22.3 How Are Glycogen and Starch Catabolized in Animals? 673 hibition by fructose-2,6-bisphosphate, whereas phosphofructokinase is allosterically activated by fructose-2,6-bisphosphate. The combination of these effects should per- mit either phosphofructokinase or fructose-1,6-bisphosphatase (but not both) to operate at any one time and should thus prevent futile cycling. For instance, in the fasting state, when food (that is, glucose) intake is zero, phosphofructokinase (and therefore glycolysis) is inactive due to the low concentration of fructose-2,6- bisphosphate. In the liver, gluconeogenesis operates to provide glucose for the brain. However, in the fed state, up to 30% of fructose-1,6-bisphosphate formed from phosphofructokinase is recycled back to fructose-6-P (and then to glucose). Because the dependence of fructose-1,6-bisphosphatase activity on fructose-1,6- bisphosphate is sigmoidal in the presence of fructose-2,6-bisphosphate (see Figure 22.9), substrate cycling occurs only at relatively high levels of fructose-1,6- bisphosphate. Substrate cycling in this case prevents the accumulation of excessively high levels of fructose-1,6-bisphosphate. 22.3 How Are Glycogen and Starch Catabolized in Animals? Dietary Starch Breakdown Provides Metabolic Energy As noted earlier, well-fed adult human beings normally metabolize about 160 grams of carbohydrates each day. A balanced diet easily provides this amount, mostly in the form of starch. If too little carbohydrate is supplied by the diet, glycogen reserves in liver and muscle tissue can also be mobilized. The reactions by which ingested starch and glycogen are digested are shown in Figure 22.11. The enzyme known as ␣-amy- lase is an important component of saliva and pancreatic juice. (-Amylase is found in plants. The ␣- and -designations for these enzymes serve only to distinguish the two and do not refer to glycosidic linkage nomenclature.) ␣-Amylase is an endogly- cosidase that hydrolyzes ␣(1⎯→4) linkages of amylopectin and glycogen at random positions, eventually producing a mixture of maltose, maltotriose [with three ␣(1⎯→4)-linked glucose residues], and other small oligosaccharides. ␣-Amylase can ␣-Amylase -Amylase ␣-(1 6)-glucosidase (a) (b) FIGURE 22.11 (a) The sites of hydrolysis of starch by ␣- and -amylase are indicated. (b) Glycogenin is a glycosyltransferase that initiates eukaryotic glycogen synthesis from glucose. Bound UDP-glucose (blue) and Mn 2ϩ ion (purple) are shown (pdb id ϭ 1LL2). 674 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway cleave on either side of an amylopectin branch point, but activity is reduced in highly branched regions of the polysaccharide and stops four residues from any branch point. The highly branched polysaccharides that are left after extensive exposure to ␣-amylase are called limit dextrins. These structures can be further degraded by the action of a debranching enzyme, which carries out two distinct reactions. The first of these, known as oligo(␣1,4→␣1,4) glucanotransferase activity, removes a trisaccha- ride unit and transfers this group to the end of another, nearby branch (Figure 22.12). This leaves a single glucose residue in ␣(1⎯→6) linkage to the main chain. The ␣(1⎯→6) glucosidase activity of the debranching enzyme then cleaves this residue from the chain, leaving a polysaccharide chain with one branch fewer. Repetition of this sequence of events leads to complete degradation of the polysaccharide. -Amylase is an exoglycosidase that cleaves maltose units from the free, nonre- ducing ends of amylopectin branches, as in Figure 22.11. Like ␣-amylase, however, -amylase does not cleave either the ␣(1⎯→6) bonds at the branch points or the ␣(1⎯→4) linkages near the branch points. Metabolism of Tissue Glycogen Is Regulated Digestion itself is a highly efficient process in which almost 100% of ingested 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 it should