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Introduction
Stores of readily available glucose to supply the tissues with an oxidizable energy source are found principally in the liver, as glycogen. A second major source of stored glucose is the glycogen of skeletal muscle. However, muscle glycogen is not generally available to other tissues, because muscle lacks the enzyme glucose-6-phosphatase.
The major site of daily glucose consumption (75%) is the brain via aerobic pathways. Most of the remainder of is utilized by erythrocytes, skeletal muscle, and heart muscle. The body obtains glucose either directly from the diet or from amino acids and lactate via gluconeogenesis. Glucose obtained from these two primary sources either remains soluble in the body fluids or is stored in a polymeric form, glycogen. Glycogen is considered the principal storage form of glucose and is found mainly in liver and muscle, with kidney and intestines adding minor storage sites. With up to 10% of its weight as glycogen, the liver has the highest specific content of any body tissue. Muscle has a much lower amount of glycogen per unit mass of tissue, but since the total mass of muscle is so much greater than that of liver, total glycogen stored in muscle is about twice that of liver. Stores of glycogen in the liver are considered the main buffer of blood glucose levels.
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Glycogenolysis
Degradation of stored glycogen (glycogenolysis) occurs through the action of glycogen phosphorylase. The action of phosphorylase is to phosphorolytically remove single glucose residues from a-(1,4)-linkages within the glycogen molecules. The product of this reaction is glucose-1-phosphate. The advantage of the reaction proceeding through a phosphorolytic step is that:
  • 1. The glucose is removed from glycogen is an activated state, i.e. phosphorylated and this occurs without ATP hydrolysis.
  • 2. The concentration of Pi in the cell is high enough to drive the equilibrium of the reaction the favorable direction since the free energy change of the standard state reaction is positive.
The glucose-1-phosphate produced by the action of phosphorylase is converted to glucose-6-phosphate by phosphoglucomutase: this enzyme, like phosphoglycerate mutase (of glycolysis), contains a phosphorylated amino acid in the active site (in the case of phosphoglucomutase it is a Ser residue). The enzyme phosphate is transferred to C-6 of glucose-1-phosphate generating glucose-1,6-phosphate as an intermediate. The phosphate on C-1 is then transferred to the enzyme regenerating it and glucose-6-phospahte is the released product.
As mentioned above the phosphorylase mediated release of glucose from glycogen yields a charged glucose residue without the need for hydrolysis of ATP. An additional necessity of releasing phosphorylated glucose from glycogen ensures that the glucose residues do not freely diffuse from the cell. In the case of muscle cells this is acutely apparent since the purpose in glycogenolysis in muscle cells is to generate substrate for glycolysis.
The conversion of glucose-6-phosphate to glucose, which occurs in the liver, kidney and intestine, by the action of glucose-6-phosphatase does not occur in skeletal muscle as these cells lack this enzyme. Therefore, any glucose released from glycogen stores of muscle will be oxidized in the glycolytic pathway. In the liver the action of glucose-6-phosphatase allows glycogenolysis to generate free glucose for maintaining blood glucose levels.
Glycogen phosphorylase cannot remove glucose residues from the branch points (a-1,6 linkages) in glycogen. The activity of phosphorylase ceases 4 glucose residues from the branch point. The removal of the these branch point glucose residues requires the action of debranching enzyme (also called glucan transferase) which contains 2 activities: glucotransferase and glucosidase. The transferase activity removes the terminal 3 glucose residues of one branch and attaches them to a free C-4 end of a second branch. The glucose in a-(1,6)-linkage at the branch is then removed by the action of glucosidase. This glucose residue is uncharged since the glucosidase-catalyzed reaction is not phosphorylytic. This means that theroretically glycogenolysis occurring in skeletal muscle could generate free glucose which could enter the blood stream. However, the activity of hexokinase in muscle is so high that any free glucose is immediately phosphorylated and enters the glycolytic pathway. Indeed, the precise reason for the temporary appearance of the free glucose from glycogen is the need of the skeletal muscle cell to generate energy from glucose oxidation, thereby, precluding any chance of the glucose entering the blood.
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Regulation of Glycogenolysis
Glycogen phosphorylase is a homodimeric enzyme that exist in two distinct conformational states: a T (for tense, less active) and R (for relaxed, more active) state. Phosphorylase is capable of binding to glycogen when the enzyme is in the R state. This conformation is enhanced by binding of AMP and inhibited by binding ATP or glucose-6-phosphate. The enzyme is also subject to covalent modification by phosphorylation as a means of regulating its activity. The relative activity of the un-modified phosphorylase enzyme (given the name phosphorylase-b) is sufficient to generate enough glucose-1-phosphate for entry into glycolysis for the production of sufficient ATP to maintain the normal resting activity of the cell. This is true in both liver and muscle cells.
