Phosphogluconate oxidative pathway

B4. Beck W. S., Occurrence and control of the phosphogluconate oxidation pathway in normal and leukemic leukocytes. J. Biol. Chem. 232, 271-283 (1958). 

Despite the limited conclusions which can be drawn from studies of the relative yields of derived from glucose-/and glucose-6-C in C 02, it would appear that the phosphogluconate oxidative pathway participates... 

The effect of an increase in G-6-Pase activity on the relative rates of the various pathways of G-6-P metabolism has been calculated, and it was found that in the normal liver in vitro 55% of the G-6-P is metabolized via the phosphatase reaction, while in the diabetic this is increased to 73 %. Not only does the increased G-6-Pase activity result in increased hydrolysis of G-6-P to glucose, but also in a reduction of glycogen formation from 18 % in the normal to 2 % in the diabetic. The data indicate that an increase in G-6-Pase does not markedly alter the relative rates of metabolism of G-6-P via the Embden-Meyerhof and the phosphogluconate oxidation pathways. 

M12. McNair Scott, D. B., and Cohen, S. S., The oxidative pathway of carbohydrate metabolism in Escherichia coli. 5. Isolation and identification of ribulose-5-phosphate produced from 6-phosphogluconate by the dehydrogenases of Escherichia coli. Biochem. J. 65, 686-689 (1957). 

Dickens also observed the oxidation and fermentation by yeast systems of n-ribose-5-phosphate, but not of the expected degradation product of 6-phosphogluconate, n-arabinose-5-phosphate. He proposed that ribose arose in some manner from phosphogluconate degradation and formulated the reaction scheme of the oxidative pathway of glucose-6-phosphate shown in Fig. 12. 

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. 

Normally, glucoso-6-phosphate is used in glycolysis and in the pentose cycle. The mutation in the Pgd gene blocks the oxidation pathway in pentose biosynthesis and, hence, 6-phosphogluconate is accumulated. This situation is lethal for an organism. If there is another termination of the pentose cycle by a mutation in gene... 

Glucose-3- H is a better precursor for measuring the participation of NADPH from the oxidative pathway to the fatty acid synthesis than glucose-l- H. The tritium is transferred to NADP during the oxidation of 6-phosphogluconate into ribose 5-phosphate, and the isotopic effect which is apparently present in the case of the glucose-1- H and which orients it preferentially toward the glycolytic pathway, disappears (Katz et al., 1965). 

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. 

An additional way of cleaving a six-carbon sugar chain provides the basis for the Entner-Doudoroff pathway which is used by Zymomonas lindneri and many other species of bacteria. Glucose 6-P is oxidized first to 6-phosphogluconate, which is converted by dehydration to a 2-oxo-3-deoxy derivative (Eq. 17-18,... 

In a certain group of bacteria, still another pathway (Entner-Doudomff pathway) for the utilization of glucose has been studied. Here glucosc-6-phosphate is oxidized to 6-phosphogluconic acid which is dehydrated to 2-keto-3-deoxy-6-phusphogluconic acid. This substance is then split to pyruvic acid and glyceraldehyde-3-phosphatc l which alsu can be converted to pyruvic acid). 

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). 

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