Pentose pathway

The large supply of NADPH required for the above reactions is obtained through the metabolism of glucose-6-phosphate via the pentose pathway. 

Figure 17-8 The pentose phosphate pathways. (A) Oxidation of a hexose (C6) to three molecules of C02 and a three-carbon fragment with the option of removing C3, C4, C5, and C7 units for biosynthesis (dashed arrows). (B) Non-oxidative pentose pathways 2 1/2 C6 —> 3 C5 or 2 C6 —> 3 C4 or 3 V2C6 —> 3 C7.

Figure 18.10 The hexose monophosphate shunt pathway. A, glucose-6-phosphate dehydrogenase B, 6-phosphogluconate dehydrogenase C, pentose-5-phosphate iso-merase D, pentose phosphate epimerase E, transaldolase F, transketolase G, phospho-hexoseisomerase. (Reproduced by permission from Williams JF. A critical examination of the evidence for the reactions of the pentose pathway in animal tissues. Trends Biochem Sri December 316, 1980.)...

Nordlie RC, Jorgenson RA. Latency and inhibitability by metabolites of glucose-6-phosphatase of permeable hepatocytes from fasted and fed rats. J. Biol. Chem. 1981 256 4768-4771. McCornuck KL, Wang X, Mick GJ. Evidence that the 11 P-hydroxysteroid dehydrogenase is regulated by pentose pathway flux. Studies in rat adipocytes and microsomes. J. Biol. Chem. 2006 281 341-347. 

Search for an Alternative Reaction Scheme for the Pentose Pathway... 

Figure 1 Reactions of the oxidative segment of the pentose phosphate pathway. The conversion of [2- C] glucose to CO2 and [1- C] ribulose S-phosphate is shown. Detail of the reaction steps are discussed in the text. Reprinted from International Journal of Biochemistry, 19. Williams John. F., Arora Krishan K. and Longnecker John P. The pentose pathway A random harvest, 69 Pages, 1987, with permission from Elsevier,...

Williams JF, Arora KK, Longnecker JP. The pentose pathway a random Harvest. Impediments which oppose acceptance of the classical (F-type) pentose cycle for liver, some neoplasms and photosynthetic tissue. The case for the L-type pentose pathway. Int. J. Biochem. 1987 19 749-817. 

BleaUey PA, Arora KK, Williams JF. Evidence that aldolase and D-arabinose 5-phosphate are components of pentose pathway reactions in liver in vitro. Biochem. Int. 1984 8 491-500. Horecker BL, Mehler AH. Carbohydrate metabolism. Ann. Rev. Biochem. 1955 24 207-274. 

Paoletti F, Williams JF, Horecker BL. Detection and estimation of sedoheptulose and octulose mono- and bisphosphates in extracts of rat liver. Archiv. Biochem. Biophys. 1979 198 620-626. Williams JF, Clark MG, Arora KK. C-labelling of octulose bisphosphates by L-type pentose pathway reactions in liver in situ and in vivo. Biochem. Int. 1985 11 97-106. 

Williams JF, Clark MG, Arora KK, Reichstein IC. Glucose 6-phosphate formation by L-pentose pathway reactions of rat liver in vitro-, further evidence. Hoppe Seyler s Zeit. Fur Biochemie. 1984 365 1425-1434. 

Ranigan IL, Grant-Collins J, Arora KK, MacLeod JK, Williams JF. Exchange reactions catalysed by group transferring enzymes oppose the quantitation and unravelling of the identity of the pentose pathway. Europ. J. Biochem. 1993 213 477-485. 

Transketolase. Another group of enzymes that catalyze the stereospecific formation and cleavage of carbohydrates in vivo are the transketolases and transaldolases. Transketolase (E.C. 2.2.1.1) is a thiamin pyrophosphate (TPP) dependent enzyme that catalyzes the transfer of a hydroxyketo group from a ketose phosphate to an aldose phosphate in the pentose pathway (Scheme 12) (36). 

Pentose Phosphate Pathway 21 Glucose-6-Phosphate Dehydrogenase 6-Phosphogluconate Dehydrogenase Metabolism of Ribulose-5-Phosphate Role of the Pentose Phosphate Pathway in Metabolism Distribution of the Pentose Pathway in the Organism... 

Intracellular Distribution of Glucuronic and Pentose Pathways 25 Tricarboxylic Acid Cycle 26 Lipoic Acid... 

In the pentose pathway, the CO2 formed is derived from the carbon 1 of glucose, in contrast to the Embden-Meyerhof pathway, which yields equal amounts of CO2 from carbons 1 and 6 of glucose. Therefore, if glucose labeled in carbon 1 or carbon 6 is oxidized by a system in which the Embden-Meyerhof pathway operates alone, equal amounts of radioactive CO2 are derived from glucose labeled in carbon 1 and carbon 6. But if glucose is also oxidized by the hexose monophosphate pathway, more CO2 is derived from carbon 1 than from carbon 6. Hence, the production of CO2 derived from glucose labeled in carbon 1 or carbon 6 is often used to evaluate the relative activity rates of each pathway. 

Fructose-6-phosphate and glyceraldehyde constitute the channels through which the metabolites of the pentose phosphate cycle are reconverted to glu-cose-6-phosphate. Indeed, phosphoglucose isomerase converts fructose-6-phosphate to glucose-6-phos-phate, and in the presence of triose isomerase and aldolase, glyceraldehyde phosphate may be converted to fructose-1,6-diphosphate. The metabolic steps involved in the pentose pathway are summarized in Fig. 1-12. 

---