Biochemistry, 4th Edition P74 docx

Problems 693 stimulates protein phosphatase 2A (PP2A), which dephosphorylates the bifunctional enzyme PFK-2/F-2,6-BPase (Figure 22.34, and see Figure 22.10). Increases in [fructose-2,6-bisphosphate] stimulate glycolysis and inhibit gluconeogenesis. At the same time, PP2A dephosphorylates carbohydrate-responsive element-binding protein (ChREBP), a transcription factor that activates expression of liver genes for lipid synthesis. These effects are a powerful combination. Increased glycolysis produces substantial amounts of acetyl-CoA, the principal substrate for lipid synthesis. The pentose phosphate pathway produces NADPH, the source of electrons for lipid biosynthesis. Elevated expression of the appropriate genes sets the stage for lipid biosynthesis in the liver, an important consequence of ingestion of carbohydrates. SUMMARY 22.1 What Is Gluconeogenesis, and How Does It Operate? Gluconeo- genesis is the generation (genesis) of new (neo) glucose. In addition to pyruvate and lactate, other noncarbohydrate precursors can be used as substrates for gluconeogenesis in animals, including most of the amino acids, as well as glycerol and all the TCA cycle intermediates. On the other hand, fatty acids are not substrates for gluconeogenesis in animals. Lysine and leucine are the only amino acids that are not substrates for gluconeogenesis. These amino acids produce only acetyl-CoA upon degradation. Acetyl-CoA can be a substrate for gluconeogenesis in plants when the glyoxylate cycle is operating. The major sites of gluconeogenesis are the liver and kidneys, which account for about 90% and 10% of the body’s gluconeogenic activity, respectively. 22.2 How Is Gluconeogenesis Regulated? Glycolysis and gluconeogenesis are under reciprocal control, so glycolysis is inhibited when gluconeogenesis is active, and vice versa. When the energy status of the cell is low, glucose is rapidly degraded to produce needed energy. When the energy status is high, pyruvate and other metabolites are uti- lized for synthesis (and storage) of glucose. The three sites of regulation in the gluconeogenic pathway are glucose-6-phosphatase, fructose- 1,6-bisphosphatase, and the pyruvate carboxylase–PEP carboxykinase pair, respectively. These are the three most appropriate sites of regulation in gluconeogenesis. Glucose-6-phosphatase is under substrate- level control by glucose-6-phosphate. Acetyl-CoA allosterically activates pyruvate carboxylase. Fructose-1,6-bisphosphatase is inhibited by AMP and activated by citrate. Fructose-2,6-bisphosphate is a powerful in- hibitor of fructose-1,6-bisphosphatase. 22.3 How Are Glycogen and Starch Catabolized in Animals? Virtually 100% of digestible food is absorbed and metabolized. Digestive breakdown of starch is an unregulated process. On the other hand, tissue glycogen represents an important reservoir of potential energy, and the reactions involved in its degradation and synthesis are carefully controlled and regulated. Glycogen reserves in liver and muscle tissue are stored in the cytosol as granules exhibiting a molecular weight range from 6 ϫ 10 6 to 1600 ϫ 10 6 . These granular aggregates contain the enzymes required to synthesize and catabolize the glycogen, as well as all the enzymes of glycolysis. The principal enzyme of glycogen catabolism is glycogen phosphorylase, a highly regulated enzyme. The glycogen phosphorylase reaction involves phosphorolysis at a nonreducing end of a glycogen polymer. 22.4 How Is Glycogen Synthesized? Luis Leloir, a biochemist in Ar- gentina, showed in the 1950s that glycogen synthesis depended upon sugar nucleotides. The glycogen polymer is built around a tiny protein core. The first glucose residue is covalently joined to the protein glycogenin via an acetal linkage to a tyrosine–OH group on the protein. Sugar units are added to the glycogen polymer by the action of glycogen synthase. The reaction involves transfer of a glucosyl unit from UDP–glucose to the C-4 hydroxyl group at a nonreducing end of a glycogen strand. The mechanism proceeds by cleavage of the COO bond between the glucose moiety and the ␤-phosphate of UDP–glucose, leaving an oxonium ion intermediate, which is rapidly attacked by the C-4 hydroxyl oxygen of a terminal glucose unit on glycogen. 