Metabolism of Glycogen 18

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Metabolism of Glycogen 18

Peter A. Mayes, PhD, DSc, & David A. Bender, PhD

BIOMEDICAL IMPORTANCE phatase catalyzes hydrolysis of pyrophosphate to 2 mol of inorganic phosphate, shifting the equilibrium of the Glycogen is the major storage carbohydrate in animals, main reaction by removing one of its products. corresponding to starch in plants; it is a branched poly- α Glycogen synthase catalyzes the formation of a gly- mer of -D-glucose. It occurs mainly in liver (up to 6%) coside bond between C of the activated glucose of and muscle, where it rarely exceeds 1%. However, be- 1 UDPGlc and C4 of a terminal glucose residue of glyco- cause of its greater mass, muscle contains about three to gen, liberating uridine diphosphate (UDP). A preexist- four times as much glycogen as does liver (Table 18–1). ing glycogen molecule, or “glycogen primer,” must be Muscle glycogen is a readily available source of glu- present to initiate this reaction. The glycogen primer cose for glycolysis within the muscle itself. Liver glyco- may in turn be formed on a primer known as glyco- gen functions to store and export glucose to maintain genin, which is a 37-kDa protein that is glycosylated blood glucose between meals. After 12–18 hours of on a specific tyrosine residue by UDPGlc. Further glu- fasting, the liver glycogen is almost totally depleted. cose residues are attached in the 1→4 position to make Glycogen storage diseases are a group of inherited dis- a short chain that is a substrate for glycogen synthase. orders characterized by deficient mobilization of glyco- In skeletal muscle, glycogenin remains attached in the gen or deposition of abnormal forms of glycogen, lead- center of the glycogen molecule (Figure 13–15), ing to muscular weakness or even death. whereas in liver the number of glycogen molecules is greater than the number of glycogenin molecules. GLYCOGENESIS OCCURS MAINLY IN MUSCLE & LIVER Branching Involves Detachment The Pathway of Glycogen Biosynthesis of Existing Glycogen Chains Involves a Special Nucleotide of Glucose The addition of a glucose residue to a preexisting glyco- (Figure 18–1) gen chain, or “primer,” occurs at the nonreducing, outer end of the molecule so that the “branches” of the As in glycolysis, glucose is phosphorylated to glucose glycogen “tree” become elongated as successive 1→4 6-phosphate, catalyzed by hexokinase in muscle and linkages are formed (Figure 18–3). When the chain has glucokinase in liver. Glucose 6-phosphate is isomer- been lengthened to at least 11 glucose residues, branch- ized to glucose 1-phosphate by phosphoglucomutase. ing enzyme transfers a part of the 1→4 chain (at least The enzyme itself is phosphorylated, and the phospho- six glucose residues) to a neighboring chain to form a group takes part in a reversible reaction in which glu- 1→6 linkage, establishing a branch point. The cose 1,6-bisphosphate is an intermediate. Next, glucose branches grow by further additions of 1→4-glucosyl 1-phosphate reacts with uridine triphosphate (UTP) to units and further branching. form the active nucleotide uridine diphosphate glu- cose (UDPGlc)* and pyrophosphate (Figure 18–2), catalyzed by UDPGlc pyrophosphorylase. Pyrophos- GLYCOGENOLYSIS IS NOT THE REVERSE OF GLYCOGENESIS BUT IS A SEPARATE * Other nucleoside diphosphate sugar compounds are known, eg, PATHWAY (Figure 18–1) UDPGal. In addition, the same sugar may be linked to different nucleotides. For example, glucose may be linked to uridine (as Glycogen phosphorylase catalyzes the rate-limiting shown above) as well as to guanosine, thymidine, adenosine, or cy- step in glycogenolysis by promoting the phosphorylytic tidine nucleotides. cleavage by inorganic phosphate (phosphorylysis; cf hy-

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Glycogen → → (1 4 and 1 6 glucosyl units)x