be no surprise that the reactions involved in its degradation and synthesis are carefully controlled and regulated. Glycogen re- serves 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 that was discussed extensively in Chapter 15. The glycogen phos- phorylase reaction (Figure 22.13) involves phosphorolysis at a nonreducing end of a glycogen polymer. The standard-state free energy change for this reaction is O O O O O O O O O O O O O O HO O O O O O O O O O O O O O O O O O O O O O O O O O O O O HO O O HO HO Limit branch Limit dextrin Debranching enzyme ␣(1 6)-glucosidase activity of debranching enzyme cleaves this residue Further cleavage by ␣-amylase FIGURE 22.12 The reactions of debranching enzyme. Transfer of a group of three ␣(1⎯→4)-linked glucose residues from a limit branch to another branch is fol- lowed by cleavage of the ␣(1⎯→6) bond of the residue that remains at the branch point. 22.4 How Is Glycogen Synthesized? 675 ϩ3.1 kJ/mol, but the intracellular ratio of [P i ] to [glucose-1-P] approaches 100, and thus the actual ⌬G in vivo is approximately Ϫ6 kJ/mol. There is an energetic ad- vantage to the cell in this phosphorolysis reaction. If glycogen breakdown were hy- drolytic and yielded glucose as a product, it would be necessary to phosphorylate the product glucose (with the expenditure of a molecule of ATP) to initiate its gly- colytic degradation. The glycogen phosphorylase reaction degrades glycogen to produce limit dex- trins, which are further degraded by debranching enzyme, as already described. 22.4 How Is Glycogen Synthesized? Animals synthesize and store glycogen when glucose levels are high, but the syn- thetic pathway is not merely a reversal of the glycogen phosphorylase reaction. High levels of phosphate in the cell favor glycogen breakdown and prevent the phos- phorylase reaction from synthesizing glycogen in vivo, despite the fact that ⌬G°Ј for the phosphorylase reaction actually favors glycogen synthesis. Hence, another reac- tion pathway must be employed in the cell for the net synthesis of glycogen. In essence, this pathway must activate glucose units for transfer to glycogen chains. Glucose Units Are Activated for Transfer by Formation of Sugar Nucleotides We are familiar with several examples of chemical activation as a strategy for group transfer reactions. Acetyl-CoA is an activated form of acetate; biotin and tetrahydro- folate activate one-carbon groups for transfer; and ATP is an activated form of phos- phate. Luis Leloir, a biochemist in Argentina, showed in the 1950s that glycogen syn- thesis depended upon sugar nucleotides, which may be thought of as activated forms of sugar units. For example, formation of an ester linkage between the C-1 hydroxyl group and the -phosphate of UDP activates the glucose moiety of UDP–glucose. HOCH 2 O HH H H HO OH OH O H P O O O – P O O O – CH 2 O H HO OH H HH N O HN O Uridine diphosphate glucose (UDPG) CH 2 OH O O CH 2 OH O O CH 2 OH O O CH 2 OH O O + CH 2 OH O CH 2 OH O O CH 2 OH O O CH 2 OH O OOPO 3 2 – P OH HO HO OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH HO Nonreducing end residues n n – 1 ␣-D-Glucose-1-phosphate residues FIGURE 22.13 The glycogen phosphorylase reaction. Uridine diphosphate glucose (UDP–glucose). 676 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway UDP–Glucose Synthesis Is Driven by Pyrophosphate Hydrolysis Sugar nucleotides are formed from sugar-1-phosphates and nucleoside triphos- phates by specific pyrophosphorylase enzymes (Figure 22.14). For example, UDP– glucose pyrophosphorylase catalyzes the formation of UDP–glucose from glucose- 1-phosphate and uridine 5Ј-triphosphate: Glucose-1-P ϩ UTP ⎯⎯→ UDP–glucose ϩ pyrophosphate The reaction proceeds via attack by a phosphate oxygen of glucose-1-phosphate on the ␣-phosphorus of UTP, with departure of the pyrophosphate anion. The reaction is a reversible one, but—as is the case for many biosynthetic reactions—it