22.5 How Is Glycogen Metabolism Controlled? Activation of glycogen phosphorylase is tightly linked to inhibition of glycogen synthase, and vice versa. Regulation involves both allosteric control and covalent modification, with the latter being under hormonal control. Glycogen synthase is also regulated by covalent modification. Storage and utilization of tissue glycogen are regulated by hormones, including insulin, glucagon, epinephrine, and the glucocorticoids. Insulin stimulates glycogen synthesis and inhibits glycogen breakdown in liver and muscle, whereas glucagon and epinephrine stimulate glycogen breakdown. 22.6 Can Glucose Provide Electrons for Biosynthesis? The pentose phosphate pathway is a collection of eight reactions that provide NADPH for biosynthetic processes and ribose-5-phosphate for nucleic acid synthesis. Several metabolites of the pentose phosphate pathway can also be shuttled into glycolysis. Utilization of glucose-6-P in the pentose phosphate pathway depends on the cell’s need for ATP, NADPH, and ribose-5-P. PROBLEMS Preparing for an exam? Create your own study path for this chapter at www.cengage.com/login. 1. Consider the balanced equation for gluconeogenesis in Section 22.1. Account for each of the components of this equation and the indi- cated stoichiometry. 2. (Integrates with Chapters 3 and 18.) Calculate ⌬G°Ј and ⌬G for gluconeogenesis in the erythrocyte, using data in Table 18.2 (as- sume NAD ϩ /NADH ϭ 20, [GTP] ϭ [ATP], and [GDP] ϭ [ADP]). See how closely your values match those in Section 22.1. 3. Use the data of Figure 22.9 to calculate the percent inhibition of fructose-1,6-bisphosphatase by 25 mM fructose-2,6-bisphosphate when fructose-1,6-bisphosphate is (a) 25 mM and (b) 100 mM. 4. (Integrates with Chapter 3.) Suggest an explanation for the exergonic nature of the glycogen synthase reaction (⌬G°ЈϭϪ13.3 kJ/mol). Consult Chapter 3 to review the energetics of high-energy phosphate compounds if necessary. 5. Using the values in Table 23.1 for body glycogen content and the data in part b of the illustration for A Deeper Look (page 680), 694 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway calculate the rate of energy consumption by muscles in heavy exercise (in J/sec). Use the data for fast-twitch muscle. 6. Which reactions of the pentose phosphate pathway would be inhibited by NaBH 4 ? Why? 7. (Integrates with Chapter 7.) Imagine a glycogen molecule with 8000 glucose residues. If branches occur every eight residues, how many reducing ends does the molecule have? If branches occur every 12 residues, how many reducing ends does it have? How many nonreducing ends does it have in each of these cases? 8. Explain the effects of each of the following on the rates of gluconeogenesis and glycogen metabolism: a. Increasing the concentration of tissue fructose-1,6-bisphosphate b. Increasing the concentration of blood glucose c. Increasing the concentration of blood insulin d. Increasing the amount of blood glucagon e. Decreasing levels of tissue ATP f. Increasing the concentration of tissue AMP g. Decreasing the concentration of fructose-6-phosphate 9. (Integrates with Chapters 3 and 15.) The free energy change of the glycogen phosphorylase reaction is ⌬G°Јϭϩ3.1 kJ/mol. If [P i ] ϭ 1 mM, what is the concentration of glucose-1-P when this reaction is at equilibrium? 