BRANCHING ENZYME Pi

(1→4 Glucosyl units) x Insulin

UDP

GLYCOGEN cAMP GLYCOGEN SYNTHASE PHOSPHORYLASE Glycogen primer

Glucagon Epinephrine GLUCAN * TRANSFERASE Glycogenin DEBRANCHING Uridine ENZYME disphosphate glucose (UDPGlc)

To uronic acid Free glucose from pathway debranching UDPGlc PYROPHOSPHORYLASE enzyme INORGANIC PYROPHOSPHATASE PPi

2 Pi Uridine UDP triphosphate (UTP) Glucose 1-phosphate

2+ Mg PHOSPHOGLUCOMUTASE

Glucose 6-phosphate To glycolysis and pentose phosphate pathway H O ADP NUCLEOSIDE 2 DIPHOSPHO- GLUCOSE-6- + ATP ADP Mg2 GLUCOKINASE KINASE PHOSPHATASE

Pi ATP Glucose

Figure 18–1. Pathway of glycogenesis and of glycogenolysis in the liver. Two high-energy phosphates are used in the incorporation of 1 mol of glucose into glycogen. + , stimulation; − , inhibition. Insulin decreases the level of cAMP only after it has been raised by glucagon or epinephrine—ie, it antagonizes their action. Glucagon is active in heart muscle but not in skeletal muscle. At asterisk: Glucan transferase and debranching enzyme ap- pear to be two separate activities of the same enzyme.

Table 18–1. Storage of carbohydrate in postabsorptive normal adult humans (70 kg). drolysis) of the 1→4 linkages of glycogen to yield glu- cose 1-phosphate. The terminal glucosyl residues from the outermost chains of the glycogen molecule are re- Liver glycogen 4.0% = 72 g1 2 moved sequentially until approximately four glucose Muscle glycogen 0.7% = 245 g → Extracellular glucose 0.1% = 10 g3 residues remain on either side of a 1 6 branch (Figure 18–4). Another enzyme (-[1v4]v-[1v4] glucan 327 g transferase) transfers a trisaccharide unit from one → 1Liver weight 1800 g. branch to the other, exposing the 1 6 branch point. 2Muscle mass 35 kg. Hydrolysis of the 1→6 linkages requires the de- 3Total volume 10 L. branching enzyme. Further phosphorylase action can ch18.qxd 2/13/2003 3:05 PM Page 147

METABOLISM OF GLYCOGEN / 147

O dephosphorylation of enzyme protein in response to 6CH2OH hormone action (Chapter 9). HN H O Uracil Cyclic AMP (cAMP) (Figure 18–5) is formed from H H 1 O N ATP by adenylyl cyclase at the inner surface of cell OH H O O membranes and acts as an intracellular second messen- HO O P O P O CH2 ger in response to hormones such as epinephrine, nor- H OH O – O – O epinephrine, and glucagon. cAMP is hydrolyzed by phosphodiesterase, so terminating hormone action. In HH Ribose liver, insulin increases the activity of phosphodiesterase. HH HO OH Phosphorylase Differs Between

Glucose Diphosphate Uridine Liver & Muscle Figure 18–2. Uridine diphosphate glucose (UDPGlc). In liver, one of the serine hydroxyl groups of active phosphorylase a is phosphorylated. It is inactivated by hydrolytic removal of the phosphate by protein phos- phatase-1 to form phosphorylase b. Reactivation re- then proceed. The combined action of phosphorylase quires rephosphorylation catalyzed by phosphorylase and these other enzymes leads to the complete break- kinase. down of glycogen. The reaction catalyzed by phospho- Muscle phosphorylase is distinct from that of liver. It glucomutase is reversible, so that glucose 6-phosphate is a dimer, each monomer containing 1 mol of pyridoxal can be formed from glucose 1-phosphate. In liver (and phosphate (vitamin B6). It is present in two forms: phos- kidney), but not in muscle, there is a specific enzyme, phorylase a, which is phosphorylated and active in either glucose-6-phosphatase, that hydrolyzes glucose the presence or absence of 5′-AMP (its allosteric modi- 6-phosphate, yielding glucose that is exported, leading fier); and phosphorylase b, which is dephosphorylated to an increase in the blood glucose concentration. and active only in the presence of 5′-AMP. This occurs during exercise when the level of 5′-AMP rises, providing, CYCLIC AMP INTEGRATES THE by this mechanism, fuel for the muscle. Phosphorylase a is REGULATION OF GLYCOGENOLYSIS the normal physiologically active form of the enzyme. & GLYCOGENESIS cAMP Activates Muscle Phosphorylase The principal enzymes controlling glycogen metabo- lism—glycogen phosphorylase and glycogen synthase— Phosphorylase in muscle is activated in response to epi- are regulated by allosteric mechanisms and covalent nephrine (Figure 18–6) acting via cAMP. Increasing modifications due to reversible phosphorylation and the concentration of cAMP activates cAMP-dependent

