Carbohydrate Metabolism 2 Supplemental Reading

Transcription

1 Carbohydrate Metabolism 2 Supplemental Reading Key Concepts - Overview of glycogen metabolism - Biochemistry and regulation glycogen degradation - Biochemistry and regulation of glycogen synthesis - Control of glycogen metabolism by intracellular signaling - Human glycogen storage diseases Figure 1. Key Question about Glycogen Metabolism: What mechanisms regulate the activity of muscle and liver glycogen phosphorylase? How do insulin and glucagon mediate recipricol reglation of glycogen metabolism? Overview of glycogen metabolism The storage form of glucose in most eukaryotic cells (except plants) is glycogen, a large highly branched polysaccharide consisting of glucose units joined by α-1,4 and α-1,6 glycosidic bonds (figure 1). Both the liver and muscle store glycogen and hence have the necessary anabolic and catabolic enzymes.glycogen degradation and synthesis occurs in the cytosol and the substrate for these reactions is the free ends of the branched polymer (nonreducing ends). The large number of branch points in glycogen results in the generation of multiple nonreducing ends that provide a highly efficient mechanism to quickly release and store glucose. The three key enzymes required for reversible degradation and synthesis of glycogen are glycogen phosphorylase, glycogen synthase and the branching/debranching enzymes. Glycogen phosphorylase and glycogen synthase modify glycogen at the nonreducing ends, whereas, the branching and debranching enzymes modify glycogen at the α-1,4 and α-1,6 glycosidic bonds (figure 2). Glycogen phosphorylase releases glucose-1- phosphate (glucose- 1P) from glycogen in a phosphorolysis reaction involving inorganic phosphate (P i ) and cleavage of the α-1,4 glycosidic bond. The glucose-1p molecules are Figure 2. converted to glucose-6-phosphate (glucose-6p), which is either used for glycolysis in muscle cells, or is dephosphorylated in liver cells so that glucose can be exported to other tissues. Glycogen synthase is responsible for adding glucose to the nonreducing ends in a reaction involving uridine diphosphate glucose (UDP-glucose). Glycogen synthase uses the bond energy available in UDP-glucose to form α-1,4 glycosidic bonds at the growing nonreducing ends. ATP hydrolysis is required to regenerate UTP for subsequent rounds of glucose addition, and therefore, glycogen synthesis requires 1 ATP/glucose residue added. As first presented in lecture 22 in the context of signal transduction, hormone signaling through glucagon, epinephrine and 1 of 12 pages

2 insulin results in reversible phosphorylation of glycogen phosphorylase and glycogen synthase. As described later, the activities of glycogen phosphorylase and glycogen synthase can also be controlled by allosteric mechanisms in response to the metabolic state of the cell. Lastly, the glycogen branching and debranching enzymes ensure that glycogen phosphorylase and glycogen synthase have access to the maximum number of non-reducing ends for the cleavage and formation of α-1,4 glycosidic bonds. Although small amounts of glycogen are synthesized in many animal cell types, it is only the liver and skeletal muscle that accumulate large amounts of glycogen. Glycogen core complexes consist of glycogenin protein and ~50,000 glucose molecules with α-1,6 branches about every 10 residues creating ~2,000 nonreducing ends. Twenty to forty glycogen core complexes associate inside liver and muscle cells to form glycogen particles containing over a million glucose molecules. These glycogen particles can be visualized by electron microscopy and account for up to 10% by weight of liver tissue (figure 3). Importantly, the physiological roles of liver and muscle glycogen are quite different. Liver glycogen is used as a source of glucose for export to other tissues when dietary glucose is limiting (between meals), whereas, the sole purpose of glycogen in muscle cells is to generate glucose-6p for use as a chemical energy source in anaerobic and aerobic Figure 3. glycolysis. Since muscle cells do not contain the enzyme glucose-6-phosphatase, all of the glucose-6p that is made available from glycogen degradation stays inside the cell. 1. What purpose does glycogen metabolism serve in animals? Liver glycogen is used as a short term energy source for the organism by providing a means to store and release glucose in response to blood glucose levels; liver cells do not use this glucose for their own energy needs (fatty acids provide chemical energy to liver cells) Muscle glycogen provides a readily available source of glucose during exercise to support anaerobic and aerobic energy conversion pathways within muscle cells; muscle cells lack the enzyme glucose-6-phosphatase and therefore cannot release glucose into the blood. 2. What are the net reactions of glycogen degradation and synthesis? Glycogen degradation: Glycogen n units of glucose + P i --> Glycogen n-1 units of glucose + glucose-6-phosphate Glycogen Synthesis: Glycogen n units of glucose + glucose-6-phosphate + ATP + H 2 O--> Glycogen n+1 units of glucose + ADP + 2P i 3. What are the key enzymes in glycogen metabolism? Glycogen phosphorylase enzyme catalyzing the phosphorolysis reaction that uses P i to remove one glucose at a time from nonreducing ends of glycogen resulting in the formation of glucose-1p. Liver and muscle glycogen phosphorylase are isozymes (two different genes) that are both activated by phosphorylation but have distinct responses to allosteric effectors. Glycogen synthase - enzyme catalyzing the addition of glucose residues to nonreducing ends of glycogen using UDP-glucose as the glucose donor. Glycogen synthase activity is inhibited by phosphorylation; binding of the allosteric activators glucose or glucose-6p promotes dephosphorylation and enzyme activation. 2 of 12 pages