10. Based on the mechanism for pyruvate carboxylase (Figure 22.3), write reasonable mechanisms for the reactions that follow: ϩϩ ϩϩ HCO 3 Ϫ Ϫ OOCCC O SCoACH H 3 C H 3 C CH 2 C C SCoA ADP P i P i P i CH CH 3 O ATP ϩϩ ϩϩ HCO 3 Ϫ O SCoA H 3 C ADP CH 3 CH 3 O SCoA H 3 C CH 3 CH 3 COO Ϫ ATP ϩ ATP ϩϩ ϩ HCO 3 ϪϪ OOCC O H 2 NNH 2 N H C NH 2 ADP O ϩ Ϫ OOC C H H 3 C SCoA C O COO Ϫ C H 3 C O ϩ Ϫ OOC H C H H 3 C SCoA H C H C O COO Ϫ C O -Methylglutaconyl-CoA ␤ -Methylcrotonyl-CoA ␤ Geranyl-CoA -Carboxygeranyl-CoA ␥ Urea N-Carboxyurea Methylmalonyl-CoA Transcarboxylase Pyruvate OxaloacetatePropionyl-CoA COO Ϫ C CH 3 O C CH 3 O COO Ϫ C CH 3 O ϩ CO 2 ϩ COO Ϫ C OH CH 3 CO HOCH HCOH HCOH CH 2 OPO 3 2Ϫ HOPO 3 2Ϫ ϩϩ H 2 COH O HCOH HCOH CH 2 OPO 3 2Ϫ HC OPO 3 2– C H 3 C O Acetolactate synthase Fructose-6-P Acet y l-P H 2 O Er y throse-4-P Phosphoketolase 11. The mechanistic chemistry of the acetolactate synthase and phosphoketolase reactions (shown here) is similar to that of the trans- ketolase reaction (Figure 22.30). Write suitable mechanisms for these reactions. Further Reading 695 12. Metaglip is a prescribed preparation (from Bristol-Myers Squibb) for treatment of type 2 diabetes. It consists of metformin (see Human Biochemistry, page 668) together with glipizide. The actions of metformin and glipizide are said to be complementary. Suggest a mechanism for the action of glipizide. 13. Study the structures of tolrestat and epalrestat in the Human Bio- chemistry box on page 687 and suggest a mechanism of action for these inhibitors of aldose reductase. 14. Based on the discussion on page 691, draw a diagram to show how several steps in the pentose phosphate pathway can be bypassed to produce large amounts of ribose-5-phosphate. Begin your diagram with fructose-6-phosphate. 15. Consider the diagram you constructed in problem 14. Which carbon atoms in ribose-5-phosphate are derived from carbon atoms in positions 1, 3, and 6 of fructose-6-phosphate? 16. As described on pages 691 and 692, the pentose phosphate pathway may be used to produce large amounts of NADPH without signifi- cant net production of ribose-5-phosphate. Draw a diagram, beginning with glucose-6-phosphate, to show how this may be ac- complished. 17. The discussion on page 692 explains that the pentose phosphate pathway and the glycolytic pathway can be combined to provide both NADPH and ATP (as well as some NADH) without net ribose- 5-phosphate synthesis. Draw a diagram to show how this may be ac- complished. 18. Consider the pathway diagram you constructed in problem 17. What is the fate of carbon from positions 2 and 4 of glucose-6- phosphate after one pass through the pathway? 19. Glycogenin catalyzes the first reaction in the synthesis of a glycogen particle, with Tyr 194 of glycogenin (page 676) combining with a glucose unit (provided by UDP-glucose) to produce a tyrosyl glucose. Write a mechanism to show how this reaction could occur. Preparing for the MCAT Exam 20. Study the graphs in the Deeper Look box (page 680) and explain the timing of the provision of energy from different metabolic sources during periods of heavy exercise. 21. (Integrates with Chapters 3 and 14.) What is the structure of crea- tine phosphate? Write reactions to indicate how it stores and provides energy for exercise. FURTHER READING Gluconeogenesis Boden, G., 2003. Effect of free fatty acids on gluconeogenesis and glycogenolysis. Life Science 72:977–988. Choe, J Y., Iancu, C. V., et al., 2003. Metaphosphate in the active site of fructose-1,6-bisphosphatase. Journal of Biological Chemistry 278: 16015–16020. Dzugaj, A., 2006. Localization and regulation of muscle fructose-1,6- bisphosphatase, the key enzyme of glyconeogenesis. Advances in En- zyme Regulation 46:51–71. Gerich, J. E., Meyer, C., et al., 2001. Renal gluconeogenesis: Its impor- tance in human glucose homeostasis. Diabetes Care 24:382–391. Hers, H G., and Hue, L., 1983. Gluconeogenesis and related aspects of glycolysis. Annual Review of Biochemistry 52:617–653. Jitrapakdee, S., and Wallace, J. C., 1999. Structure, function, and regulation of pyruvate carboxylase. Biochemical Journal 348:1–16. Kondo, S., Nakajima, Y., et al., 2007. Structure of the biotin carboxylase domain of pyruvate carboxylase from Bacillus thermodenitrificans. Acta Crystallographica D 63:885–890. Regulation of Gluconeogenesis Alves, G., and Sola-Penna, M., 