1→4- Glucosidic bond Unlabeled glucose residue 1→6- Glucosidic bond 14C-labeled glucose residue

14C-Glucose → added New 1 6- bond

GLYCOGEN BRANCHING SYNTHASE ENZYME

Figure 18–3. The biosynthesis of glycogen. The mechanism of branching as revealed by adding 14C-labeled glucose to the diet in the living animal and examining the liver glycogen at further intervals. ch18.qxd 2/13/2003 3:05 PM Page 148

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types of subunits—α, β, γ, and δ—in a structure repre- αβγδ α β sented as ( )4. The and subunits contain serine residues that are phosphorylated by cAMP-dependent protein kinase. The δ subunit binds four Ca2+ and is identical to the Ca2+-binding protein calmodulin (Chapter 43). The binding of Ca2+ activates the cat- alytic site of the γ subunit while the molecule remains in the dephosphorylated b configuration. However, the phosphorylated a form is only fully activated in the presence of Ca2+. A second molecule of calmodulin, or TpC (the structurally similar Ca2+-binding protein in muscle), can interact with phosphorylase kinase, caus- ing further activation. Thus, activation of muscle con-

PHOSPHORYLASE GLUCAN DEBRANCHING traction and glycogenolysis are carried out by the same TRANSFERASE ENZYME Ca2+-binding protein, ensuring their synchronization.

Glucose residues joined by 1 → 4- glucosidic bonds Glycogenolysis in Liver Can Glucose residues joined by Be cAMP-Independent → 1 6- glucosidic bonds In addition to the action of glucagon in causing forma- tion of cAMP and activation of phosphorylase in liver, Figure 18–4. Steps in glycogenolysis. 1-adrenergic receptors mediate stimulation of glyco- genolysis by epinephrine and norepinephrine. This in- volves a cAMP-independent mobilization of Ca2+ protein kinase, which catalyzes the phosphorylation by from mitochondria into the cytosol, followed by the ATP of inactive phosphorylase kinase b to active stimulation of a Ca2+/calmodulin-sensitive phosphory- phosphorylase kinase a, which in turn, by means of a lase kinase. cAMP-independent glycogenolysis is also further phosphorylation, activates phosphorylase b to caused by vasopressin, oxytocin, and angiotensin II act- phosphorylase a. ing through calcium or the phosphatidylinositol bis- phosphate pathway (Figure 43–7). Ca2+ Synchronizes the Activation of Phosphorylase With Muscle Contraction Protein Phosphatase-1 Glycogenolysis increases in muscle several hundred-fold Inactivates Phosphorylase immediately after the onset of contraction. This in- Both phosphorylase a and phosphorylase kinase a are volves the rapid activation of phosphorylase by activa- dephosphorylated and inactivated by protein phos- tion of phosphorylase kinase by Ca2+, the same signal as phatase-1. Protein phosphatase-1 is inhibited by a that which initiates contraction in response to nerve protein, inhibitor-1, which is active only after it has stimulation. Muscle phosphorylase kinase has four been phosphorylated by cAMP-dependent protein ki- nase. Thus, cAMP controls both the activation and in- activation of phosphorylase (Figure 18–6). Insulin re- NH2 inforces this effect by inhibiting the activation of N N phosphorylase b. It does this indirectly by increasing uptake of glucose, leading to increased formation of N N glucose 6-phosphate, which is an inhibitor of phosphor- 5′ ylase kinase.