3 Branching and debranching enzymes - these two enzymes are responsible for adding (branching) and removing (debranching) glucose residues to the glycogen complex through the cleavage and formation of α-1,6 glycosidic bonds. 4. What are examples of glycogen metabolism in real life? The performance of elite endurance athletes can benefit from a diet regimen of carbohydrate "loading" prior to competition. Recent studies indicate that a short period of intense exercise to deplete muscle glycogen stores, followed by ingestion of 10 g/kg body mass of high-carbohydrate foods, can lead to a doubling of muscle glycogen in 24 hours. Carbohydrate loading regimens can result in a build-up of stored muscle glycogen that is sometimes higher than what can be obtained by simply following a high carbohydrate diet. Biochemistry and regulation of glycogen Figure 4. degradation Glycogen degradation is initiated by glycogen phosphorylase, a homodimer that catalyzes a phosphorolysis cleavage reaction of the α-1,4 glycosidic bond at the nonreducing ends of the glycogen molecule. Inorganic phosphate (P i ) attacks the glycosidic oxygen using an acid catalysis mechanism that releases glucose-1p as the product (figure 4). Glycogen phosphorylase is a processive enzyme which means it stays Figure 5. attached to the glycogen substrate and continues to cleave α-1,4 glycosidic bonds and release glucose-1p until the enzyme gets too close to an α-1,6 branch point. A required cofactor for the glycogen phosphorylase reaction is pyridoxal phosphate (derived from vitamin B6) which is covalently bound to the enzyme through a lysine residue. Although the standard free energy change for this phosphorolysis reaction is positive (ΔGº' = +3.1 kj/mol), making the reaction unfavorable, the actual change in free energy is favorable (ΔG = -6 kj/mol) due to the high concentration of P i relative to glucose-1p inside the cell (ratio of close to 100). The structure of a glycogen Figure 6 phosphorylase dimer in figure 5 shows binding sites for glycogen and catalytic sites that contain pyridoxal phosphate. The critical P i substrate is bound to the active site by interactions with pyridoxal phosphate and active site amino acids. The glucose-1p product of the glycogen phosphorylase reaction is 3 of 12 pages