2003. Epinephrine modulates cellular dis- tribution of muscle phophofructokinase. Molecular Genetics and Me- tabolism 78:302–306. Arden, C., Hampson, L., et al., 2008. A role for PFK-2/FBPase-2, as dis- tinct from fructose-2,6-bisphosphate, in regulation of insulin secre- tion in pancreatic ␤-cells. Biochemical Journal 411:41–51. Moller, D. E., 2001. New drug targets for type 2 diabetes and the metabolic syndrome. Nature 414:821–827. Newsholme, E. A., and Leech, A. R., 1983. Biochemistry for the Medical Sci- ences. New York: John Wiley and Sons. Newsholme, E. A., and Leech, A. R., 1983b. Substrate cycles: Their role in improving sensitivity in metabolic control. Trends in Biochemical Sciences 9:277–280. Rider, M., Bertrand, L., et al., 2004. 6-Phosphofructo-2-kinase/fructose- 2,6-bisphosphatase: Head-to-head with a bifunctional enzyme that controls glycolysis. Biochemical Journal 381:561–579. Rolfe, D. J., and Brown, G. C., 1997. Cellular energy utilization and molecular origin of standard metabolic rate in mammals. Physiolog- ical Reviews 77:731–758. Van Schaftingen, E., and Hers, H. G., 1981. Inhibition of fructose-1,6- bisphosphatase by fructose-2,6-bisphosphate. Proceedings of the Na- tional Academy of Sciences U.S.A. 78:2861–2863. Exercise Physiology Akermark, C., Jacobs, I., et al., 1996. Diet and muscle glycogen concentration in relation to physical performance in Swedish elite ice hockey players. International Journal of Sport Nutrition 6:272–284. Hargreaves, M., 1997. Interactions between muscle glycogen and blood glucose during exercise. Exercise and Sport Science Reviews 25:21–39. Horton, E. S., and Terjung, R. L., 1988. Exercise, Nutrition and Energy Me- tabolism. New York: Macmillan. Rhoades, R., and Pflanzer, R., 1992. Human Physiology . Philadelphia: Saunders College Publishing. Gluc ose Homeostasis Dalsgaard, M. K., 2006. Fuelling cerebral activity in exercising man. Jour- nal of Cerebral Blood Flow and Metabolism 26:731–750. Dalsgaard, M. K., and Secher, N. H., 2007. The brain at work: A cerebral metabolic manifestation of central fatigue? Journal of Neuroscience Re- search 85:3334–3339. Feinman, R. D., and Fine, E. J., 2007. Nonequilibrium thermodynamics and energy efficiency in weight loss diets. Theoretical Biology and Med- ical Modelling 4:1–13. Huan g, S., and Czech, M. P., 2007. The GLUT4 glucose transporter. Cell Metabolism 5:237–252. Watson, R. T., and Pessin, J. E., 2006. Bridging the GAP between insulin signaling and GLUT4 translocation. Trends in Biochemical Sciences 31:215–222. Glycogen Metabolism Browner, M. F., and Fletterick, R. J., 1992. Phosphorylase: A biological transducer. Trends in Biochemical Sciences 17:66–71. Delibegovic, M., Armstrong, C. J., et al., 2003. Disruption of the striated muscle glycogen targeting subunit PPP1R3A of protein phosphatase 1 leads to increased weight gain, fat deposition, and devel- opment of insulin resistance. Diabetes 52:506–604. Foster, J. D., and Nordlie, R. C., 2002. The biochemistry and molecular biology of the glucose-6-phosphatase system. Experimental Biology and Medicine 227:601–608. Horcajada, C., Guinovart, J. J., et al., 2006. Crystal structure of an archaeal glycogen synthase. Journal of Biological Chemistry 281:2923–2931. Hurley, T. D., Stout, S., et al., 2005. Requirements for catalysis in mammalian glycogenin. Journal of Biological Chemistry 280:23892–23899. Johnson, L. N., 1992. Glycogen phosphorylase: Control by phosphorylation and allosteric effectors. FASEB Journal 6:2274–2282. 