6-Phosphogluconate pentose phosphate pathway

Fig. 1-4A. Embden-Meyerhof-Parnas (EMP) pathway for utilization of glucose by homolactic bacteria, (b) 6-Phosphogluconate (pentose phosphate) pathway for utilization of sugar by heterolactic bacteria.

Fig. 1-4B. 6-Phosphogluconate (pentose phosphate) pathway for utilization of sugar by heterolactic bacteria.

Cells require a constant supply of N/ X)PH for reductive reactions vital to biosynthetic purposes. Much of this requirement is met by a glucose-based metabolic sequence variously called the pentose phosphate pathway, the hexose monophosphate shunt, or the phosphogluconate pathway. In addition to providing N/VDPH for biosynthetic processes, this pathway produces ribos 5-phosphate, which is essential for nucleic acid synthesis. Several metabolites of the pentose phosphate pathway can also be shuttled into glycolysis. 

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... 

The oxidative pentose phosphate pathway (phosphogluconate pathway, or hexose monophosphate pathway) brings about oxidation and decarboxylation at C-l of glucose 6-phosphate, reducing NADP+ to NADPH and producing pentose phosphates. 

Pentose phosphate pathway Summary of the path way Reduced coenzymes produced by the pathway PENTOSE PHOSPHATE PATHWAY (p. 143) Also called the hexose monophosphate shunt, or6-phosphogluconate pathway, the pentose phosphate pathway is found in all cells. It consists of two irreversible oxidative reac tions followed by a series of reversible sugar-phosphate interconversions. No ATP is directly consumed or produced in the cycle, and two NADPH are produced for each glu cose 6-phosphate entering the oxidative part of the pathway. 

Phosphogluconate pathway. Another name for the pentose phosphate pathway. This name derives from the fact that 6-phosphogluconate is an intermediate in the formation of pentoses from glucose. 

Some mammalian cells have the ability to metabolize glucose 6-phosphate in a pathway that involves the production of C3, C4, C5, C6, and C7 sugars. This process also yields the reduced coenzyme, NADPH, which is oxidized in the biosynthesis of fatty acids and steroids (Chap. 13). Consequently, this metabolic pathway is of major importance in those cells involved in fatty acid and steroid production, such as the liver, lactating mammary gland, adrenal cortex, and adipose tissue. The pentose phosphate pathway, which does not require oxygen and which occurs in the cytoplasm of these cells, has two other names the phosphogluconate pathway (after the first product in the pathway) and the hexose monophosphate shunt (since the end products of the pathway can reenter glycolysis). 

Figure 6.4. Role of transketolase in the pentose phosphate pathway. Glucose 6-phosphate dehydrogenase, EC 1.1.1.49 phosphogluconate dehydrogenase, EC 1.1.1.44 rihulose-phosphate epimerase, EC 5.1.3.1 phosphorihose isomerase, EC 5.3.1,6 transketolase, EC 2.2.1.1 and transaldolase, EC 2.2.I.2.

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.

Phosphogluconate is an important intermediate in the pentose phosphate pathway. 

NADPH, which provides the reducing equivalents for fatty acid synthesis, is produced by the inducible malic enzyme and by the inducible enzymes of the pentose phosphate pathway, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. 

The series of cytoplasmic reactions known as the pentose phosphate pathway is also called the hexose monophosphate (HMP) shunt (or cycle) or the phosphogluconate pathway. The qualitative interconversions that take place... 

The NADPH is provided by the pentose phosphate pathway. Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are inhibited by NADPH at the concentrations found in the unstimulated phagocyte. As NADPH is consumed by NADPH oxidase, inhibition of the pentose phosphate pathway is reduced, increasing the rate of formation of NADPH. 

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... 

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