4 not an intermediate in glycolysis and liver cells do not contain a glucose-1p-phosphatase. Therefore, the next reaction in the glycogen degradation pathway is the conversion of glucose-1p to glucose-6p by the enzyme phosphoglucomutase. As shown in figure 6, this reaction is similar to the phosphoglycerate mutase reaction in glycolysis in that the enzyme first donates a phosphate group to the substrate to generate an intermediate bisphosphate compound, and then the bisphosphate compound is dephosphorylated to regenerate the phosphoenzyme and release the product. In liver cells, glucose-6p is dephosphorylated by glucose-6-phosphatase to generate glucose for export, whereas in muscle cells which lack glucose-6-phosphatase, the glucose-6p is used as a source of chemical energy in glycolysis. Glycogen phosphorylase removes glucose units from the nonreducing end until it reaches within four glucose units of an α-1,6 branchpoint. As shown in figure 7, the glycogen debranching enzyme (also called α-1,6-glucosidase) recognizes the partially degraded branch structure and remodels the substrate in a two step reaction. First, the debranching enzyme transfers three glucose units to the nearest nonreducing end to generate a new substrate for glycogen phosphorylase. In the next step, the bifunctional debranching enzyme cleaves the α-1,6 glycosidic bond to release free glucose. In liver cells, the glucose is directly exported to the blood and in muscle cells it is phosphorylated by hexokinase as a first step on the glycolytic pathway. Since α-1,6 branch points occur about every 10 glucose residues in glycogen, complete degradation releases ~90% glucose-1p and 10% glucose molecules. Figure 8. Figure 7. Glycogen degradation is dependent on active glycogen phosphorylase enzyme, which is itself regulated by both covalent modification (phosphorylation) and by allosteric control (energy charge). As shown schematically in figure 8, glycogen phosphorylase is found in cells in two conformations; an active conformation, R form, and an inactive conformation, T form. Molecular analysis of glycogen phosphorylase structure and function revealed that phosphorylation of serine 14 (Ser 14) on the enzyme shifts the equilibrium in favor of the active R state. This phosphorylated form of glycogen phosphorylase is called the a form, or simply phosphorylase a, and the unphosphorylated form is the b form, or phosphorylase b. The enzyme responsible for phosphorylating glycogen phosphorylase b is phosphorylase kinase which is a downstream target of glucagon and epinephrine signaling in liver cells, and epinephrine signaling in muscle cells (muscle cells do not contain glucagon receptors). In contrast, insulin signaling stimulates the activity of protein phosphatase-1 (PP-1) leading to inactivation of glycogen phosphorylase (generates the unphosphorylated glycogen phosphorylase b form). Protein phosphatase 1 is the 4 of 12 pages

5 Figure 9. Figure 10. Bioc Dr. Miesfeld Spring 2008 same insulin-regulated enzyme that dephosphorylates PFK-2/FBPase-2, the enzyme responsible for fructose-2,6-bp levels in the cell (see fig. 17 in lecture 33). In addition to phosphorylation, the activity of glycogen phosphorylase can also be controlled by allosteric regulators which bind to the enzyme and shift the equilibrium. However, the liver and muscle isozymes of glycogen phosphorylase are allosterically-regulated in different ways, which reflects the unique functions glycogen in these two tissues. As shown in figure 9, muscle glycogen phosphorylase b can be shifted from the T to R state by binding of the allosteric activator AMP. AMP activation of muscle glycogen phosphorylase b results in an R state conformation that is 80% as active as the a form of the enzyme. In contrast, ATP and glucose-6p function as allosteric inhibitors of muscle glycogen phosphorylase b by competing with AMP for binding to the allosteric site, thereby pushing the equilibrium back to the T state. Importantly, allosteric activation of glycogen phosphorylase b (unphosphorylated form) provides a mechanism whereby the energy charge of the cell (relative levels of AMP and ATP) is able to control glycogen degradation independent of hormone signaling. This makes sense because muscle contraction requires ATP hydrolysis, which increases the level of AMP in the cell, most often under conditions where epinpherine signaling is not activated. As shown in figure 10, the activity of liver glycogen phosphorylase a, but not muscle glycogen phosphorylase a is subject to allosteric control by glucose binding which shifts the equilibrium from the R to T state. When liver glycogen phosphorylase a (phosphorylated form) is shifted to the T state, it is a better substrate for dephosphorylation by PP-1 than is the R state. Therefore in the presence of insulin signaling which activates PP-1, glucose binding to liver glycogen phosphorylase a leads to rapid inhibition of glycogen degradation. Biochemistry and regulation of glycogen synthesis Pathways for glycogen degradation and glycogen synthesis are separate. The phosphorylase reaction is not readily reversible because the concentration of P i in cells is 100-fold higher than that of Glucose-1-P. Glycogen synthesis is primarily regulated by hormone signaling in response to glucose levels in the blood. When glucose levels are high, insulin signaling in muscle and liver cells activates glycogen synthase to stimulate 5 of 12 pages