696 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway Jope, R. S., Yuskaitis, C., et al., 2007. Glycogen synthase kinase-3 (GSK3): Inflammation, diseases, and therapeutics. Neurochemical Research 32:577–595. Kerkela, R., Woulfe, K., et al., 2007. Glycogen synthase kinase-3␤: Ac- tively inhibiting hypertrophy. Trends in Cardiovascular Medicine 17: 91–96. Kotova, O., Al-Khalili, L., et al., 2006. Cardiotonic steroids stimulate glycogen synthesis in human skeletal muscle cells via a Src- and ERK1/2-dependent mechanism. Journal of Biological Chemistry 281: 20085–20094. Larner, J., 1990. Insulin and the stimulation of glycogen synthesis: The road from glycogen structure to glycogen synthase to cyclic AMP- dependent protein kinase to insulin mediators. Advances in Enzy- mology 63:173–231. Lerin, C., Montell, E., et al., 2003. Regulation and function of the muscle glycogen-targeting subunit of protein phosphatase-1 (G M ) in human muscle cells depends on the COOH-terminal region and glycogen content. Diabetes 52:2221–2226. Montori-Grau, M., Guitart, M., et al., 2007. Expression and glycogenic effect of glycogen-targeting protein phosphatase 1 regulatory subunit G L in cultured human muscle. Biochemical Journal 405:107–113. Ozen, H., 2007. Glycogen storage diseases: New perspectives. World Jour- nal of Gastroenterology 13:2541–2553. Paterson, J., Kelsall, I. R., et al., 2008. Disruption of the striated muscle glycogen-targeting subunit of protein phosphatase 1: Influence of the genetic background. Journal of Molecular Endocrinology 40: 47–59. Saeed, Y. A., and Barger, S. W., 2007. Glycogen synthase kinase-3 in neu- rodegeneration and neuroprotection: Lessons from lithium. Cur- rent Alzheimer Research 4:21–31. Stalmans, W., Cadefau, J., et al., 1997. New insight into the regulation of liver glycogen metabolism by glucose. Biochemical Society Transactions 25:19–25. Yamamoto-Honda, R., Honda, Z., et al., 2000. Overexpression of the glycogen targeting (G M ) subunit of protein phosphatase-1. Biochem- ical and Biophysical Research Communications 275:859–864. Two Hummingbirds Lithograph/The Academy of Natural Sciences of Philadelphia/CORBIS 23 Fatty Acid Catabolism 23.1 How Are Fats Mobilized from Dietary Intake and Adipose Tissue? Modern Diets Are Often High in Fat Fatty acids are acquired readily in the diet and can also be made from carbohydrates and the carbon skeletons of amino acids. Fatty acids provide 30% to 60% of the calo- ries in the diets of most Americans. For our caveman and cavewoman ancestors, the figure was probably closer to 20%. Dairy products were apparently not part of their diet, and the meat they consumed (from fast-moving animals) was low in fat. In con- trast, modern domesticated cows and pigs are actually bred for high fat content (and better taste). However, woolly mammoth burgers and saber-toothed tiger steaks are hard to find these days—even in the gourmet sections of grocery stores—and so, by default, we consume (and metabolize) large quantities of fatty acids. Triacylglycerols Are a Major Form of Stored Energy in Animals Although some of the fat in our diets is in the form of phospholipids, triacylglycerols are a major source of fatty acids. Triacylglycerols are also our principal stored energy reserve. As shown in Table 23.1, the energy available in stores of fat in the average person far exceeds the energy available from protein, glycogen, and glucose. Overall, fat accounts for approximately 83% of available energy, partly because more fat is stored than protein and carbohydrate and partly because of the sub- stantially higher energy yield per gram for fat compared with protein and carbohydrate. Complete combustion of fat yields about 37 kJ/g, compared with about 16 to 17 kJ/g for sugars, glycogen, and amino acids. In animals, fat is stored mainly as triacylglycerols in specialized cells called adipocytes or adipose cells. As shown in Fig- ure 23.1, triacylglycerols, aggregated to form large globules, occupy most of the volume of adipose cells. Much smaller amounts of triacylglycerols are stored as small, aggregated globules in muscle tissue. Hormones Trigger the Release of Fatty Acids from Adipose Tissue The pathways for liberation of fatty acids from triacylglycerols, either from adipose cells or from the diet, are shown in Figures 23.2 and 23.3. Fatty acids are mobilized from adipocytes in response to hormone messengers such as adrenaline, glucagon, and adrenocorticotropic hormone (ACTH). These signal molecules bind to re- ceptors on the plasma membrane of adipose cells and lead to the activation of The hummingbird’s tremendous capacity to store and use fatty acids enables it to make migratory journeys of remarkable distances. The fat is in the fire. John Heywood Proverbs (1497–1580) KEY QUESTIONS 23.1 How Are Fats Mobilized from Dietary Intake and Adipose Tissue? 