Pathways involved in the regulation of glycogen phosphorylase. See the text for details of the regulatory mechanisms. PKA is cAMP-dependent protein kinase. PPI-1 is phosphoprotein phosphatase-1 inhibitor. Whether a factor has positive (+ve) or negative (-ve) effects on any enzyme is indicated. Briefly, phosphorylase b is phosphorylated, and rendered highly active, by phosphorylase kinase. Phosphorylase kinase is itself phosphorylated, leading to increased activity, by PKA (itself activated through receptor mediated mechanisms). PKA also phosphorylates PPI-1 leading to an inhibition of phosphate removal allowing the activated enzymes to remain so longer. Calcium ions can activate phosphorylase kinase even in the absence of the enzyme being phosphorylated. This allows neuromuscular stimulation by acetylcholine to lead to increased glycogenolysis in the absence of receptor stimulation.
In response to lowered blood glucose the a cells of the pancreas secrete glucagon which binds to cell surface receptors on liver and several other cells. Liver cells are the primary target for the action of this peptide hormone. The response of cells to the binding of glucagon to its cell surface receptor is the activation of the enzyme adenylate cyclase which is associated with the receptor. Activation of adenylate cyclase leads to a large increase in the formation of cAMP. cAMP binds to an enzyme called cAMP-dependent protein kinase, PKA. Binding of cAMP to the regulatory subunits of PKA leads to the release and subsequent activation of the catalytic subunits. The catalytic subunits then phosphorylate a number of proteins on serine and threonine residues. Of significance to this discussion is the PKA-mediated phosphorylation of phosphorylase kinase as shown in the diagram above. Phosphorylation of phosphorylase kinase activates the enzyme which in turn phosphorylates the b form of phosphorylase. Phosphorylation of phosphorylase-b greatly enhances its activity towards glycogen breakdown. The modified enzyme is called phosphorylase-a. The net result is an extremely large induction of glycogen breakdown in response to glucagon binding to cell surface receptors.
This identical cascade of events occurs in skeletal muscle cells as well. However, in these cells the induction of the cascade is the result of epinephrine binding to receptors on the surface of muscle cells. Epinephrine is released from the adrenal glands in response to neural signals indicating an immediate need for enhanced glucose utilization in muscle, the so called fight or flight response. Muscle cells lack glucagon receptors. The presence of glucagon receptors on muscle cells would be futile anyway since the role of glucagon release is to increase blood glucose concentrations and muscle glycogen stores cannot contribute to blood glucose levels.
Regulation of phosphorylase kinase activity is also affected by two distinct mechanisms involving Ca2+ ions. The ability of Ca2+ ions to regulate phosphorylase kinase is through the function of one of the subunits of this enzyme. One of the subunits of this enzyme is the ubiquitous protein, calmodulin. Calmodulin is a calcium binding protein. Binding induces a conformational change in calmodulin which in turn enhances the catalytic activity of the phosphorylase kinase towards its substrate, phosphorylase-b. This activity is crucial to the enhancement of glycogenolysis in muscle cells where muscle contraction is stimulated acetylcholine stimulation of neuromuscular junctions. The effect of acetylcholine release from nerve terminals at a neuromuscular junction is to depolarize the muscle cell leading to increased release of sarcoplasmic Ca2+, thereby activating phosphorylase kinase.Thus, not only does the increased intracellular calcium increase the rate of muscle contraction it increases glycogenolysis which provides the muscle cell with the increased ATP it also needs for contraction.
The second Ca2+ ion-mediated pathway to phosphorylase kinase activation is through activation of a-adrenergic receptors by epinephrine.

Pathways involved in the regulation of glycogen phosphorylase by epinephrine activation of a-adrenergic receptors. See the text for details of the regulatory mechanisms. PLC-g is phospholipase C-g. The substrate for PLC-g is phosphatidylinositol-4,5-bisphosphate (PIP2) and the products are IP3, inositol trisphosphate and DAG, diacylglycerol.
Unlike b-adrenergic receptors which are coupled to activation of adenylate cyclase, a-adrenergic receptors are coupled through G-proteins that activate phospholipase-C-g (PLC-g). Activation pf PLC-g leads to increased hydrolysis of membrane phosphatidylinositol-4,5-bisphosphate (PIP2), the products of which are inositol trisphosphate (IP3) and diacylglycerol (DAG). DAG binds to and activates protein kinase C (PKC) an enzyme that phosphorylates numerous substrate, one of which is glycogen synthase (see below). IP3 binds to receptors on the surface of the endoplasmic reticulum leading to release of Ca2+ ions. The Ca2+ ions then interact the calmodulin subunits of phosphoryase kinase resulting in its' activation. Additionally, the Ca2+ ions activate PKC in conjunction with DAG.