is driven forward by subsequent hydrolysis of pyrophosphate: Pyrophosphate ϩ H 2 O ⎯⎯→ 2 P i The net reaction for sugar nucleotide formation (combining the preceding two equations) is thus Glucose-1-P ϩ UTP ϩ H 2 O ⎯⎯→ UDP–glucose ϩ 2 P i Sugar nucleotides of this type act as donors of sugar units in the biosynthesis of oligosaccharides and polysaccharides. In animals, UDP–glucose is the donor of glu- cose units for glycogen synthesis, but ADP–glucose is the glucose source for starch synthesis in plants. Glycogen Synthase Catalyzes Formation of ␣(1⎯→4) Glycosidic Bonds in Glycogen The very large glycogen polymer is built around a tiny protein core. The first glu- cose residue is covalently joined to the protein glycogenin (see Figure 22.11b) via an acetal linkage to a tyrosine–OH group on the protein. Sugar units can then be added by the action of glycogen synthase. The reaction involves transfer of a glu- P P CH 2 OH O OH HO HO OP O – O O – – OPO O – O P O O – O P OCH 2 O – O O H HO OH H HH N HN O O P 2 CH 2 OH O OH HO HO OP O O – O P OCH 2 O – O O H HO OH H HH N HN O O UTP Glucose-1-P UDP–glucose pyrophosphorylase UDP–glucose ANIMATED FIGURE 22.14 The UDP– glucose pyrophosphorylase reaction is a phospho- anhydride exchange, with a phosphoryl oxygen of glucose-1-P attacking the ␣-phosphorus of UTP to form UDP–glucose and pyrophosphate. See this figure ani- mated at www.cengage.com/login. Glycogen synthase from Agrobacterium tumefaciens consists of two Rossman folds (see Chapter 16) sepa- rated by a deep cleft that includes the active site (shown here with bound ADP,purple) (pdb id ϭ 1RZU). 22.4 How Is Glycogen Synthesized? 677 cosyl 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 glu- cose unit on glycogen (Figure 22.15). The reaction is exergonic and has a ⌬G°Ј of Ϫ13.3 kJ/mol. Glycogen Branching Occurs by Transfer of Terminal Chain Segments Glycogen is a branched polymer of glucose units. The branches arise from ␣(1⎯→6) linkages, which occur every 8 to 12 residues. As noted in Chapter 7, the branches provide multiple sites for rapid degradation or elongation of the poly- mer and also increase its solubility. Glycog en branches are formed by amylo- HUMAN BIOCHEMISTRY Advanced Glycation End Products—A Serious Complication of Diabetes Covalent linkage of sugars to proteins to form glycoproteins nor- mally occurs through the action of enzymes that use sugar nucleo- tides as substrates. However, sugars may also react nonenzymati- cally with proteins. The C-1 carbonyl group of glucose forms Schiff base linkages with lysine side chains of proteins. These Schiff base adducts undergo Amadori rearrangements and subsequent oxida- tions to form irreversible “glycation” products, including carboxy- methyllysine and pentosidine derivatives (see accompanying fig- ure). These advanced glycation end products (AGEs) can alter the function of the protein. Such AGE-dependent changes are thought to contribute to circulation, joint, and vision problems in people with diabetes. Nonenzymatic glycation of hemoglobin is a better diagnostic yardstick for type-2 diabetes than serum glucose levels. Red blood cells have an average life expectancy of about 4 months. By mea- suring the concentration of “glycated hemoglobin” in a patient, it is possible to determine the average glucose concentration in the blood over the past several months. CH 2 OH (CH 2 ) 4 (CH 2 ) 3 CH H CHO OH C N O NH 2 HOH C HOH C CH HHO C CH 2 OH HOH C H C HOH C HOH C HHO C Protein+ + + Protein N CH 2 OH CH 2 O C H HOH C HOH C HHO C Protein H N NNN H O C HN C N (CH 2 ) 4 NH N HO H C C CH 2 COO – Rearrangement leads to irreversibly glycated proteins Schiff base Pentosidine Carboxymethyllysine Other advanced glycation end products 678 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway (1,4⎯→1,6)-transglycosylase, also known as branching enzyme. The reaction involves the transfer of a 6- or 7-residue segment from the nonreducing end of a linear chain at least 11 residues in length to the C-6 hydroxyl of a glucose residue of the same chain or another chain (Figure 22.16). For each branching reaction, the resulting polymer has gained a new terminus at which growth can occur. 