6 Figure 12. Figure 13. Bioc Dr. Miesfeld Spring 2008 glucose storage. The addition of glucose units to the nonreducing ends of glycogen by the enzyme glycogen synthase requires the synthesis of an activated form of glucose called uridine diphosphate glucose (UDP-glucose). Activated nucleotides such as UDP-glucose serve a special role in metabolic pathways by providing high energy compounds that are destined for specific pathways. Figure 11 shows that glucose-6p is first converted to glucose-1p by phosphoglucomutase, and then the enzyme UDP-glucose pyrophosphorylase catalyzes a reaction involving the attack of a phosphoryl oxygen from glucose-1p on the uridine Figure 11. triphosphate (UTP). This leads to the formation of UDP-glucose and the release of phosphate of pyrophosphate (PP i ). Although the standard free energy change of the UDP-glucose pyrophosphorylase reaction is close to zero, the rapid hydrolysis of PP i by the abundant cellular enzyme pyrophosphatase results in a highly favorable coupled reaction. As shown in figure 12, glycogen synthase transfers the glucose unit of UDP-glucose to the C-4 carbon of the terminal glucose at the nonreducing end of a glycogen chain. This reaction leads to the formation of a new α-1,4 glycosidic bond and extension of the chain by one glucose. The UDP moiety is released and UTP is regenerated in a reaction involving ATP and the enzyme nucleoside diphosphate kinase. Once the chain reaches a length of 11 glucose residues, the glycogen branching enzyme transfers seven glucose units from the end of the chain to an internal position by creating a new α-1,6 branchpoint. As shown in figure 13, this new branchpoint must be at least four glucose residues away from the nearest branch. The addition of branchpoints to glycogen is very important because it not only generates the necessary 6 of 12 pages

7 nonreducing ends for rapid turnover of glucose during cycles of glycogen synthesis and degradation, but branching also increases the solubility of glycogen. Glycogen synthase can only add glucose to the nonreducing end of a pre-existing chain containing at least seven glucose residues. The initiation of a new glycogen core requires the anchoring protein glycogenin (figure 14) which has both glucosyltransferase and glycogen synthase activities. Glycogenin transfers the glucose moiety from UDP-glucose to a tyrosine residue in the protein to form the initial glucoseprotein anchor. The glucose chain is then extended out to seven residues by the glycogen synthase activity of glycogenin using UDPglucose as the substrate. This glucoseglycogenin primer is a substrate for glycogen synthase and the glycogen branching enzyme which complete the process of adding another ~50,000 glucose units to generate the glycogen core complex. The energetic cost of storing glucose in glycogen particles is relatively small. Glycogen degradation is essentially "free" because the phosphorolysis reaction utilizes P i to generate glucose-1p which is easily converted to glucose-6p. Likewise, glycogen synthesis only requires the input of one ATP for each glucose residue added to the growing chain (regeneration of UTP by nucleoside diphosphate kinase reaction requires ATP hydrolysis). Figure 14. Nevertheless, in order for the liver to maintain safe blood glucose levels, and for muscle cells to replenish glycogen stores as quickly as possible after physical exertion, flux through the glycogen degradation and synthesis pathways needs to be stringently controlled. Similar to glycogen phosphorylase, the activity of glycogen synthase is also primarily controlled by reversible phosphorylation. The effect of phosphorylation on the activity of glycogen synthase is however the reverse of what we saw with glycogen phosphorylase. As shown in figure 15, dephosphorylation activates glycogen synthase, whereas, glycogen phosphorylase is activated by phosphorylation (see figure 8). In this case, the active glycogen synthase a form is dephosphorylated and favors the R state, whereas, the inactive glycogen synthase b form is phosphorylated and favors the T state. Hormone activation of glycogen synthase activity is mediated by insulin which promotes the dephosphorylation (and activation) of glycogen synthase by stimulating PP1 activity. When blood glucose levels are high, stimulation of glycogen synthesis promotes glucose uptake into liver and muscle cells. As shown in figure 15, numerous kinases have been shown to be activated by epinephrine and glucagon signaling resulting in phosphorylation and inactivation of glycogen synthase. One of these kinases is protein Figure of 12 pages