23.2 How Are Fatty Acids Broken Down? 23.3 How Are Odd-Carbon Fatty Acids Oxidized? 23.4 How Are Unsaturated Fatty Acids Oxidized? 23.5 Are There Other Ways to Oxidize Fatty Acids? 23.6 What Are Ketone Bodies, and What Role Do They Play in Metabolism? ESSENTIAL QUESTIONS Fatty acids represent the principal form of stored energy for many organisms.There are two important advantages to storing energy in the form of fatty acids. (1) The carbon in fatty acids (mostly OCH 2 O groups) is almost completely reduced compared to the carbon in other simple biomolecules (sugars, amino acids).Therefore, oxidation of fatty acids will yield more energy (in the form of ATP) than any other form of carbon. (2) Fatty acids are not generally as hydrated as monosaccharides and polysac- charides are, and thus they can pack more closely in storage tissues. How are fatty acids catabolized, and how is their inherent energy captured by organisms? Create your own study path for this chapter with tutorials, simulations, animations, and Active Figures at www.cengage.com/login. 698 Chapter 23 Fatty Acid Catabolism Energy Dry Weight Available Energy Constituent (kJ/g dry weight) (g) (kJ) Fat (adipose tissue) 37 15,000 555,000 Protein (muscle) 17 6,000 102,000 Glycogen (muscle) 16 120 1,920 Glycogen (liver) 16 70 1,120 Glucose (extracellular fluid) 16 20 320 Total 660,360 Sources: Owen, O. E.,and Reichard, G. A.,Jr., 1971.Fuels consumed by man:The interplay between carbohydrates and fatty acids. Progress in Biochemistry and Pharmacology 6:177; and Newsholme, E. A.,and Leech, A. R., 1983.Biochemistry for the Medical Sciences. New York:Wiley. TABLE 23.1 Stored Metabolic Fuel in a 70-kg Person FIGURE 23.1 Scanning electron micrograph of an adipose cell (fat cell). Globules of triacylglycerols occupy most of the volume of such cells. Prof. P. Motta, Dept. of Anatomy, University “La Sapienza,” Rome/Science Photo Library/ Photo Researchers, Inc. P P P Adenylyl cyclase Receptor cAMP Hormone Protein kinase (inactive) Protein kinase (active) Triacylglycerol lipase (inactive) Triacylglycerol lipase (active) Phosphatase P Triacylglycerol Diacylglycerol Monoacylglycerol Glycerol MAG lipase DAG lipase Fatty acid Fatty acid Adipose cell Plasma membrane Fatty acid ADP ATP ATP FIGURE 23.2 Liberation of fatty acids from triacylglycerols in adipose tissue is hormone-dependent. 23.1 How Are Fats Mobilized from Dietary Intake and Adipose Tissue? 699 O C H CH 2 H 2 C O O CO CO CO 12 3 O C H CH 2 H 2 C OH O CO CO CO 12 3 O – + O C H CH 2 H 2 C OH OH CO CO 12 3 O – + CO O – + O C H CH 2 H 2 C O OH CO CO 12 3 CO O – + Triacylglycerol Pancreatic lipase Pancreatic lipase Pancreatic lipase Diacylglycerol␤-Monoacylglycerol Diacylglycerol 2 fatty acid 2 CoA 2 fatty acyl CoA Monoacylglycerol Triacylglycerol Protein Chylomicrons ChylomicronsLymph duct Epithelial cells of intestinal wall Pancreatic duct Entry of pancreatic juice into duodenum Small intestine Large intestine Pancreas Stomach Duodenum Food containing triacylglycerols (b)(a) FIGURE 23.3 (a) The pancreatic duct secretes digestive fluids into the duodenum, the first portion of the small intestine. (b) Hydrolysis of triacylglycerols by pancreatic and intestinal lipases. Pancreatic lipases cleave fatty acids at the C-1 and C-3 positions. Resulting monoacylglycerols with fatty acids at C-2 are hydrolyzed by intestinal lipases. Fatty acids and monoacylglycerols are absorbed through the intestinal wall and assembled into lipoprotein aggregates termed chylomicrons (discussed in Chapter 24). 