Biochemistry, 4th Edition P73 pdf

22.6 Can Glucose Provide Electrons for Biosynthesis? 683 turn activate some 800 molecules of phosphorylase. Each of these catalyzes the for- mation of many molecules of glucose-1-P. The Difference Between Epinephrine and Glucagon Although both epinephrine and glucagon exert glycogenolytic effects, they do so for quite different reasons. Ep- inephrine is secreted as a response to anger or fear and may be viewed as an alarm or danger signal for the organism. Called the “fight or flight” hormone, it prepares the organism for mobilization of large amounts of energy. Among the many physi- ological changes elicited by epinephrine, one is the initiation of the enzyme cas- cade, as in Figure 15.17, which leads to rapid breakdown of glycogen, inhibition of glycogen synthesis, stimulation of glycolysis, and production of energy. The burst of energy produced is the result of a 2000-fold amplification of the rate of glycolysis. Because a fear or anger response must include generation of energy (in the form of glucose)—both immediately in localized sites (the muscles) and eventually throughout the organism (as supplied by the liver)—epinephrine must be able to activate glycogenolysis in both liver and muscles. Glucagon is involved in the long-term maintenance of steady-state levels of glu- cose in the blood. It performs this function by stimulating the liver to release glu- cose from glycogen stores into the bloodstream. To further elevate glucose levels, gluca gon also stimulates liver gluconeogenesis by activating F-2,6-BPase activity (see Figure 22.10). It is important to note, however, that stabilization of blood glucose levels is managed almost entirely by the liver. Glucagon does not activate the phos- phorylase cascade in muscle (muscle membranes do not contain glucagon recep- tors). Muscle glycogen breakdown occurs only in response to epinephrine release, and muscle tissue does not participate in maintenance of steady-state glucose levels in the blood. Glucagon and epinephrine both trigger glycogen breakdown and inhibit glyco- gen synthesis (in liver and muscles, respectively), but their other effects on meta- bolic pathways are adapted exquisitely to the needs of the tissues involved. The liver must export glucose to the bloodstream to support other tissues. Thus, in the liver, PFK-2 is phosphorylated and inhibited by protein kinase A (PKA), lowering [fruc- tose-2,6-bisphosphate], inhibiting glycolysis, and activating gluconeogenesis (see Figure 22.20). In muscles, the glucose provided by glycogen breakdown is used im- mediately and locally to provide ATP energy for contraction. Therefore, glycolysis should be activated in concert with glycogen breakdown in muscles. Activation of glycolysis is accomplished in different ways, depending on the muscle. Heart mus- cle PFK-2 is activated upon phosphorylation at Ser 466 and Ser 483 by PKA in response to epinephrine, thus activating glycolysis (see Figure 22.20). Skeletal muscle PFK-2, however, is not a substrate for PKA. Instead, skeletal muscle PFK-1 is phosphorylated and activated by PKA (see Figure 22.20). Cortisol and Glucocorticoid Effects on Glycogen Metabolism Glucocorticoids are a class of steroid hormones that exert distinct effects on liver, skeletal muscle, and adipose tissue. The effects of cortisol, a typical glucocorticoid, are best de- scribed as catabolic because cortisol promotes protein breakdown and decreases protein synthesis in skeletal muscle. In the liver, however, it stimulates gluconeo- genesis and increases glycogen synthesis. Cortisol-induced gluconeogenesis results primarily from increased conversion of amino acids into glucose (Figure 22.21). Specific effects of cortisol in the liver include increased expression of several genes encoding enzymes of the gluconeogenic pathway, activation of enzymes involved in amino acid metabolism, and stimulation of the urea cycle, which disposes of nitro- gen liberated during amino acid catabolism (see Chapter 25). 