8 kinase A (PKA) which is activated by glucagon or epinephrine signaling. In addition, glycogen synthase is also inactivated by phosphorylation through the activity of protein kinase C (PKC), glycogen synthase kinase 3 (GSK3) and calmodulin dependent kinase (CAMK). Unlike the single phosphorylation site on glycogen phosphorylase, glycogen synthase is phosphorylated on multiple serine residues (up to nine) by these different kinases, some of which only target the enzyme if it has already been phosphorylated. For example, GSK3 phosphorylation of glycogen synthase requires that it first be phosphorylated by CAMK which functions as a priming event. Carl and Gerty Cori were a husband and wife research team that shared the 1947 Nobel Prize in Physiology and Medicine for their work elucidating glycogen metabolism. Based on what she knew about the enzymes required for glycogen metabolism, Gerty Cori correctly reasoned that several metabolic disorders were due to defects in liver and muscle glycogen phosphorylase. Figure 16. Recipricol control of glycogen metabolism by phosphorylation Since glycogen phosphorylase and glycogen synthase have opposing effects on glycogen metabolism, it is critical that their activities be reciprocally regulated to avoid futile cycling and to efficiently control glucose-6p concentrations Figure 17. within the cell. As shown in figure 16, infusion of glucose into mouse liver results in a rapid decrease in glycogen phosphorylase activity within 1 minute, followed by a dramatic increase in glycogen synthase activity by 4 minutes. This makes sense because when glucose is available, liver cells need to stop degrading glycogen and instead store the glucose by activating glycogen synthase. Figure 17 summarizes the effects of glucagon and insulin signaling on glycogen metabolism in liver cells where it can be seen that glucagon stimulates glucose efflux and insulin stimulates glucose influx through the GLUT2 glucose transporter protein. It can be seen that net phosphorylation drives glycogen degradation and net dephosphorylation drives glycogen synthesis. Control of glycogen metabolism through intracellular signaling pathways As illustrated in figure 17, glucagon signaling in liver cells results in Gs α mediated stimulation of adenylate cyclase activity leading to the production of the second messenger cyclic AMP (camp). As shown in figure 18, activation of protein kinase A (PKA) activity by camp triggers two types of phosphorylation 8 of 12 pages

9 circuits in muscle cells; one that stimulates glycogen degradation and a second that inhibits glycogen synthesis. PKA Figure 18. phosphorylates phosphorylase kinase to convert it to the active form, which in turn leads to the phosphorylation of glycogen phosphorylase b to generate glycogen phosphorylase a. This activated form of glycogen phosphorylase catalyzes a phosphorolysis reaction that removes glucose units from nonreducing ends of the glycogen core complex to produce glucose- 1P which is converted to glucose-6p. As shown in figure 19, insulin signaling results in a net increase in dephosphorylation of glycogen metabolizing enzymes and elevated rates of glycogen synthesis. First, activation of insulin receptor tyrosine kinase activity by hormone binding results in the phosphorylation of insulin receptor substrate (IRS) proteins. Phosphorylated IRS proteins activate phosphoinositide-3-kinase (PI-3K) which phosphorylates phosphatidylinositol-4-5-bisphosphate (PIP2) to generate phosphatidylinositol 3,4,5-triphosphate (PIP 3 ) and create a docking site for phosphoinositol-dependent kinase (PDK1). Second, activated PDK1 phosphorylates AKT kinase (also known as PKB) which phosphorylates a variety of target proteins in the cell, one of which is glycogen synthase kinase 3 (GSK3) which is inactivated by phosphorylation. Without active GSK3 around to maintain glycogen synthase in the inactive phosphorylated state, the level of active dephosphorylated glycogen synthase increases. More importantly, IRS proteins also activate insulin-stimulated protein kinase 1 (ISPK) which phosphorylates the G M subunit of PP1 on site 1 resulting in increased PP1 activity. Dephosphorylation of glycogen synthase b by PP1 generates the active form of the enzyme (glycogen synthase a) which is then able to initiate glycogen synthesis. In addition, insulinmediated activation of PP1 also leads to the dephosphorylation and inactivation of both phosphorylase kinase and glycogen phosphorylase. Taken together, insulin-mediated inhibition of 9 of 12 pages

10 Bioc Dr. Miesfeld Spring 2008 GSK3 activity, and stimulation of PP1 activity, results in activation of glycogen synthesis and inhibition of glycogen degradation. Figure 19. Human glycogen storage diseases A number of human diseases have been identified that affect glycogen metabolism. Disease symptoms in many cases include liver dysfunction due to excess glycogen, fasting-induced hypoglycemia (low blood glucose levels), and in the most severe diseases, death at an early age. Figure 20 depicts enzymes that are deficient in several human glycogen storage diseases. The metabolic symptoms are listed in figure 21. Two of these diseases, von Gierke's disease and Hers' disease, result from enzyme defects that restrict the use of glycogen reserves in the liver. von Gierke's disease is due to a deficiency in the enzyme glucose-6-phosphatase which causes a build-up of glycogen in the liver because glucose-6p accumulates and activates glycogen synthase. Since glucose-6p cannot be dephosphorylated and released into the blood, the patient requires a nearly continuous supply of dietary glucose which is especially difficult at night when liver glycogen would normally function to maintain blood glucose levels. In the 1950s, Gerty Cori applied what she had learned about glycogen metabolizing enzymes to studies aimed at discovering the molecular basis of human glycogen storage diseases. One of these diseases, known as Hers' disease, is due to defects in liver glycogen phosphorylase which prevents normal degradation of liver glycogen in Figure of 12 pages