22.5 How Is Glycogen Metabolism Controlled? Glycogen Metabolism Is Highly Regulated Synthesis and degradation of glycogen must be carefully controlled so that this im- portant energy reservoir can properly serve the metabolic needs of the organism. Glu- cose is the principal metabolic fuel for the brain, and the concentration of glucose in circulating blood must be maintained at about 5 mM for this purpose. Glucose derived from glycogen breakdown is also a primary energy source for muscle contraction. Con- trol of glycogen metabolism is effected via reciprocal regulation of glycogen phospho- rylase and glycogen synthase. Thus, activation of glycogen phosphorylase is tightly linked to inhibition of glycogen synthase, and vice versa. Regulation involves both al- losteric control and covalent modification, with the latter being under hormonal con- trol. The regulation of glycogen phosphorylase is discussed in detail in Chapter 15. Glycogen Synthase Is Regulated by Covalent Modification Glycogen synthase also exists in two distinct forms that can be interconverted by the action of specific enzymes: active, dephosphorylated glycogen synthase I (glucose- 6-P–independent) and less active, phosphorylated glycogen synthase D (glucose-6- H + O OH HO CH 2 OH HO O P O O – O P O CH 2 O – O O N O O HN HH OH OH HH O OH O CH 2 OH HO O O OH O CH 2 OH HO O OH O CH 2 OH HO O OH HO CH 2 OH HO O OH O CH 2 OH HO O O OH O CH 2 OH HO O OH HO CH 2 OH HO O OH HO CH 2 OH HO + UDP–glucose UDP Glycogen (n residues) Oxonium ion intermediate Glycogen (n + 1 residues) ANIMATED FIGURE 22.15 The glycogen synthase reaction. Cleavage of the COO bond of UDP–glucose yields an oxonium intermediate. Attack by the hydroxyl oxygen of the terminal residue of a glycogen molecule completes the reaction. See this figure animated at www.cengage.com/login. (1 4)-terminal chains of glycogen Branching enzyme cuts here and transfers a seven-residue terminal segment to a C(6)–OH group FIGURE 22.16 Formation of glycogen branches by the branching enzyme. Six- or seven-residue segments of a growing glycogen chain are transferred to the C-6 hydroxyl group of a glucose residue on the same or a nearby chain. 22.5 How Is Glycogen Metabolism Controlled? 679 P–dependent). The phosphorylated form can be allosterically activated by glucose-6- phosphate, but the unphosphorylated enzyme is insensitive to this allosteric effector (Figure 22.17). The nature of phosphorylation is complex (Figure 22.17a). At least nine serine residues on the enzyme appear to be subject to phosphorylation, each site’s phosphorylation having some effect on enzyme activity. Four protein kinases are involved in phosphorylation of glycogen synthase: casein kinase, AMP-dependent protein kinase, protein kinase A, and glycogen synthase kinase 3 (GSK3). Dephosphorylation of both glycogen phosphorylase and glycogen synthase is car- ried out by phosphoprotein phosphatase-1 (PP1). The action of PP1 inactivates glycogen phosphorylase and activates glycogen synthase. Insulin receptor Insulin (a) Glucose Glucose Glucose-6-P Allosteric activation ATP ADP P P P P P Plasma membrane GLUT4 vesicle Protein kinase cascade Protein kinase cascade GSK3 (inactive) GSK3 (active) Glycogen synthase I (active) Glycogen synthase D (inactive) PP-1 + + ++ + (b) Protein phosphorylation and second messenger modulation GluconeogenesisActive transport Glycogen synthesis Lipid synthesis Lipid breakdown Glycolysis Protein synthesis Insulin receptor Insulin FIGURE 22.17 (a) Binding of insulin to plasma mem- brane receptors in the liver and muscles triggers protein kinase cascades that stimulate glycogen synthesis. Insulin’s effects include inactivation of GSK3 and stimu- lation of PP1, both actions activating glycogen synthase, as well as recruitment of GLUT4 to the plasma mem- brane. Glucose uptake provides substrate for glycogen synthesis and glucose-6-phosphate, which allosterically activates the otherwise inactive form of glycogen synthase. (b) The metabolic effects of insulin are medi- ated through protein phosphorylation and second mes- senger modulation. 