22.6 Can Glucose Provide Electrons for Biosynthesis? Cells require a constant supply of NADPH 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 684 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway the phosphogluconate pathway. In addition to providing NADPH for biosynthetic processes, this pathway produces ribose-5-phosphate, which is essential for nucleic acid synthesis. Several metabolites of the pentose phosphate pathway can also be shuttled into glycolysis. The Pentose Phosphate Pathway Operates Mainly in Liver and Adipose Cells The pentose phosphate pathway begins with glucose-6-phosphate, a six-carbon sugar, and produces three-, four-, five-, six-, and seven-carbon sugars (Figure 22.22). As we will see, two successive oxidations lead to the reduction of NADP ϩ to NADPH and the re- lease of CO 2 . Five subsequent nonoxidative steps produce a variety of carbohydrates, some of which may enter the glycolytic pathway. The enzymes of the pentose phosphate pathway are particularly abundant in the cytoplasm of liver and adipose cells. These en- zymes are largely absent in muscle, where glucose-6-phosphate is utilized primarily for energy production via glycolysis and the TCA cycle. These pentose phosphate pathway enzymes are located in the cytosol, which is the site of fatty acid synthesis, a pathway heavily dependent on NADPH for reductive reactions. The Pentose Phosphate Pathway Begins with Two Oxidative Steps 1. Glucose-6-Phosphate Dehydrogenase The pentose phosphate pathway begins with the oxidation of glucose-6-phosphate. The products of the reaction are a cyclic ester (the lactone of phosphogluconic acid) and NADPH (Figure 22.23). Glucose- 6-phosphate dehydrogenase, which catalyzes this reaction, is highly specific for NADP ϩ . As the first step of a major pathway, the reaction is irreversible and highly regulated. Glucose-6-phosphate dehydrogenase is strongly inhibited by the product coenzyme, NADPH, and also by fatty acid esters of coenzyme A (which are inter- mediates of fatty acid biosynthesis). Inhibition due to NADPH depends upon the cytosolic NADP ϩ /NADPH ratio, which in the liver is about 0.015 (compared to about 725 for the NAD ϩ /NADH ratio in the cytosol). 2. Gluconolactonase The gluconolactone produced in step 1 is hydrolytically un- stable and readily undergoes a spontaneous ring-opening hydrolysis, although an enzyme, gluconolactonase, accelerates this reaction (Figure 22.24). The linear product, the sugar acid 6-phospho- D-gluconate, is further oxidized in step 3. 3. 6-Phosphogluconate Dehydrogenase The oxidative decarboxylation of 6- phosphogluconate by 6-phosphogluconate dehydrogenase yields D-ribulose-5- phosphate and another equivalent of NADPH. There are two distinct steps in this + + + + Glucose A mino acids Plasma Liver Amino acid metabolizing enzymes Gluconeogenesis Nitrogen Glycogen synthesis Urea cycle Cortisol Urea FIGURE 22.21 The effects of cortisol on carbohydrate and protein metabolism in the liver. Go to CengageNOW and click CengageInteractive to explore the reaction mecha- nism for 6-phosphogluconate dehydrogenase. 