Glucose pentose-phosphate shunt

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... 

Fig. 8.2 Glycolysis and related pathways. Glycolysis is a central metabolic machinery in which one mole of glucose is catabolized to two moles of pyruvate, NADH, and ATP. Under aerobic conditions, pyruvate is further oxidized by mitochondrial system. In erythrocytes DHAP is a dead-end product however, in brain it can be converted into direction of lipid synthesis. Glycolysis and the pentose phosphate pathway (pentosePP) are interconnected via fructose-6-P and glyceral-dehyde-3-P. A high level of NADPH favors lipid synthesis via pentose phosphate shunt (pentosePP). At TPI inhibition (TPI deficiency), glyceraldehyde-3-Pcan be produced via G6PDH as well, to contribute to the glycolytic flux. a-GDH catalyzes the...

The rest of the Calvin cycle is involved in interconversion of carbohydrates to make glucose (or starch) and the regeneration of the ribulose-bisphosphate acceptor. The reactions are also found in the pathways for gluconeogenesis and the pentose phosphate shunt (see Volume 1, Chapters 10 and 12). The first step is the phosphorylation of 3-phosphoglycerate by the same reactions involved in gluconeogenesis. 

Insulin release Glucose oxidation Pentose phosphate shunt... 

Fig. 18. Effect of glucose on insulin release, glucose oxidation, pentose phosphate shunt, NADH/NAD +, NADPH/NADP"1", GSH/GSSG ratios and Ca + uptake of rat pancreatic islets. In a concentration range up to 16.7 mM, glucose-mediated stimulation of insulin secretion is closely parallelled by increases in glucose oxidation, pentose phosphate shunt activity, the NADH/NAD+, NADPH/NADP+, GSH/GSSG ratios and Ca2+ uptake (Ammon and Wahl, 1994).

Glucose can serve as substrate in the pentose-phosphate shunt (Figure 1). After transformation to glucose-6P, the latter compound is not converted to fructose-6P but to 6-phosphoglucono-5-lactone, the committed step in the pentose-phosphate shunt. This metabolic pathway provides NADPH, which is required for the biosynthesis of fatty acids (e.g., in liver cells and adipocytes) and to keep glutathione in the reduced state (e.g., in red blood cells). Moreover, the pentose-phosphate shunt provides ribose-5P which is necessary for the production of nucleotides. 

Pentose phosphate shunt Glucose-6-phosphate dehydrogenase Phosphogluconate dehydrogenase Transketolase Transaldolase... 

This enzyme also catalyses reactions with /3-D-glucose and other sugars. It catalyses the reaction whereby glucose-6-phosphate enters the pentose phosphate shunt. It may be used as an indicator reaction for glucokinase, or hexokinase. It is assayed by u.v. spectroscopy. 

Xylitol is obtained by hydrogenation of the pentose sugar xylose. It is as sweet as sucrose, and new relatively inexpensive methods have been devised for its production. Xylulose occurs naturally on the pathway by which breakdown products of gluconic acid (page 232) are fed into the pentose phosphate shunt. When administered, xylitol is readily metabolized but differs from glucose in the initial steps prior to the point at which it enters the glycolytic pathway. 

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