700 Chapter 23 Fatty Acid Catabolism adenylyl cyclase, which forms cyclic AMP from ATP. (Second messengers and hormonal signaling are discussed in Chapter 32.) In adipose cells, cAMP activates protein kinase A, which phosphorylates and activates a triacylglycerol lipase (also termed hormone-sensitive lipase) that hydrolyzes a fatty acid from C-1 or C-3 of triacylglycerols. Subsequent actions of diacylglycerol lipase and monoacylglycerol lipase yield fatty acids and glycerol. The cell then releases the fatty acids into the blood, where they are bound to serum albumin (the most abundant protein in blood serum). Serum albumin transports free fatty acids to sites of utilization. Degradation of Dietary Fatty Acids Occurs Primarily in the Duodenum Dietary triacylglycerols are degraded to a small extent (via fatty acid release) by lipases in the low-pH environment of the stomach, but mostly they pass untouched into the duodenum. Alkaline pancreatic juice secreted into the duodenum (Figure 23.3a) raises the pH of the digestive mixture, allowing hydrolysis of the triacylglycerols by pancreatic lipase and by nonspecific esterases, which hydrolyze the fatty acid ester linkages. Pancreatic lipase cleaves fatty acids from the C-1 and C-3 positions of triacylglycerols, and other lipases and esterases attack the C-2 position (Figure 23.3b). These processes depend upon the presence of bile salts, a family of car- boxylic acid salts with steroid backbones (see also Chapter 24). These agents act as detergents to emulsify the triacylglycerols and facilitate the hydrolytic activity of the lipases and esterases. Short-chain fatty acids (ten or fewer carbons) released in this way are absorbed directly into the villi of the intestinal mucosa, whereas long-chain fatty acids, which are less soluble, form mixed micelles with bile salts and are carried in this fashion to the surfaces of the epithelial cells that cover the villi (Figure 23.4). The fatty acids pass into the epithelial cells, where they are condensed with glycerol to form new triacylglycerols. These triacylglycerols aggregate with lipoproteins to form particles called chylomicrons, which are then transported into the lymphatic system and on to the bloodstream, where they circulate to the liver, lungs, heart, muscles, and other organs (see Chapter 24). At these sites, the triacylglycerols are microvilli or the lymphatic system, tissues throughout the body. FIGURE 23.4 In the small intestine, fatty acids combine with bile salts in mixed micelles, which deliver fatty acids to epithelial cells that cover the intestinal villi.Tri- acylglycerols are formed within the epithelial cells. 23.2 How Are Fatty Acids Broken Down? 701 hydrolyzed to release fatty acids, which can then be oxidized in a highly exergonic metabolic pathway known as ␤-oxidation. 23.2 How Are Fatty Acids Broken Down? Knoop Elucidated the Essential Feature of ␤-Oxidation The earliest clue to the secret of fatty acid oxidation and breakdown came in the early 1900s, when Franz Knoop carried out experiments in which he fed modified fatty acids to dogs. Knoop’s experiments showed that fatty acids must be degraded by oxidation at the ␤-carbon (Figure 23.5), followed by cleavage of the C ␣ OC ␤ bond. Repetition of this process yielded two-carbon units, which Knoop assumed must be acetate. Much later, Albert Lehninger showed that this degradative process took place in the mitochondria, and F. Lynen and E. Reichart showed that the two- carbon unit released is acetyl-CoA, not free acetate. Because the entire process begins with oxidation of the carbon that is “␤” to the carboxyl carbon, the process has come to be known as ␤-oxidation. In mammalian cells, ␤-oxidation take place primarily in mitochondria, but a similar pathway occurs in peroxisomes. In yeast and other lower eukaryotes, ␤-oxidation is confined exclusively to peroxisomes. Mitochondrial ␤-oxidation provides energy to the organism (Figure 23.6), whereas peroxisomal ␤-oxidation is responsible for H 3 C S CoA O C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C ␥␣ C H 2 ␤ H 2 C C H 3 C S