22.6 Can Glucose Provide Electrons for Biosynthesis? 685 H + H 2 O P H C O C H OH CHO H CH OH CH OH C H 2 H 2 H 2 H 2 H 2 H 2 H 2 H 2 H 2 H 2 O P O – C O C H OH CHO H CH OH CH OH C O P C H H OH CO CH OH CH OH C O H 2 P C H H OH CO CH OH CH OH C O P H C O C H OH CH OH CH OH C O P C H H OH CO CH OH CH OH C O CH OH CHO H P H O C CH OH CH OH C O P C H H OH CO CH OH C O CHO H P H O C C O CH OH P C H H OH CO CH OH C O CHO H CH OH P H O C C O CH OH 1 , 2 3 4 5 4 5 6 7 8 CO 2 NADP + NADP + NADPH + H + NADPH + Reductive anabolic pathways Glucose-6- phosphate 6-Phospho- gluconate Ribulose-5- phosphate Ribulose-5- phosphate Ribose-5- phosphate Nucleic acid biosynthesis Sedoheptulose- 7-phosphate Erythrose-4- phosphate Another Xylulose-5- phosphate Xylulose-5- phosphate Glyceraldehyde- 3-phosphate Fructose-6- phosphate Glyceraldehyde- 3-phosphate Glycolytic intermediates ACTIVE FIGURE 22.22 The pentose phosphate pathway.The numerals in the blue circles indicate the steps discussed in the text. Test yourself on the concepts in this figure at www.cengage.com/login. 686 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway reaction (Figure 22.25): The initial NADP ϩ -dependent dehydrogenation yields a ␤-keto acid, 3-keto-6-phosphogluconate, which is very susceptible to decarboxy- lation (the second step). The resulting product, D-ribulose-5-P, is the substrate for the nonoxidative reactions composing the rest of this pathway. There Are Four Nonoxidative Reactions in the Pentose Phosphate Pathway This portion of the pathway begins with an isomerization and an epimerization, and it leads to the formation of either D-ribose-5-phosphate or D-xylulose-5-phosphate. These intermediates can then be converted into glycolytic intermediates or directed to biosynthetic processes. 4. Phosphopentose Isomerase This enzyme interconverts ribulose-5-P and ribose- 5-P via an enediol intermediate (Figure 22.26). The reaction (and mechanism) is quite + H + O OH OH OH HO CH 2 2 – O 3 PO CH 2 2 – O 3 PO O OH OH HO O NADPH NADP + ␣- D-Glucose-6-phosphate Glucose-6-P dehydrogenase 6-Phospho-D-gluconolactone Step 1 FIGURE 22.23 The glucose-6-phosphate dehydroge- nase reaction is the committed step in the pentose phosphate pathway. 2– O 3 POCH 2 O OH OH HO O HCOH COO – HOCH HCOH HCOH CH 2 OPO 3 2 – 6-P-D-Gluconolactone Gluconolactonase 6-P- D-Gluconate Step 2 H + H 2 O FIGURE 22.24 The gluconolactonase reaction. HCOH COO – HOCH HCOH HCOH CH 2 OPO 3 2 – HCOH COO – C HCOH HCOH CH 2 OPO 3 2 – O CH 2 OH HCOH HCOH CH 2 OPO 3 2 – C O + 6-P-D-Gluconate Step 3 6-Phosphogluconate dehydrogenase H + H + 3-Keto-6-P-D-Gluconate D-Ribulose-5-P NADPHNADP + CO 2 FIGURE 22.25 The 6-phosphogluconate dehydrogenase reaction. 22.6 Can Glucose Provide Electrons for Biosynthesis? 687 similar to the phosphoglucoisomerase reaction of glycolysis, which interconverts glucose-6-P and fructose-6-P. The ribose-5-P produced in this reaction is utilized in the biosynthesis of coenzymes (including NADH, NADPH, FAD, and B 12 ), nucleotides, and nucleic acids (DNA and RNA). The net reaction for the first four steps of the pen- tose phosphate pathway is Glucose-6-P ϩ 2 NADP ϩ ϩ H 2 O ⎯⎯→ ribose-5-P ϩ 2 NADPH ϩ 2 H ϩ ϩ CO 2 5. Phosphopentose Epimerase This reaction converts ribulose-5-P to another ketose, namely, xylulose-5-P. This reaction also proceeds by an enediol intermediate but involves an inversion at C-3 (Figure 22.27). In the reaction, an acidic proton located ␣- to a carbonyl carbon is removed to generate the enediolate, but