O CH2

O Glycogen Synthase & Phosphorylase – O PO Activity Are Reciprocally Regulated HH (Figure 18–7) HH Like phosphorylase, glycogen synthase exists in either a O 3′ OH phosphorylated or nonphosphorylated state. However, unlike phosphorylase, the active form is dephosphory- Figure 18–5. 3′,5′-Adenylic acid (cyclic AMP; cAMP). lated (glycogen synthase a) and may be inactivated to ch18.qxd 2/13/20033:05PMPage149

Epinephrine

β Receptor +

Inactive Active adenylyl adenylyl cyclase cyclase

Glycogen(n) + + Glycogen Glucose 1-phosphate PHOSPHODIESTERASE (n+1) ATP cAMP 5′-AMP

+ Pi Inactive Active cAMP-DEPENDENT cAMP-DEPENDENT PROTEIN KINASE PROTEIN KINASE ADP 149 PHOSPHORYLASE a (active) CALMODULIN ADP H2O COMPONENT OF Inhibitor-1 PHOSPHORYLASE KINASE (inactive) ATP – + PHOSPHORYLASE KINASE b PHOSPHORYLASE KINASE a PROTEIN 2+ G6P Insulin (inactive) Ca (active) PHOSPHATASE-1 ATP

– 2+ + –Ca ATP Pi PHOSPHORYLASE b ADP (inactive) Pi H2O PROTEIN PHOSPHATASE-1

– Inhibitor-1-phosphate (active)

Figure 18–6. Control of phosphorylase in muscle. The sequence of reactions arranged as a cascade allows amplification of the hormonal signal at each step. (n = number of glucose residues; G6P, glucose 6-phosphate.) 5475ch18.qxd_ccI 2/26/03 8:06 AM Page 150

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Epinephrine

β Receptor +

Inactive Active adenylyl adenylyl cyclase cyclase

+

PHOSPHODIESTERASE ATP cAMP 5′-AMP

PHOSPHORYLASE Ca2+ + KINASE +

Inactive Active cAMP-DEPENDENT cAMP-DEPENDENT PROTEIN KINASE PROTEIN KINASE ATP Glycogen(n+1) Inhibitor-1 GSK (inactive) ADP CALMODULIN-DEPENDENT PROTEIN KINASE

ATP GLYCOGEN SYNTHASE + GLYCOGEN SYNTHASE b a (inactive) Ca2+ (active) + Insulin G6P + + ADP PROTEIN PHOSPHATASE Glycogen H O P (n) 2 i + UDPG Inhibitor-1-phosphate PROTEIN (active) PHOSPHATASE-1 –

Figure 18–7. Control of glycogen synthase in muscle (n = number of glucose residues). The sequence of reac- tions arranged in a cascade causes amplification at each step, allowing only nanomole quantities of hormone to cause major changes in glycogen concentration. (GSK, glycogen synthase kinase-3, -4, and -5; G6P, glucose 6-phosphate.)

glycogen synthase b by phosphorylation on serine REGULATION OF GLYCOGEN residues by no fewer than six different protein kinases. METABOLISM IS EFFECTED BY 2+ Two of the protein kinases are Ca /calmodulin- A BALANCE IN ACTIVITIES dependent (one of these is phosphorylase kinase). An- other kinase is cAMP-dependent protein kinase, which BETWEEN GLYCOGEN allows cAMP-mediated hormonal action to inhibit SYNTHASE & PHOSPHORYLASE glycogen synthesis synchronously with the activation of (Figure 18–8) glycogenolysis. Insulin also promotes glycogenesis in muscle at the same time as inhibiting glycogenolysis by Not only is phosphorylase activated by a rise in concen- raising glucose 6-phosphate concentrations, which tration of cAMP (via phosphorylase kinase), but glyco- stimulates the dephosphorylation and activation of gen synthase is at the same time converted to the glycogen synthase. Dephosphorylation of glycogen syn- inactive form; both effects are mediated via cAMP- thase b is carried out by protein phosphatase-1, which dependent protein kinase. Thus, inhibition of gly- is under the control of cAMP-dependent protein ki- cogenolysis enhances net glycogenesis, and inhibition of nase. glycogenesis enhances net glycogenolysis. Furthermore, ch18.qxd 2/13/2003 3:05 PM Page 151