11 Figure 21. Bioc Dr. Miesfeld Spring 2008 response to hormonal signaling. She also found that patients suffering from McArdle's disease harbor defects in muscle glycogen phosphorylase. These individuals suffer from exercise-induced cramps and muscle pain due to their inability to degrade muscle glycogen. Cori's disease, also described by Gerty Cori, results from defects in the glycogen debranching enzyme. The structure of both liver and muscle glycogen in these individuals contains short outer chains in the glycogen particle that cannot be removed, thus blocking complete degradation). Andersen's disease, which is due to defects in glycogen branching enzyme, results in the synthesis of large unbranched glycogen molecules that are insoluble and cause the immune system to attack and destroy liver cells. This is one of the most severe glycogen storage diseases because liver damage occurs early in life causing problems with normal physical and mental development. Unlike other glycogen storage diseases that can be treated to some extent with dietary glucose, the only treatment for Andresen's disease is a liver transplant. ANSWER TO KEY QUESTIon ABOUT GLYCOGEN METABOLISM: The activity of the two isozymes of muscle and liver glycogen phosphorylase is regulated by phosphorylation and allosteric effectors. Both muscle and liver glycogen phosphorylase are homodimers that exist in two conformations, the active R state and the inactive T state. Phosphorylation of these two glycogen phosphorylase enzymes shifts the conformational equilibrium toward the active R state, whereas, dephosphorylation shifts the conformational equilibrium toward the inactive T state. Phosphorylated glycogen phosphorylase is called the a form (active) and the unphosphorylated enzyme is called the b form (inactive). Epinephrine and glucagon signaling through activation of phosphorylase kinase which converts the inactive glycogen phosphorylase b form (unphosphorylated) into the active glycogen phosphorylase a form (phosphorylated). In contrast, insulin signaling through protein phosphatase 1, dephosphorylates glycogen phosphorylase a and converts it into the inactive b form. Because of the unique energy requirements of muscle cells (muscle contraction requires lots of ATP), muscle glycogen phosphorylase b can also be allosterically activated by AMP which shifts the conformational equilibrium toward the active R state. Both ATP and glucose-6p function as allosteric inhibitors in muscle cells by competing with AMP for binding. Importantly, liver glycogen phosphorylase b is not allosterically activated by AMP, which makes sense because liver glycogen is reserved for providing glucose to the body in response to hormone signaling. Finally, liver glycogen 11 of 12 pages

12 phosphorylase a, but not muscle glycogen phosphorylase a, can be allosterically inhibited by glucose which shifts the conformational equilibrium from the active R state to the inactive T state. This regulatory mechanism in liver cells shuts down glycogen degradation as soon as glucose levels begin to rise which occurs before the insulin signaling pathway is fully stimulated. Insulin and glucagon mediate recipricol regulation of glycogen metabolism through reversible phosphorylation of glycogen phosphorylase and glycogen synthase. Insulin signaling initiates a series of events culminating in net dephosphorylation of glycogen synthase (converting glycogen synthase b into glycogen synthase a) and glycogen phosphorylase (converting glycogen phosphorylase a into glycogen phosphorylase b). The unphosphorylated glycogen synthase a conformation is predominantly in the active R state and stimulates glycogen synthesis, whereas, the unphosphorylated glycogen phosphorylase b conformation is predominantly in the T state and glycogen degradation is inhibited. In contrast, glucagon signaling leads to net phosphorylation of glycogen phosphorylase (converting glycogen phosphorylase b into glycogen phosphorylase a) and glycogen synthase (converting glycogen synthase a into glycogen synthase b). The phosphorylated glycogen phosphorylase a conformation is predominantly in the R state and stimulates glycogen degradation, whereas, the phosphorylated glycogen synthase b conformation is predominantly in the inactive T state and glycogen synthesis is inhibited. Taken together, insulin and glucagon signaling controls carbohydrate metabolism by reversible phosphorylation events that simultaneously activate and inhibit opposing pathways, specifically, glycolysis/ gluconeogenesis and glycogen degradation/glycogen synthesis. 12 of 12 pages