the pro- ton is added back to the same carbon from the opposite side. Note the distinction in nomenclature here. Interchange of groups on a single carbon is an epimeriza- tion, and interchange of groups between carbons is an isomerization. To this point, the pathway has generated a pool of pentose phosphates. The ⌬G°Ј for each of the last two reactions is small, and the three pentose-5-phosphates CH 2 OH HCOH HCOH CH 2 OPO 3 2 – C O HC HCOH HCOH CH 2 OPO 3 2 – C OH OH HC HCOH HCOH CH 2 OPO 3 2 – O HCOH Step 4 D-Ribulose-5-P (ketose) Enediol Ribose-5-P (aldose) FIGURE 22.26 The phosphopentose isomerase reaction involves an enediol intermediate. HUMAN BIOCHEMISTRY Aldose Reductase and Diabetic Cataract Formation The complications of diabetes include a high propensity for cataract formation in later life, both in type 1 and type 2 diabetics. Hyper- glycemia is the suspected cause, but by what mechanism? Several lines of evidence point to the polyol pathway, in which glucose and other simple sugars are reduced in NADPH-dependent reactions. Glucose, for example, is reduced by aldose reductase to sorbitol (see accompanying figure), which accumulates in lens fiber cells, in- creasing the intracellular osmotic pressure and eventually rupturing the cells. The involvement of aldose reductase in this process is supported by the fact that animals that have high levels of this en- zyme in their lenses (such as rats and dogs) are prone to develop diabetic cataracts, whereas mice that have low levels of lens aldose reductase activity are not. Moreover, aldose reductase inhibitors such as tolrestat and epalrestat suppress cataract formation. These drugs or derivatives from them may represent an effective preven- tive therapy against cataract formation in people with diabetes. CH 2 OH H CHO OH C HOH C HOH C HHO C CH 2 OH H CH 2 OH OH C HOH C HOH C HHO C H 2 C CF 3 CH 3 C O H S N COO – CH 2 CH 3 N O S COO – (a) NADPH + H + NADP + Glucose Sorbitol Aldose reductase (b) Tolrestat Epalrestat 688 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway coexist at equilibrium. The pathway has also produced two molecules of NADPH for each glucose-6-P converted to pentose-5-phosphate. The next three steps rearrange the five-carbon skeletons of the pentoses to produce three-, four-, six-, and seven- carbon units, which can be used for various metabolic purposes. Why should the cell do this? Very often, the cellular need for NADPH is considerably greater than the need for ribose-5-phosphate. The next three steps thus return some of the five- carbon units to glyceraldehyde-3-phosphate and fructose-6-phosphate, which can enter the glycolytic pathway. The advantage of this is that the cell has met its needs for NADPH and ribose-5-phosphate in a single pathway, yet at the same time it can return the excess carbon metabolites to glycolysis. 6 and 8.Transketolase The transketolase enzyme acts at both steps 6 and 8 of the pentose phosphate pathway. In both cases, the enzyme catalyzes the transfer of two- carbon units. In these reactions (and also in step 7, the transaldolase reaction, which transfers three-carbon units), the donor molecule is a ketose and the recipient is an aldose. In step 6, xylulose-5-phosphate transfers a two-carbon unit to ribose-5-phosphate to form glyceraldehyde-3-phosphate and sedoheptulose-7- phosphate (Figure 22.28). Step 8 involves a two-carbon transfer from xylulose- 5-phosphate to erythrose-4-phosphate to produce another glyceraldehyde-3- phosphate and a fructose-6-phosphate (Figure 22.29). Three