METABOLISM OF GLYCOGEN / 151

Epinephrine PHOSPHODIESTERASE (liver, muscle) Inhibitor-1 cAMP 5′-AMP Inhibitor-1 Glucagon phosphate (liver)

GLYCOGEN PHOSPHORYLASE SYNTHASE b KINASE b

cAMP- PROTEIN PROTEIN DEPENDENT PHOSPHATASE-1 PHOSPHATASE-1 PROTEIN KINASE

GLYCOGEN PHOSPHORYLASE SYNTHASE a KINASE a

Glycogen

Glycogen PHOSPHORYLASE PHOSPHORYLASE UDPGIc cycle a b

Glucose 1-phosphate PROTEIN PHOSPHATASE-1

Glucose (liver) Glucose Lactate (muscle)

Figure 18–8. Coordinated control of glycogenolysis and glycogenesis by cAMP-dependent protein ki- nase. The reactions that lead to glycogenolysis as a result of an increase in cAMP concentrations are shown with bold arrows, and those that are inhibited by activation of protein phosphatase-1 are shown as broken arrows. The reverse occurs when cAMP concentrations decrease as a result of phosphodiesterase activity, leading to glycogenesis.

the dephosphorylation of phosphorylase a, phosphory- more, they allow insulin, via glucose 6-phosphate eleva- lase kinase a, and glycogen synthase b is catalyzed by tion, to have effects that act reciprocally to those of a single enzyme of wide specificity—protein phos- cAMP (Figures 18–6 and 18–7). phatase-1. In turn, protein phosphatase-1 is inhibited by cAMP-dependent protein kinase via inhibitor-1. Thus, glycogenolysis can be terminated and glycogenesis CLINICAL ASPECTS can be stimulated synchronously, or vice versa, because Glycogen Storage Diseases Are Inherited both processes are keyed to the activity of cAMP-depen- dent protein kinase. Both phosphorylase kinase and “Glycogen storage disease” is a generic term to describe glycogen synthase may be reversibly phosphorylated in a group of inherited disorders characterized by deposi- more than one site by separate kinases and phosphatases. tion of an abnormal type or quantity of glycogen in the These secondary phosphorylations modify the sensitivity tissues. The principal glycogenoses are summarized in of the primary sites to phosphorylation and dephos- Table 18–2. Deficiencies of adenylyl kinase and phorylation (multisite phosphorylation). What is cAMP-dependent protein kinase have also been re- ch18.qxd 2/13/2003 3:05 PM Page 152

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Table 18–2. Glycogen storage diseases.

Glycogenosis Name Cause of Disorder Characteristics Type I Von Gierke’s disease Deficiency of glucose-6-phosphatase Liver cells and renal tubule cells loaded with glycogen. Hypoglycemia, lactic- acidemia, ketosis, hyperlipemia. Type II Pompe’s disease Deficiency of lysosomal α-1→4- and Fatal, accumulation of glycogen in lyso- 1→6-glucosidase (acid maltase) somes, heart failure. Type III Limit dextrinosis, Forbes’ or Absence of debranching enzyme Accumulation of a characteristic Cori’s disease branched polysaccharide. Type IV Amylopectinosis, Andersen’s Absence of branching enzyme Accumulation of a polysaccharide hav- disease ing few branch points. Death due to cardiac or liver failure in first year of life. Type V Myophosphorylase deficiency, Absence of muscle phosphorylase Diminished exercise tolerance; muscles McArdle’s syndrome have abnormally high glycogen con- tent (2.5–4.1%). Little or no lactate in blood after exercise. Type VI Hers’ disease Deficiency of liver phosphorylase High glycogen content in liver, ten- dency toward hypoglycemia. Type VII Tarui’s disease Deficiency of phosphofructokinase As for type V but also possibility of he- in muscle and erythrocytes molytic anemia. Type VIII Deficiency of liver phosphorylase As for type VI. kinase

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