680 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway Hormones Regulate Glycogen Synthesis and Degradation Storage and utilization of tissue glycogen, maintenance of blood glucose concentra- tion, and other aspects of carbohydrate metabolism are meticulously regulated by hor- mones, including insulin, glucagon, epinephrine, and the glucocorticoids. Insulin Is a Response to Increased Blood Glucose The primary hormone responsi- ble for conversion of glucose to glycogen is insulin (see Figure 5.8). Insulin is secreted by the -cells in the pancreas within the islets of Langerhans. Secretion of insulin is a re- sponse to increased glucose in the blood. When blood glucose levels rise (after a meal, for example), insulin is secreted from the pancreas into the pancreatic vein, which emp- ties into the portal vein system (Figure 22.18), so insulin traverses the liver before it enters the systemic blood supply. Insulin acts to rapidly lower blood glucose concen- tration in several ways. Insulin stimulates glycogen synthesis and inhibits glycogen breakdown in liver and muscle. Insulin Triggers Glycogen Synthesis When Blood Glucose Rises The action of insulin when blood glucose rises is immediate and powerful. During periods be- tween meals, typical human blood glucose levels are 70 to 90 mg/dL. Glucose lev- els normally rise to about 150 mg/dL within the first hour following a carbohydrate- rich meal (Figure 22.19) and then return to normal within 2 to 3 hours. (For diabetic subjects, whose insulin response is impaired, glucose levels rise after a meal to 250 mg/dL or even higher and remain high for much longer times.) A DEEPER LOOK Carbohydrate Utilization in Exercise Animals have a remarkable ability to “shift gears” metabolically dur- ing periods of strenuous exercise or activity. Metabolic adaptations allow the body to draw on different sources of energy (all of which produce ATP) for different types of activity. During periods of short-term, high-intensity exercise (such as a 100-m dash), most of the required energy is supplied directly by existing stores of ATP and creatine phosphate (see figure, part a). Long-term, low- intensity exercise (such as a 10-km run or a 42.2-km marathon) is fueled almost entirely by aerobic metabolism. Between these ex- tremes is a variety of activities (an 800-m run, for example) that rely on anaerobic glycolysis—conversion of glucose to lactate in the muscles and utilization of the Cori cycle. For all these activities, breakdown of muscle glycogen provides much of the needed glucose. The rate of glycogen consumption depends on the intensity of the exercise (see figure, part b). By contrast, glucose derived from gluconeogenesis makes only small contributions to total glucose consumed during exercise. During prolonged mild exercise, gluconeogenesis accounts for only about 8% of the total glucose consumed. During heavy exercise, this percentage becomes even lower. Choice of diet has a dramatic effect on glycogen recovery fol- lowing exhaustive exercise. A diet consisting mainly of protein and fat results in very little recovery of muscle glycogen, even af- ter 5 days (see figure, part c). On the other hand, a high-carbo- hydrate diet provides faster restoration of muscle glycogen. Even in this case, however, complete recovery of glycogen stores takes about 2 days. 100 % of total energy 75 50 25 0 (a) 0 Duration of work (sec) 30 60 90 120 from phosphocreatine Anaerobic metabolism Aerobic metabolism from ATP 24 Muscle