Pentose phosphate pathway NADPH produced

PM A). Neutrophils stimulated in this way produced a 40-minute heat burst of 23.5 nW per cell, approximately fourfold more than resting cells over a similar period. The rate of glycolysis, as measured by lactate evolution, was not altered substantially but the flux through the pentose phosphate pathway was increased by nearly 500% to give the necessary NADPH. All of the heat dissipated was accounted for by measurements of oxygen consumption, lactate production and C02 evolution in the glucose carbon 1/carbon 6 ratio. Enthalpy recovery was, therefore, 1.0 which indicated that no other pathway was induced by neutrophil activation—an enhanced flux in the pentose phosphate pathway to produce NADPH was responsible solely for the observed metabolic burst. 

The pentose phosphate pathway would produce the necessary NADPH, and the ribose 5-phosphate would be converted into the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. 

In the pentose phosphate pathway, NADPH is produced at the step catalyzed by ... 

Most of the enzymes mediating the reactions of the Calvin cycle also participate in either glycolysis (Chapter 19) or the pentose phosphate pathway (Chapter 23). The aim of the Calvin scheme is to account for hexose formation from 3-phosphoglycerate. In the course of this metabolic sequence, the NADPH and ATP produced in the light reactions are consumed, as indicated earlier in Equation (22.3). 

BOTH RIBOSE-5-P AND NADPH ARE NEEDED BY THE CELL In this case, the first four reactions of the pentose phosphate pathway predominate (Figure 23.37). N/VDPH is produced by the oxidative reactions of the pathway, and ribose-5-P is the principal product of carbon metabolism. As stated earlier, the net reaction for these processes is... 

MORE NADPH THAN RmOSE-5-P IS NEEDED BY THE CELL Large amounts of N/VDPH can be supplied for biosynthesis without concomitant production of ribose-5-P, if ribose-5-P produced in the pentose phosphate pathway is recycled to produce glycolytic intermediates. As shown in Figure 23.39, this alternative involves a complex interplay between the transketolase and transaldolase reac-... 

NADPH can be produced in the pentose phosphate pathway as well as by malic enzyme (Figure 25.1). Reducing equivalents (electrons) derived from glycolysis in the form of NADH can be transformed into NADPH by the combined action of malate dehydrogenase and malic enzyme ... 

How many of the 14 NADPH needed to form one palmitate (Eq. 25.1) can be made in this way The answer depends on the status of malate. Every citrate entering the cytosol produces one acetyl-CoA and one malate (Figure 25.1). Every malate oxidized by malic enzyme produces one NADPH, at the expense of a decarboxylation to pyruvate. Thus, when malate is oxidized, one NADPH is produced for every acetyl-CoA. Conversion of 8 acetyl-CoA units to one palmitate would then be accompanied by production of 8 NADPH. (The other 6 NADPH required [Eq. 25.1] would be provided by the pentose phosphate pathway.) On the other hand, for every malate returned to the mitochondria, one NADPH fewer is produced. 

The pentose phosphate pathway, present in the cytosol, can account for the complete oxidation of glucose, producing NADPH and COj but not ATP. 

The pentose phosphate pathway is operative in the RBC (it metabolizes about 5-10% of the total flux of glucose) and produces NADPH hemolytic anemia due to a deficiency of the activity of glucose-6-phosphate dehydrogenase is common. 

The NADPH is produced from glucose 6-phosphate in the first three reactions in the pentose phosphate pathway (see below). Hence the pentose phosphate pathway is essential in the erythrocyte and glycolysis provides the substrate glucose 6-phosphate. Individuals with a reduced amount of glucose 6-phosphate dehydrogenase can suffer from oxidative damage to their cells and hence haemolysis. 

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. 

Glucose 6-phosphate dehydrogenase, the first enzyme in the oxidative pentose phosphate pathway, is also regulated by this light-driven reduction mechanism, but in the opposite sense. During the day, when photosynthesis produces plenty of NADPH, this enzyme is not needed for NADPH production. Reduction of a critical disulfide bond by electrons from ferredoxin inactivates the enzyme. 

In hepatocytes and adipocytes, cytosolic NADPH is largely generated by the pentose phosphate pathway (see Fig. 14-21) and by malic enzyme (Fig. 21-9a). The NADP-linked malic enzyme that operates in the carbon-assimilation pathway of C4 plants (see Fig. 20-23) is unrelated in function. The pyruvate produced in the reaction shown in Figure 21-9a reenters the mitochondrion. In hepatocytes and in the mammary gland of lactating animals, the NADPH required for fatty acid biosynthesis is supplied primarily by the pentose phosphate pathway (Fig. 21-9b). 

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. 

The NADPH required is produced by several enzyme systems but the best known is the oxidative pentose phosphate pathway. Glutathione peroxidase and catalase thus co-operate to remove hydrogen peroxide in vivo. 

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...

Answer The reductive pentose phosphate pathway regenerates ribulose 1,5-bisphosphate from triose phosphates produced during photosynthesis, in a series of reactions involving sugars of three, four, five, six, and seven carbons and the enzymes transaldolase and transketo-lase. The oxidative pentose phosphate pathway plays a different metabolic role it provides NADPH for reductive biosynthesis and pentose phosphates for nucleotide synthesis. 

The requirement for NADPH far exceeds an equal requirement for ribose 5-phosphate (necessary for the production of nucleic acids and nucleotides), and so the second phase of the pentose phosphate pathway converts the C5 sugar, by a series of reversible reactions, into the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. This interconversion is shown in Fig. 11-27. Not only does the second phase of the pathway conserve all the carbon atoms of the C5 sugar, but it produces erythrose 4-phosphate (C4), xylulose 5-phosphate (C5), and sedoheptulose 7-phosphate (C7), which are available to other metabolic processes. 

When glucose is converted to ribulose 5-phosphate. NADPH is produced in the redox reactions of the oxidative part of the pentose phosphate pathway. For every molecule of glucose entering the pathway, two NADP+ molecules are reduced. Some of this NADPH is utilized in the synthesis of palmitic acid. To calculate the amount consider the following ... 

Glucose is oxidized in the pentose phosphate pathway (Chap. 11), to produce NADPH that can enter cholesterol synthesis. One molecule of glucose is required per molecule of lanosterol synthesized. (The reactions that convert lanosterol to cholesterol are outside the scope of Chap. 13.)... 

Thus, the equivalent of glucose 6-phosphate can be completely oxidized to CO 2 with the concomitant generation of NADPH. In essence, ribose 5-phosphate produced by the pentose phosphate pathway is recycled into glucose 6-phosphate by transketolase, transaldolase, and some of the enzymes of the gluconeogenic pathway. 

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