Hexose-phosphate shunt

The additional pathway diverts glucose 6-phosphate from the first step of glycolysis and reduces NADP" in two redox reactions (Fig. 11-25). This reaction sequence is called the oxidative part of the pathway or the oxidative hexose phosphate shunt. It is especially active in lipogenic cells such as hepatocytes and adipocytes. 

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). 

The oxidative pathway for the metabolism of D-glucose 6-phosphate (XLV), distinctive from the glycolytic, Embden-Meyerhof route (see p. 200) and known as the hexose monophosphate shunt, was suggested by certain experiments of Warburg,200 Gerischer,207 Lipmann,208 and Dickens209... 

The hexose monophosphate pathway has several names just to confuse you. It s called the hexose monophosphate shunt or pathway (HMP shunt or pathway), or the pentose phosphate pathway, or the phospho-gluconate pathway (Fig. 15-1). The pathway in its full form is complicated and has complicated stoichiometry. Usually it s not necessary to remember all of it. The important points are that it makes NADPH for biosynthesis and riboses (C-5 sugars) for DNA and RNA synthesis. 

Under basal conditions 5% of brain glucose is metabolized via the pentose phosphate shunt (PPS), also termed the hexose monophosphate pathway [66], a pathway active in both neurons and astrocytes. The PPS has... 

Pentose phosphate pathway (= hexose monophosphate shunt) generation of NADPH G6PDH deficiency... 

A, The pentose phosphate pathway (PPP), also called the hexose monophosphate shunt, is an alternate pathway of glucose metabolism that supplies the NAD PH required by many biosynthetic pathways. 

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. 

FMN was first identified as the coenzyme of an enzyme system that catalyzes the oxidation of the reduced nicotinamide coenzyme, NADPH (reduced NADP), to NADP (nicotinamide adenine dinucleotide phosphate). NADP is an essential coenzyme for glucose-6-phosphate dehydrogenase which catalyzes the oxidation of glucose-6-phosphate to 6-phosphogluconrc acid. This reaction initiates the metabolism of glucose by a pathway other than the TCA cycle (citric acid cycle). The alternative route is known as the phosphoglneonate oxidative pathway, or the hexose monophosphate shunt. The first step is ... 

Alternatively, it is possible to write a sequence of reactions, including the action of phosphofructokinase and aldolase on seven-carbon intermediates, in which the carbon of ribulose-5-phosphate is converted mainly to glyceralde-hyde-3-phosphate. Such a pathway, with the triose phosphate entering the glycolytic sequence, amounts to a bypass, or shunt, around the first reactions of glycolysis, and the name hexose monophosphate shunt is sometimes used. Any amount of ribose-5-phosphate or erythrose-4-phosphate that may be needed for biosynthetic sequences can also be ob-... 

Enzymes of KDO Synthesis and Metabolism and Their Inhibition. The KDO pathway can be thought of as a minor branched pathway in carbohydrate metabolism initiating with the key intermediate in the hexose-monophosphate shunt, D-ribulose-5-phosphate. As shown in Figure 2 the biosynthesis and utilization are known to involve at least five sequential reactions ... 

Understand the physiologic importance of the hexose monophosphate shunt and understand reactions catalyzed by glu-cose-6-phosphate and 6-phosphogluconate dehydrogenases, transaldolase, and transketolase discuss the importance of the hexose monophosphate shunt in red cell physiology. 

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.)...

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 synthesis of Oj makes possible acceleration of glucose metabolism in the hexose monophosphate shunt, yielding NADP+, which is then used for dehydrogenation of glucose 6-phosphate to 6-phosphogluconic aldehyde (K15). 

Regeneration of ribulose bisphosphate occurs by the same reactions that occur in the hexose-monophosphate shunt. The hexose monophosphate shunt interconverts 3-, 4-, 5-, 6- and 7-carbon sugar phosphates. In the formation of one molecule of glucose from CO2, carrying out the previous reactions six times is necessary. Making the six molecules of ribulose-bisphosphate required for fixing six COi s occurs by the following sets of reactions ... 

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