glycogen content (grams/kg of muscle) 20 16 12 0 (c) 0 Hours of recovery 10 30 40 5 days 8 4 20 50 2 hours of exercise High-carbohydrate diet No food Fat & protein diet Light exercise 100 % glycogen content 75 50 25 0 (b) 0 Exercise time (min) 30 60 90 120 Moderate exercise Heavy exercise ᮡ (a) Contributions of the various energy sources to muscle activity during mild exercise. (b) Con- sumption of glycogen stores in fast-twitch muscles during light, moderate, and heavy exercise. (c) Rate of glycogen replenishment following exhaustive exercise. (a and c adapted from Rhodes, R., and Pflanzer, R. G., 1992. Human Physiology. Philadelphia: Saunders College Publishing; b adapted from Horton, E. S., and Terjung, R. L., 1988. Exercise, Nutrition and Energy Metabolism. New York: Macmillan.) Liver Spleen Splenic vein Pancreatic veins Pancreas Portal vein FIGURE 22.18 The portal vein system carries pancreatic secretions such as insulin and glucagon to the liver and then into the rest of the circulatory system. 22.5 How Is Glycogen Metabolism Controlled? 681 Insulin lowers blood glucose by triggering several cascades of reactions that result in glucose uptake and glycogen synthesis (see Figure 22.17a). An insulin- triggered protein kinase cascade increases glucose transport into muscle, liver, and adipose tissues by stimulating exocytotic processes that translocate GLUT4, a glu- cose transporter, from intracellular vesicles to the plasma membrane (see Figure 22.17a). Large amounts of glucose thus transported into the cell are converted to glucose-6-P, which can be directed to glycogen synthesis (by conversion to glucose- 1-P). Also, glucose-6-P is the allosteric effector that activates the otherwise inactive, phosphorylated form of glycogen synthase. Binding of insulin to the plasma membrane, in either liver or muscle cells, trig- gers another protein kinase cascade (see Figure 15.17 and Chapter 32) that results in phosphorylation and inactivation of glycogen synthase kinase 3 (GSK3). This ki- nase normally phosphorylates and inactivates glycogen synthase. Inhibition of GSK3 means that more of the cell’s glycogen synthase will remain in the unphosphory- lated, active state (see Figure 22.17a). Insulin also stimulates PP1, which dephos- phorylates (and activates) glycogen synthase. Several other physiological effects of insulin also serve to lower blood and tissue glucose levels (see Figure 22.17b). Insulin increases cellular utilization of glucose by inducing the synthesis of several important glycolytic enzymes, namely, glucokinase, phosphofructokinase, and pyruvate kinase. In addition, insulin acts to inhibit sev- eral enzymes of g luconeogenesis. These various actions enable the organism to re- spond quickly to increases in blood glucose levels. Glucagon and Epinephrine Stimulate Glycogen Breakdown Catabolism of tissue glycogen is triggered by the actions of the hormones epinephrine and glucagon (Fig- ure 22.20). In response to decreased blood glucose, glucagon is released from the ␣-cells in pancreatic islets of Langerhans. This peptide hormone travels through the blood to specific receptors on liver cell membranes. (Glucagon acts on liver and adipose tissue but not other tissues.) Similarly, signals from the central nervous system cause release of epinephrine—also known as adrenaline—from the adrenal glands into the bloodstream. Epinephrine acts on liver and muscles. When either hormone binds to its receptor on the outside surface of the cell membrane, a cascade is initiated that activates glycogen phosphorylase and inhibits glycogen synthase (Figure 22.20). The result of these actions is tightly coordinated stimulation of glycogen breakdown and inhibi- tion of glycogen synthesis. The Phosphorylase Cascade Amplifies the Hormonal Signal Stimulation of glyco- gen breakdown involves consumption of molecules of ATP at three different steps in the hormone-sensitive adenylyl cyclase