of these products CH 2 OH COH COH CH 2 OPO 3 2 – C O H H B CH 2 OH OH OH CH 2 OPO 3 2 – CO – C HC HB + CH 2 OH C C CH 2 OPO 3 2 – C O HO H H OH E E Phosphopentose epimerase Ribulose-5-P Step 5 Xylulose-5-PEnediolate FIGURE 22.27 The phosphopentose epimerase reaction interconverts ribulose-5-P and xylulose-5-phosphate. The mechanism involves an enediol intermediate and occurs with inversion at C-3. CH 2 OH CH CH 2 OPO 3 2 – C O HO + CHO CH 2 OPO 3 2 – HCOH HCOH HCOH HCOH Transketolase CH 2 OPO 3 2 – CHO HCOH + CH 2 OH CH 2 OPO 3 2 – C O HCOH HCOH HCOH HOCH Xylulose-5-P Ribose-5-P Glyceraldehyde-3-P Sedoheptulose-7-P Step 6 FIGURE 22.28 The transketolase reaction of step 6 in the pentose phosphate pathway. Go to CengageNOW and click CengageInteractive to learn more about the reaction of the transketolase enzyme. CH 2 OH C CH 2 OPO 3 2 – C O HO H HCOH + CHO CH 2 OPO 3 2 – HCOH HCOH Transketolase CH 2 OPO 3 2 – CHO HCOH + CH 2 OH CH 2 OPO 3 2 – C O HCOH HCOH CHHO Xylulose-5-P Step 8 Erythrose-4-P Glyceraldehyde-3-P Fructose-6-P FIGURE 22.29 The transketolase reaction of step 8 in the pentose phosphate pathway. 22.6 Can Glucose Provide Electrons for Biosynthesis? 689 enter directly into the glycolytic pathway. (The sedoheptulose-7-phosphate is taken care of in step 7, as we shall see.) Transketolase is a thiamine pyrophosphate– dependent enzyme, and the mechanism (Figure 22.30) involves abstraction of the acidic thiazole proton of TPP, attack by the resulting carbanion at the carbonyl carbon of the ketose phosphate substrate, expulsion of the glyceraldehyde-3- phosphate product, and transfer of the two-carbon unit. Transketolase can process a variety of 2-keto sugar phosphates in a similar manner. It is specific for ketose substrates with the configuration shown but can accept a variety of aldose phos- phate substrates. 7. Transaldolase The transaldolase functions primarily to make a useful gly- colytic substrate from the sedoheptulose-7-phosphate produced by the first trans- ketolase reaction. This reaction (Figure 22.31) is quite similar to the aldolase re- action of glycolysis, involving formation of a Schiff base intermediate between the C CH 2 OH HOCH O S N R" R R' + HCOH CH 2 OPO 3 2 – S N R" R + C CH 2 OH HCOH CH 2 OPO 3 2 – OH B CHO HCOH CH 2 OPO 3 2 – S N + C CH 2 OH OH HCOH HCOH CH 2 OPO 3 2 – HCOH HC O S N C CH 2 OH OH S N R + + HCOH HCOH CH 2 OPO 3 2 – HCOH OC CH 2 OH S N R + HCOH HCOH CH 2 OPO 3 2 – HCOH C CH 2 OH O H – E – R'' R" R R" R R" R' R' R' R' R' HOCH HOCH CHOH B E H – H + D-Xylulose-5-P D-Ribose-5-P Sedoheptulose-7-P Glyceraldehyde-3-P FIGURE 22.30 The mechanism of the TPP-dependent transketolase reaction. Ironically, the group transferred in the transketolase reaction might best be described as an aldol, whereas the transferred group in the transaldolase reaction is actually a ketol. Despite the irony, these names persist for historical reasons. 690 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway sedoheptulose-7-phosphate and an active-site lysine residue (Figure 22.32). Elim- ination of the erythrose-4-phosphate product leaves an enamine of dihydroxy- acetone, which remains stable at the active site (without imine hydrolysis) until the other substrate comes into position. Attack of the enamine carbanion at the carbonyl carbon of glyceraldehyde-3-phosphate is followed by hydrolysis of the Schiff base (imine) to yield the product fructose-6-phosphate. Step 7 CH 2 OH CH 2 OPO 3 2 – C O HCOH HCOH CHHO HCOH + CH 2 OPO 3 2 – CHO HCOH CH 2 OPO 3 2 – HCOH CHO HCOH + CH 2 OH CH 2 OPO 3 2 – C O HOCH HCOH HCOH Sedoheptulose-7-P Glyceraldehyde-3-P Erythrose-4-P Fructose-6-P Transaldolase FIGURE 22.31 The transaldolase reaction. – CH 2 OH C O CHO Lys NH 2 H CH OH R N H CH 2 OH C + + C HO H CH O R H N H CH 2 OH C C HO H H N CH 2 OH C HO H C RCHO (Erythrose-4-P) H C O CH OH CH 2 OH C N CHO H CH OH CH OH CH 2 OPO 3 2 – CH 2 OH C O CHO H CH OH CH OH CH 2 OPO 3 2 – CH 2 OPO 3 2 – E E E E E H 2 O Enamine Glyceraldehyde-3-P Fructose-6-P ANIMATED FIGURE 22.32 The trans- aldolase mechanism involves attack on the substrate by an active-site lysine. Departure of erythrose-4-P leaves the reactive enamine, which attacks the aldehyde car- bon of glyceraldehyde-3-P. Schiff base hydrolysis yields the second product, fructose-6-P. See this figure ani- mated at www.cengage.com/login. 22.6 Can Glucose Provide Electrons for Biosynthesis? 691 Utilization of Glucose-6-P Depends on the Cell’s Need for ATP, NADPH, and Ribose-5-P It is clear that glucose-6-phosphate can be used as a substrate either for glycolysis or for the pentose phosphate pathway. The cell makes this choice on the basis of its rel- ative needs for biosynthesis and for energy from metabolism. ATP can be produced in abundance if glucose-6-phosphate is channeled into glycolysis. On the other hand, if NADPH or ribose-5-phosphate is needed, glucose-6-phosphate can be directed to the pentose phosphate pathway. The molecular basis for this regulatory decision de- pends on the enzymes that metabolize glucose-6-phosphate in glycolysis and the pen- tose phosphate pathway. In glycolysis, phosphoglucoisomerase converts glucose- 6-phosphate to fructose-6-phosphate, which is utilized by phosphofructokinase (a highly regulated enzyme) to produce fructose-1,6-bisphosphate. In the pentose phosphate pathway, glucose-6-phosphate dehydrogenase (also highly regulated) produces 6-phosphogluconolactone from glucose-6-phosphate. Thus, the fate of glucose-6-phosphate is determined to a large extent by the relative activities of phos- phofructokinase and glucose-6-P dehydrogenase. Recall from Chapter 18 that PFK is inhibited when the ATP/AMP ratio increases and that it is inhibited by citrate but ac- tivated by fructose-2,6-bisphosphate. Thus, when the energy charge is high, glycolytic flux decreases. Glucose-6-P dehydrogenase, on the other hand, is inhibited by high levels of NADPH and also by the acyl-CoA intermediates of fatty acid biosynthesis. Both of these are indicators that biosynthetic demands have been satisfied. If that is the case, glucose-6-phosphate dehydrogenase and the pentose phosphate pathway are inhibited. If NADPH levels drop, the pentose phosphate pathway turns on and NADPH and ribose-5-phosphate are made for biosynthetic purposes. Even when the latter choice has been made, however, the cell must still be re- sponsive to the relative needs for ribose-5-phosphate and NADPH (as well as ATP). Depending on these relative needs, the reactions of glycolysis and the pentose phos- phate pathway can be combined in novel ways to emphasize the synthesis of needed metabolites. There are four principal possibilities. 