual to find NADP+ acting as an oxidant. However, in mammals the enzymes of the pentose phosphate pathway are specific for NADP+. The reason is thought to lie in the need of NADPH for biosynthesis (Section I). On this basis, the occurrence of the pentose phosphate pathway in tissues having an unusually active biosynthetic function (liver and mammary gland) is understandable. 

Fig. 8.2 Glycolysis and related pathways. Glycolysis is a central metabolic machinery in which one mole of glucose is catabolized to two moles of pyruvate, NADH, and ATP. Under aerobic conditions, pyruvate is further oxidized by mitochondrial system. In erythrocytes DHAP is a dead-end product however, in brain it can be converted into direction of lipid synthesis. Glycolysis and the pentose phosphate pathway (pentosePP) are interconnected via fructose-6-P and glyceral-dehyde-3-P. A high level of NADPH favors lipid synthesis via pentose phosphate shunt (pentosePP). At TPI inhibition (TPI deficiency), glyceraldehyde-3-Pcan be produced via G6PDH as well, to contribute to the glycolytic flux. a-GDH catalyzes the...

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. 

Fig. 1. Pathways of glucose metabolism in eubacteria and eukaryotes. The three major catabolic pathways are the Embden-Meyerhof glycolytic sequence (solid lines), the Entner-Doudoroff pathway (heavy solid lines) and the pentose phosphate pathway (dashed lines). The sequence from glyceraldehyde 3-phosphate to pyruvate is common to all three pathways.

While the main function of glycolysis is to produce ATP, there are minor catabolic pathways that produce specialized products for cells. One, the pentose phosphate pathway, produces NADPH and the sugar ribose 5-phosphate. NADPH is used to reduce substrates in the synthesis of fatty acids, and ribose 5-phosphate is used in the synthesis of nucleic acids. 

The Pentose Phosphate Pathway is an alternate pathway for glucose oxidation which is used to provide reducing equivalents in support of biosynthesis. Thus although it involves the catabolism of glucose, it is generally going to be active only when anabolism is taking place (Fig. 9.9). 

NAD tends to be an electron acceptor in catabolic reactions involving the degradation of carbohydrates, fatty acids, ketone bodies, amino acids, and alcohol. NAD is used in energy-producing reactions. NADP, which is cytosolic, tends to be involved in biosynthetic reactions. Reduced NADP is generated by the pentose phosphate pathway (cytosolic) and used by cytosolic pathways, such as fatty acid biosynthesis and cholesterol synthesis, and by ribonucleotide reductase. The niacin coenzymes are used for two-electron transfer reactions. The oxidized form of NAD is NAD". There is a positive charge on the cofactor because the aromatic amino group is a quaternary amine. A quaternary amine participates in four... 

H8. Hanks, L. V., Politzer, W. M., Touster, O., et al., Myoinositol catabolism in human pentosurics The predominant role of the glucuronate-xyulose-pentose phosphate pathway. Ann. N.Y. Acad. Sci. 165, 564-576 (1969). 

See also Figure 13.12, Glycolysis, Gluconeogenesis, Pentose Phosphate Pathway, Calvin Cycle, Catabolism of Other Saccharides... 

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