6-Phosphogluconate pathway

Metabolic Functions. The formation of phosphate esters is the essential initial process in carbohydrate metaboHsm (see Carbohydrates). The glycolytic, ie, anaerobic or Embden-Meyerhof pathway comprises a series of nine such esters. The phosphogluconate pathway, starting with glucose, comprises a succession of 12 phosphate esters. 

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. 

There is known one more catabolic route for carbohydrates commonly referred to as the pentose phosphate cycle (also called hexose mono phosphate shunt, or phosphogluconate pathway). 

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). 

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 second half of this chapter examines a pathway common to all organisms, known variously as the pentose phosphate pathway, the hexose monophosphate pathway, the phosphogluconate pathway, or the pentose shunt. The pathway provides a means by which glucose can be oxidized to generate NADPH and is the source of much of the NADPH that is needed for the biosynthesis of many biomolecules, most notably fats. We will observe the use of... 

Polyhydroxyalkanoates biosynthesis is regulated, on one hand, by the activity of 3-ketothiolase (EC 2.3.1.16), and on the other hand of acetoacetyl-CoA reductase (EC 1.1.1.36) intracellular PHA breakdown is dependent on the activity of 3-hydroxybutyrate dehydrogenase (EC 1.1.1.30). Besides these three enzymes, the following compounds can be pointed out as major factors responsible of the activities of the key enzymes acetyl-CoA, free CoA, NAD(P) + (or NAD(P)H2, respectively) and, to a lower extent, ATP, pyruvate and oxalacetate. In any case, acetyl-CoA can be considered as the central metabolite both for biomass formation and PHB biosynthesis. This compound stems from the catabolic break down of carbon substrates like sugars (mainly catabolized by the 2-Keto-3-desoxy-6-phosphogluconate pathway) or fatty acids (converted by 6-oxidation). 

Catabolism via e.g. 2-keto-3-desoxy-6-phosphogluconate pathway (KDPa)... 

Although most of the glucose catabolised in animal tissues is via glycolysis to pyruvate which then enters the Krebs cycle, there are some minor metabolic pathways which lead to alternative products. One of the most important of these is the Pentose Phosphate pathway (also known as the pentose shunt or the phosphogluconate pathway). In this pathway, glucose-6-phosphate is oxidised to ribose-5-phosphate with the generation of two molecules of NADPH. The overall equation may be written as... 

Functionally, the 6-phosphogluconate pathway, when operative, accomplishes two important goals for the AAB (1) It overcomes the obstacle of not having a functional EMP for oxidation of glucose and (2) it provides a... 

Fig. 10.20. Glucose metabolism in lactic acid bacteria. A homofermentation, B Bifidus pathway, and C heterofermentation (6-phosphogluconate pathway)...

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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