Cytosol pentose phosphate pathway reactions

REACTIONS OF THE PENTOSE PHOSPHATE PATHWAY OCCUR IN THE CYTOSOL... 

The final reactions to be considered in the metabolism of ethanol in the liver are those involved in reoxidation of cytosolic NADH and in the reduction of NADP. The latter is achieved by the pentose phosphate pathway which has a high capacity in the liver (Chapter 6). The cytosolic NADH is reoxidised mainly by the mitochondrial electron transfer system, which means that substrate shuttles must be used to transport the hydrogen atoms into the mitochondria. The malate/aspartate is the main shuttle involved. Under some conditions, the rate of transfer of hydrogen atoms by the shuttle is less than the rate of NADH generation so that the redox state in the cytosolic compartment of the liver becomes highly reduced and the concentration of NAD severely decreased. This limits the rate of ethanol oxidation by alcohol dehydrogenase. 

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). 

How is the cytoplasmic [NADPH]/[NADP+] ratio maintained at a value higher than that of [NADH]/ [NAD+] Part of the answer is from operation of the pentose phosphate pathway (Section E,3). The reactions of Eq. 17-12, if they attained equilibrium, would give a ratio of cytosolic [NADPH]/[NADP+] > 2000 at 0.05 atm C02. Compare this with the ratio 1 /634 for [NADH/[NAD+] deduced from the observation on the reactions of Eq. 17-42. 

NAD tends to be an electron acceptor in catabolic reactions involving the degradation of carbohydrates, fatty acids, ketone bodies, amino acids, and alcohol. NAD is used in energy-producing reactions. NADP, which is cytosolic, tends to be involved in biosynthetic reactions. Reduced NADP is generated by the pentose phosphate pathway (cytosolic) and used by cytosolic pathways, such as fatty acid biosynthesis and cholesterol synthesis, and by ribonucleotide reductase. The niacin coenzymes are used for two-electron transfer reactions. The oxidized form of NAD is NAD". There is a positive charge on the cofactor because the aromatic amino group is a quaternary amine. A quaternary amine participates in four... 

The pentose phosphate pathway also catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions. All these reactions occur in the cytosol, and in plants part of the pentose phosphate pathway also participates in the formation of hexoses from CO2 in photosynthesis. Thus, D-ribulose 5-phosphate can be directly converted into D-ribose 5-phosphate by phosphopentose isomerase, or to D-xylulose 5-phosphate by phosphopentose epimerase. D-Xylulose 5-phosphate can then be combined with D-ribose 5-phosphate to give rise to sedoheptulose 7-phosphate and glyceraldehyde-3-phosphate. This reaction is a transfer of a two-carbon unit catalyzed by transketolase. Both products of this reaction can be further converted into erythrose 4-phosphate and fructose 6-phosphate. The four-carbon sugar phosphate erythrose 4-phosphate can then enter into another transketolase-catalyzed reaction with the D-xylulose 5-phosphate to form glyceraldehyde 3-phosphate and fructose 6-phosphate, both of which can finally enter glycolysis. 

B. The synthesis of fatty acids from glucose occurs in the cytosol, except for the mitochondrial reactions in which pyruvate is converted to citrate. Biotin is required for the conversion of pyruvate to oxaloacetate, which combines with acetyl CoA to form citrate. Biotin is also required by acetyl CoA carboxylase. Pantothenic acid is covalently bound to the fatty acid synthase complex as part of a phosphopantetheinyl residue. The growing fatty acid chain is attached to this residue during the sequence of reactions that produce palmitic acid. NADPH, produced by the malic enzyme as well as by the pentose phosphate pathway, provides reducing equivalents. Citrate, not isocitrate, is a key regulatory compound. 

Transketolase is a TPP-dependent enzyme found in the cytosol of many tissues, especially hver and blood cells, in which principal carbohydrate pathways exist. In the pentose phosphate pathway, which additionally supplies reduced nicotinamide-adenine dinucleotide phosphate (NADPH) necessary for biosynthetic reactions, this enzyme catalyzes the reversible transfer of a glycoaldehyde moiety from the first two carbons of a donor ketose phosphate to the aldehyde carbon of an aldose phosphate. 

Fig. 8. Pathways involved in the conversion of glucose to fatty acid. Reaction (1) is catalyzed by cytosolic malate dehydrogenase. Reaction (2) is catalyzed by mitochondrial malate dehydrogenase. (T) designates tricarboxylate anion transporter. Reactions catalyzed by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in the pentose phosphate pathway produce NADPH. CS, citrate synthase ACL, ATP citrate lyase PDH, pyruvate dehydrogenase complex ACC, acetyl-CoA carboxylase FAS, fatty acid synthase.

The pentose phosphate pathway is an important pathway for generating NADPH (for biosynthetic reactions) and pentose sugars (for nucleotide biosynthesis). It operates exclusively in the cytosol. Be aware that in contrast to pathways, such as glycolysis (with a linear sequence of reactions) or the citric acid cycle (with a circular sequence of reactions), the pentose phosphate pathway has several possible "branches" that can be taken to allow it to supply the cell with different products as needed. The primary products of the pathway include NADPH (from the oxidative reactions), pentoses (used in nucleotide synthesis), and miscellaneous other sugar phosphates. 

The net effect of these reactions is that acetyl-CoA is moved out of the mitochondrion and cytosolic NADH is converted to cytosolic NADPH. NADPH is used, in turn, for the biosynthesis of fatty acids. Recall (see here) that the synthesis of 1 mole of palmitate requires 14 moles of NADPH, most of which is generated in the cytosol via the pentose phosphate pathway. 

Finally, oxaloacetate is simultaneously decarboxylated and phosphorylated by phosphoenolpyruvate carboxykinase in the cytosol. The CO2 that was added to pyruvate by pyruvate carboxylase comes off in this step. Recall that, in glycolysis, the presence of a phosphoryl group traps the unstable enol isomer of pyruvate as phosphoenolpyruvate (Section 16.1.7). In gluconeogenesis, the formation of the unstable enol is driven by decarboxylation—the oxidation of the carboxylic acid to CO2—and trapped by the addition of a phosphate to carbon 2 from GTP. The two-step pathway for the formation of phosphoenolpyruvate from pyruvate has a AG° of + 0.2 kcal mol ( + 0.13 kj moP ) in contrast with +7.5 kcal mol ( + 31 kj mol ) for the reaction catalyzed by pyruvate kinase. The much more favorable AG° for the two-step pathway results from the use of a molecule of ATP to add a molecule of CO2 in the carboxylation step that can be removed to power the formation of phosphoenolpyruvate in the decarboxylation step. Decarboxylations often drive reactions otherwise highly endergonic. This metabolic motif is used in the citric acid cycle (Section IS.x.x), the pentose phosphate pathway (Section 17.x.x), and fatty acid synthesis (Section 22.x.x). 

The regulation of the pentose phosphate pathway has not been as fully studied as that of glycolysis. The rate of the pathway appears to be controlled by glucose-6-phosphate dehydrogenase. The major regulatory factor governing this essentially irreversible reaction is the cytosolic ratio of [NADP+]/[NADPH],... 

The oxidative component of the pentose phosphate pathway produces ribulose 5-phosphate, CO2 and two molecules of NADH from each molecule of glucose 6-phosphate entering the pathway. The reactions take place in the cytosol. The pathway is important in producing the five-carbon sugars used in nucleotide synthesis. The reactions can take place in anaerobic conditions. 

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