cascade (see Figure 15.17). Note that the cas- cade mechanism is a means of chemical amplification, because the binding of just a 300 Normal Diabetic 250 0 0 100 200 50 100 150 200 Blood glucose Time, minutes FIGURE 22.19 A glucose tolerance test involves inges- tion of a glucose solution followed by measurements of blood glucose for about 3 hours. Normal subjects exhibit a rise in blood glucose to about 150 mg/dL, followed by a decline to normal values over a 3-hour period. In diabetic subjects, blood glucose rises to higher values and remains high for longer periods. HUMAN BIOCHEMISTRY von Gierke Disease—A Glycogen-Storage Disease In 1929, the physician Edgar von Gierke treated a patient with a very enlarged abdomen. The patient’s liver and kidneys were severely enlarged due to massive accumulations of glycogen, and von Gierke appropriately called the condition “hepato-nephromegalia glyco- genica.” Now termed von Gierke’s disease, or Type Ia glycogen stor- age disease, this condition results from the absence of glucose-6- phosphatase activity in the affected organs. This simple genetic defect causes a host of difficult complications, including a striking el- evation of serum triglycerides, excess adipose tissue in the cheeks, thin extremities, short stature, excessive curvature of the lumbar spine, and delay of puberty. The absence of glucose-6-phosphatase activity in the liver blocks the last steps of glycogenolysis and gluconeogenesis, inter- rupting the recycling of glucose and causing affected individuals to be hypoglycemic. The accumulation of glucose-6-phosphate in the liver leads to greatly increased glycolytic activity, with conse- quent elevation of lactic acid, a condition known more commonly as lactic acidosis. Large amounts of uric acid and lipids are pro- duced, and the high rates of glycolysis produce excess NADH. The treatment of von Gierke’s disease consists of trying to maintain normal levels of glucose in the patient’s serum. This of- ten requires oral administration of large amounts of glucose, in its various forms, including, for example, uncooked cornstarch, which acts as a slow-release form of glucose. 682 Chapter 22 Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway few molecules of epinephrine or glucagon results in the synthesis of many molecules of cyclic AMP, which, through the action of cAMP-dependent protein kinase, can ac- tivate many more molecules of phosphorylase kinase and even more molecules of phosphorylase. For example, an extracellular level of 10 Ϫ10 to 10 Ϫ8 M epinephrine prompts the formation of 10 Ϫ6 M cyclic AMP, and for each protein kinase activated by cyclic AMP, approximately 30 phosphorylase kinase molecules are activated; these in OH OH C HHO CH 2 NH 2 + CH 3 Liver Heart muscle Skeletal muscle Epinephrine + Adenylyl cyclase cAMP ↑ + cAMP-dependent protein kinase (PKA) PFK-2 F-2,6-BPase + PFK-1 F-1,6-BPase + Gluconeogenesis + Blood glucose + F-2,6-BP ↓ + Phosphorylase kinase + Glycogen breakdown Glycogen synthesis Glycolysis PFK-1 + PFK-2 Heart Skeletal muscle F-2,6-BPase + PFK-1 + Glycolysis + Glycolysis + F-2,6-BP ↑ + Glycogen phosphorylase + GSK3 PFK-1 association with actin Glucagon FIGURE 22.20 Glucagon and epinephrine each activate a cascade of reactions that stimulate glycogen break- down and inhibit glycogen synthesis in liver and muscles,respectively.The effects of these hormones on other metabolic pathways depend on the tissue. In liver, glucagon inhibits glycolysis and stimulates gluconeogenesis, facilitating export of glucose into the bloodstream. In muscles, epinephrine stimulates glycolysis to provide energy for contraction.These effects all depend on protein phosphorylations by cAMP-dependent protein kinase. Note that the liver and heart isoforms of PFK-2/F-2,6-BPase respond oppositely to phosphorylation by PKA. Glucagon is a 29-residue peptide with the sequence H 3 ϩ N-HSEGTFTSDYSKYLDSRRAQDFVQWLMNT-COO Ϫ .