In order to terminate the activity of the enzymes of the glycogen phosphorylase activation cascade, once the needs of the body are met, the modified enzymes need to be un-modified. In the case of Ca2+ induced activation, the level of Ca2+ ion release from muscle stores will terminate when the incoming nerve impulses cease. The removal of the phosphates on phosphorylase kinase and phosphorylase-a is carried out by phosphoprotein phosphatase-1 (PP-1). In order that the phosphate residues placed on these enzymes by PKA and phosphorylase kinase are not immediately removed, the activity of PP-1 must also be regulated. This is accomplished by the binding of PP-1 to phosphoprotein phosphatase inhibitor (PPI-1). This protein also is phosphorylated by PKA and dephosphorylated by PP-1 (see diagram above). The phosphorylation of PPI allows it to bind to PP-1, an activity it is incapable of carrying out when not phosphorylated. When PPI binds PP-1 its phosphorylations are removed by PP-1 but at a much reduced rate than by free PP-1 thus temporarily trapping PP-1 from other substrates.
The effects of the activation of this regulatory phosphorylation cascade on the rate of glycogen synthesis is described below.
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Glycogen Synthesis
Synthesis of glycogen from glucose is carried out the enzyme glycogen synthase. This enzyme utilizes UDP-glucose as one substrate and the non-reducing end of glycogen as another. The activation of glucose to be used for glycogen synthesis is carried out by the enzyme UDP-glucose pyrophosphorylase. This enzyme exchanges the phosphate on C-1 of glucose-1-phosphate for UDP. The energy of the phospho-glycosyl bond of UDP-glucose is utilized by glycogen synthase to catalyze the incorporation of glucose into glycogen. UDP is subsequently released from the enzyme. The a-1,6 branches in glucose are produced by amylo-(1,4 - 1,6)-transglycosylase, also termed the branching enzyme. This enzyme transfers a terminal fragment of 6-7 glucose residues (from a polymer at least 11 glucose residues long) to an internal glucose residue at the C-6 hydroxyl position.
Until recently, the source of the first glycogen molecule that might act as a primer in glycogen synthesis was unknown. Recently it has been discovered that a protein known as glycogenin is located at the core of glycogen molecules. Glycogenin has the unusual property of catalyzing its own glycosylation, attaching C-1 of a UDP-glucose to a tyrosine residue on the enzyme. The attached glucose is believed to serve as the primer required by glycogen synthase.
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Regulation of Glycogen Synthesis
Glycogen synthase ia a tetrameric enzyme consisting of 4 identical subunits. The activity of glycogen synthase is regulated by phosphorylation of serine residues in the subunit proteins. Phosphorylation of glycogen synthase reduces its activity towards UDP-glucose. When in the non-phosphorylated state, glycogen synthase does not require glucose-6-phosphate as an allosteric activator---when phosphorylated it does. The two forms of glycogen synthase are identifed by the same nomenclature as used for glycogen phosphorylase. The unphosphorylated and most active form is synthase-a and the phosphorylated glucose-6-phosphate-dependent form is synthase-b shift.
Pathways involved in the regulation of glycogen synthase. See the text for details of the regulatory mechanisms. PKA is cAMP-dependent protein kinase. PPI-1 is phosphoprotein phosphatase-1 inhibitor. Whether a factor has positive (+ve) or negative (-ve) effects on any enzyme is indicated. Briefly, glycogen synthase a is phosphorylated, and rendered much less active and requires glucose-6-phosphate to have any activity at all. Phosphorylation of glycogen synthase is accomplished by several different enzymes. The most important is synthase-phosphorylase kinase the same enzyme responsible for phosphorylation (and activation) of glycogen phosphorylase. PKA (itself activated through receptor mediated mechanisms) also phosphorylates glycogen synthase directly. The effects of PKA on PPI-1 are the same as those described above for the regulation of glycogen phosphorylase. The other enzymes shown to directly phosphorylate glycogen synthase are protein kinase C (PKC), calmodulin-dependent protein kinase, glycogen synthase kinase-3 (GSK-3) and two forms of casein kinase (CK-I and CK-II). The enzyme PKC is activated by Ca2+ ions and phospholipids, primarily diacylglycerol, DAG. DAG is formed by receptor-mediated hydrolysis of membrane phosphatidylinositol bisphosphate (PIP2).
Phosphorylation of synthase occurs primarily in response to hormonal activation of PKA. One of the major kinases active on synthase is synthase-phosphorylase kinase; the same enzyme that phosphorylates glycogen phosphorylase. However, at least 5 additional enzymes have been identified that phosphorylate glycogen synthase directly. One of of these glycogen synthase phosphorylating enzymes is PKA itself. One important glycogen synthase phosphorylating enzyme is active independently of increases in cAMP levels. This enzyme is glycogen synthase kinase 3 (GSK-3). Each phosphorylation event occurs at distinct serine residues which can result in a progressively increased state of synthase phosphorylation.