CoA OO C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C ␥␣ C ␤ H 2 C C H 3 C S CoA + H 3 CCSCoA O O C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C H 2 H 2 C C [ FAD ] , NAD + [ FADH 2 ] , NADH + H + CoASH FIGURE 23.5 Fatty acids are degraded by repeated cycles of oxidation at the ␤-carbon and cleavage of the C ␣ OC ␤ bond to yield acetate units, in the form of acetyl-CoA. Each cycle of ␤-oxidation yields four electrons, captured as FADH 2 and NADH, which drive electron transport and oxidative phosphorylation pathways to produce ATP. COO – C + + O SCoA ++ + –32.3 +31.5 –0.8 = = = P P P P P P ATP ATP CoASH AMP AMP kJ mol kJ mol kJ mol ΔG ␱ ' for ΔG ␱ ' for acyl-CoA synthesis Net ΔG ␱ ' ΔG o ' = –33.6 H 2 O kJ mol FIGURE 23.6 The acyl-CoA synthetase reaction activates fatty acids for ␤-oxidation.The reaction is driven by hydrolysis of ATP to AMP and pyrophosphate and by the subsequent hydrolysis of pyrophosphate. 702 Chapter 23 Fatty Acid Catabolism shortening long-chain fatty acids that are poor substrates for mitochondrial ␤-oxidation. Such shortened fatty acids then become substrates for mitochondrial ␤-oxidation. Coenzyme A Activates Fatty Acids for Degradation The process of ␤-oxidation begins with the formation of a thiol ester bond between the fatty acid and the thiol group of coenzyme A. This reaction, shown in Figure 23.6, is catalyzed by acyl-CoA synthetase, which is also called acyl-CoA ligase or fatty acid thiokinase. This condensation with CoA activates the fatty acid for reaction in the ␤-oxidation pathway. For long-chain fatty acids, this reaction normally occurs at the outer mitochondrial membrane in higher eukaryotes before entry of the fatty acid into the mitochondrion, but it may also occur at the surface of the endoplas- mic reticulum. Short- and medium-length fatty acids undergo this activating reaction in the mitochondria. In all cases, the reaction is accompanied by the hydrolysis of ATP to form AMP and pyrophosphate. As shown in Figure 23.6, the overall reaction has a net ⌬G°Ј of about Ϫ0.8 kJ/mol, so the reaction is favorable but eas- ily reversible. However, there is more to the story. As we have seen in several similar cases, the pyrophosphate produced in this reaction is rapidly hydrolyzed by inor- ganic pyrophosphatase to two molecules of phosphate, with a net ⌬G°Ј of about Ϫ33.6 kJ/mol. Thus, pyrophosphate is maintained at a low concentration in the cell (usually less than 10 ␮M), and the synthetase reaction is strongly promoted. The mechanism of the acyl-CoA synthetase reaction is shown in Figure 23.7 and involves attack of the fatty acid carboxylate on ATP to form an acyladenylate intermediate, which is subsequently attacked by CoA, forming a fatty acyl-CoA thioester. Carnitine Carries Fatty Acyl Groups Across the Inner Mitochondrial Membrane All of the other enzymes of the ␤-oxidation pathway are located in the mitochondrial matrix. Short-chain fatty acids, as already mentioned, are transported into the matrix as free acids and form the acyl-CoA derivatives there. However, long-chain fatty acyl-CoA derivatives cannot be transported into the matrix directly. These long- OO O – O – P O – O O O – P O O – P O Adenosine O – C O O O – P O – O O – P O C O R O P O O – O Adenosine H S O P O O – O – O O AdenosineC S R C R S + – OP O O – O Adenosine CoA CoA CoA Fatty acid Pyrophosphate Enzyme-bound acyl-adenylate intermediate Fatty acyl-CoA AMP ATP Transient tetrahedral intermediate ANIMATED FIGURE 23.7 The mechanism of the acyl-CoA synthetase reaction involves fatty acid carboxylate attack on ATP to form an acyl-adenylate intermediate.The fatty acyl CoA thioester product is formed by CoA attack on this intermediate. See this figure animated at www.cengage.com/login. . (from Bristol-Myers Squibb) for treatment of type 2 diabetes. It consists of metformin (see Human Biochemistry, page 668) together with glipizide. The actions of metformin and glipizide are said

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