1. Both Ribose-5-P and NADPH Are Needed by the Cell In this case, the first four reac- tions of the pentose phosphate pathway predominate (Figure 22.33). NADPH is pro- duced by the oxidative reactions of the pathway, and ribose-5-P is the principal prod- uct of carbon metabolism. As stated earlier, the net reaction for these processes is Glucose-6-P ϩ 2 NADP ϩ ϩ H 2 O ⎯⎯→ ribose-5-P ϩ CO 2 ϩ 2 NADPH ϩ 2 H ϩ 2. More Ribose-5-P Than NADPH Is Needed by the Cell Synthesis of ribose-5-P can be accomplished without production of NADPH if the oxidative steps of the pen- tose phosphate pathway are bypassed. The key to this route is the withdrawal of fructose-6-P and glyceraldehyde-3-P, but not glucose-6-P, from glycolysis. The ac- tion of transketolase and transaldolase on fructose-6-P and glyceraldehyde-3-P produces three molecules of ribose-5-P from two molecules of fructose-6-P and one of glyceraldehyde-3-P. In this route, as in case 1, no carbon metabolites are re- turned to glycolysis. The net reaction for this route is 5 Glucose-6-P ϩ ATP ⎯⎯→ 6 ribose-5-P ϩ ADP ϩ H ϩ 3. More NADPH Than Ribose-5-P Is Needed by the Cell Large amounts of NADPH 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 1 2 3 4 Glucose-6-P 6-Phosphogluconate Reactions , Ribulose-5-phosphate Ribose-5-phosphate NADPH NADP + H 2 O + NADPH NADP + FIGURE 22.33 When biosynthetic demands dictate, the first four reactions of the pentose phosphate pathway predominate and the principal products are ribose-5-P and NADPH. 692 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway glycolytic intermediates. This alternative involves a complex interplay between the transketolase and transaldolase reactions to convert ribulose-5-P to fructose-6-P and glyceraldehyde-3-P, which can be recycled to glucose-6-P via gluconeogenesis. The net reaction for this process is 6 Glucose-6-P ϩ 12 NADP ϩ ϩ 6 H 2 O ⎯⎯→ 6 ribulose-5-P ϩ 6 CO 2 ϩ 12 NADPH ϩ 12 H ϩ 6 Ribulose-5-P ⎯⎯→ 5 glucose-6-P ϩ P i Net: Glucose-6-P ϩ 12 NADP ϩ ϩ 6 H 2 O ⎯⎯→ 6 CO 2 ϩ 12 NADPH ϩ 12 H ϩ ϩ P i Note that in this scheme, the six hexose sugars have been converted to six pen- tose sugars with release of six molecules of CO 2 , and the six pentoses are recon- verted to five glucose molecules. 4. Both NADPH and ATP Are Needed by the Cell, but Ribose-5-P Is Not Under some conditions, both NADPH and ATP must be provided in the cell. This can be accomplished in a series of reactions similar to case 3 if the fructose-6-P and glyceraldehyde-3-P produced in this way proceed through glycolysis to produce ATP and pyruvate, which itself can yield even more ATP by continuing on to the TCA cycle. The net reaction for this alternative is 3 Glucose-6-P ϩ 5 NAD ϩ ϩ 6 NADP ϩ ϩ 8 ADP ϩ 5 P i ⎯⎯→ 5 pyruvate ϩ 3 CO 2 ϩ 5 NADH ϩ 6 NADPH ϩ 8 ATP ϩ 2 H 2 O ϩ 8 H ϩ Note that, except for the three molecules of CO 2 , all the other carbon from glucose- 6-P is recovered in pyruvate. Xylulose-5-Phosphate Is a Metabolic Regulator In addition to its role in the pentose phosphate pathway, xylulose-5-phosphate serves as a signaling molecule. When blood [glucose] rises (for example, follow- ing a carbohydrate-rich meal), glycolysis and the pentose phosphate pathway are activated in the liver, and xylulose-5-phosphate produced in the latter pathway CH 2 OH CO HOCH CH 2 OPO 3 2 – HOCH Xylulose-5-P Protein phosphatase 2A 2 H 2 O 2 ADP 2 ATP ChREBP ChREBP 2 P i Pyruvate PDH Acetyl-CoA P P Lipid synthesis PKA + F-2,6-BP + Glycolysis + PFK-2 F-2,6-BPase + FIGURE 22.34 Xylulose-5-phosphate is a regulator of multiple metabolic pathways. Activation of PP2A triggers dephosphorylation of the bifunctional PFK-2/F2,6-BPase, which raises fructose-2,6-BP levels, in turn activating glycolysis and inhibiting gluconeogenesis. Simultaneously, ChREBP is dephosphorylated, leading to elevated expression of genes encoding enzymes for lipogenesis.These effects combine to trigger lipid biosynthesis in response to a carbohydrate-rich meal.