Glycogen synthase activity can also be affected by epinephrine binding to a-adrenergic receptors through a pathway like that described above for regulation of glycogen phosphorylase.

Pathways involved in the regulation of glycogen synthase by epinephrine activation of a-adrenergic receptors. See the text for details of the regulatory mechanisms. PKC is protein kinase C. PLC-g is phospholipase C-g. The substrate for PLC-g is phosphatidylinositol-4,5-bisphosphate (PIP2) and the products are IP3, inositol trisphosphate and DAG, diacylglycerol.
When a-adrenergic receptors are stimulated there is an increase in the activity of PLC-g with a resultant increase in PIP2 hydrolysis. The products of PIP2 hydrolysis are DAG and IP3. As described above for glycogen phoshorylase, DAG and the Ca2+ ions released by IP3 activate PKC which phosphorylates and inactivates glycogen synthase. Additional responses of calcium are the activation of calmodulin-dependent protein kinase (calmodulin is a component of many enzymes that are responsive to Ca2+) which also phosphorytes glycogen synthase.
The effects of these phosphorylations leads to:
  • 1. Decreased affinity of synthase for UDP-glucose.
  • 2. Decreased affinity of synthase for glucose-6-phosphate.
  • 3. Increased affinity of synthase for ATP and Pi.
Reconversion of synthase-b to synthase-a requires dephosphorylation. This is carried out predominately by protein phosphatase-1 (PP-1) the same phosphatase involved in dephosphorylation of phosphorylase.
The activity of PP-1 is also affected by insulin. The pancreatic hormone exerts an opposing effect to that of glucagon and epinephrine. This should appear obvious since the role of insulin is to increase the uptake of glucose from the blood.
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Glycogen Storage Diseases
Since glycogen molecules can become enormously large, an inability to degrade glycogen can cause cells to become pathologically engorged; it can also lead to the functional loss of glycogen as a source of cell energy and as a blood glucose buffer. Although glycogen storage diseases are quite rare, their effects can be most dramatic. The debilitating effect of many glycogen storage diseases depends on the severity of the mutation causing the deficiency. In addition, although the glycogen storage diseases are attributed to specific enzyme deficiencies, other events can cause the same characteristic symptoms. For example, Type I glycogen storage disease (von Gierke's disease) is attributed to lack of glucose-6-phosphatase. However, this enzyme is localized on the cisternal surface of the endoplasmic reticulum (ER); in order to gain access to the phosphatase, glucose-6-phosphate must pass through a specific translocase in the ER membrane. Mutation of either the phosphatase or the translocase makes transfer of liver glycogen to the blood a very limited process. Thus, mutation of either gene leads to symptoms associated with von Gierke's disease, which occurs at a rate of about 1 in 200,000 people.
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Table of Glycogen Storage Diseases
Type: Name Enzyme Affected Primary Organ Manifestations
Type 0 glycogen synthase liver hypoglycemia, early death, hyperketonia
Type Ia:
von Gierke's		 glucose-6-phosphatase liver hepatomegaly, kidney failure, thrombocyte dysfunction
Type Ib microsomal glucose-6-phosphate translocase liver like Ia, also neutropenia, bacterial infections
Type Ic microsomal Pi transporter liver like Ia
Type II:
Pompe's		 lysosomal a-1,4-glucosidase,
lysosomal acid a-glucosidase
acid maltase		 skeletal and cardiac muscle infantile form = death by 2; juvenile form = myopathy; adult form = muscular dystrophy-like
Type IIIa:
Cori's or Forbe's		 liver and muscle debranching enzyme liver, skeletal and cardiac muscle infant hepatomegaly, myopathy
Type IIIb liver debranching enzyme
normal muscle enzyme		 liver, skeletal and cardiac muscle liver symptoms same as type IIIa
Type IV:
Anderson's		 branching enzyme liver, muscle hepatosplenomegaly, cirrhosis
Type V:
McArdle's		 muscle phosphorylase skeletal muscle excercise-induced cramps and pain, myoglobinuria
Type VI:
Her's		 liver phosphorylase liver hepatomegaly, mild hypoglycemia, hyperlipidemia and ketosis, improvement with age
Type VII:
Tarui's		 muscle PFK-1 muscle, RBC's like V, also hemolytic anemia
Type VIb, VIII or Type IX phosphorylase kinase liver, leukocytes, muscle like VI
Type XI:
Fanconi-Bickel		 glucose transporter-2 (GLUT-2) liver failure to thrive, hepatomegaly, rickets, proximal renal tubular dysfunction
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This article has been modified by Dr. M. Javed Abbas.
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20:39 21/12/2002