Glycogen metabolism

04/11/2023Biochemistry For Medics 1

Glycogen Metabolism

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Glycogen- IntroductionGlycogen is a readily mobilized storage form of glucose. It is a very large, branched polymer of glucose residues that can be broken down to yield glucose molecules when energy is needed. Most of the glucose residues in glycogen are linked by α-1,4-glycosidic bonds.Branches at about every tenth residue are created by α-1,6-glycosidic bonds.


Glycogen-Chemistry


Glycogen-ChemistryGlycogen is present in the cytosol in the form of granules ranging in diameter from 10 to 40 nm. It has a molecular mass of 107 Da and consists of polysaccharide chains, each containing about 13 glucose residues.The chains are either branched or unbranched and are arranged in 12 concentric layers. The branched chains (each has two branches) are found in the inner layers and the unbranched chains in the outer layer. (G, Glycogenin, the primer molecule for glycogen synthesis.)


Glycogen- ChemistryThe highly branched structure of glycogen provides a large number of sites for glycogenolysis, permitting rapid release of glucose 1-phosphate for muscle activity.

Glycogenin


Glycogen Storage Sites

It is stored mainly in liver and muscle The liver content of glycogen is greater than that of muscle, Since the muscle mass of the body is considerably greater than that of the liver, about three-quarters of total body glycogen is in muscle


Storage of Carbohydrate in a 70-Kg Human

Percentage of Tissue Weight

Tissue Weight

Body Content

Liver glycogen5.0

1.8 kg 90 g

Muscle glycogen

0.7 35 kg 245 g

Extracellular glucose

0.1 10 L 10 g


Reasons for storing Glycogen as a fuel-Glycogen serves as a buffer to maintain blood-glucose levels. Glucose is virtually the only fuel used by the brain, except during prolonged starvation.The glucose from glycogen is readily mobilized and is therefore a good source of energy for sudden, strenuous activity. Unlike fatty acids, the released glucose can provide energy in the absence of oxygen and can thus supply energy for anaerobic activity.


Glycogenesis Glycogenesis is the synthesis of glycogen from glucose. Glycogenesis mainly occurs in muscle and liver. Muscle glycogen provides a readily available source of glucose for glycolysis within the muscle itself. Liver glycogen functions to store and export glucose to maintain blood glucose between meals.


Liver and Muscle glycogen


Indirect contribution by muscle to maintain blood glucose levels

oAlanine and lactate transported from muscle are used for glucose production in liver by gluconeogenesis.oGlucose is poured in blood to maintain homeostasis.


Steps of Glycogenesis

1) Activation of Glucose2) Initiation3) Elongation 4) Glycogen branching


Activation of Glucose

Synthesis of glycogen from glucose is carried out by the enzyme glycogen synthase. This enzyme utilizes UDP-glucose as one substrate and the non-reducing end of glycogen as another. UDP-glucose, the glucose donor in the biosynthesis of glycogen, is an activated form of glucose.


Activation of Glucose (Contd.)UDP-glucose is formed from glucose-1-phosphate:

Spontaneous hydrolysis of the ~P bond in PPi (P~P) drives the overall reaction.

Cleavage of PPi is the only energy cost for glycogen synthesis (one ~P bond per glucose residue).


Activation of Glucose (Contd.)

Glucose 1-phosphate reacts with uridine triphosphate (UTP) to form the active nucleotide uridine diphosphate glucose (UDPGlc) and pyrophosphate. The reaction is catalyzed by UDPGlc pyro phosphorylase.

As in glycolysis, glucose is phosphorylated to glucose 6-phosphate, catalyzed by hexokinase in muscle and glucokinase in liver. Glucose 6-phosphate is isomerized to glucose 1-phosphate by Phosphoglucomutase.


2) Initiation

Glycogen synthase can add glucosyl residues only if the polysaccharide chain already contains more than four residues. Thus, glycogen synthesis requires a primer.This priming function is carried out by glycogenin, A protein composed of two identical 37-kd subunits, each bearing an oligosaccharide of alpha-1,4-glucose units.


Initiation (Contd.)

A glycosidic bond is formed between the anomeric C1 of the glucose moiety derived from UDP-glucose and the hydroxyl oxygen of a tyrosine side-chain of Glycogenin.UDP is released as a product.

Each subunit of glycogenin catalyzes the addition of eight glucose units to its partner in the glycogenin dimer. At this point, glycogen synthase takes over to extend the glycogen molecule.


3) Elongation

New glucosyl units are added to the nonreducing terminal residues of glycogen.

The activated glucosyl unit of UDP glucose is transferred to the hydroxyl group at a C-4 terminus of glycogen to form an α-1,4-glycosidic linkage.

In elongation, UDP is displaced by the terminal hydroxyl group of the growing glycogen molecule.

This reaction is catalyzed by glycogen synthase, the key regulatory enzyme in glycogen synthesis.


3) Elongation Both synthesis & breakdown of glycogen are spontaneous. If both pathways were active simultaneously in a cell, there would be a "futile cycle" with cleavage of one ~P bond per cycle (in forming UDP-glucose).To prevent this both pathways are reciprocally regulated

Glycogen Synthesis

UTP UDP + 2 P i

glycogen(n) + glucose-1-P glycogen(n + 1)

Glycogen Phosphorylase Pi


4) Glycogen branching

Glycogen synthase catalyzes only the synthesis of α-1,4 linkages. Another enzyme is required to form the α-1,6 linkages that make glycogen a branched polymer.Branching occurs after a number of glucosyl residues are joined in α-1,4 linkage by glycogen synthase. A branch is created by the breaking of an α-1,4 link and the formation of an α-1,6 link.


4) Glycogen branching(Contd.)

A block of residues, typically 7 in number, is transferred to a more interior site. The branching enzyme that catalyzes this reaction is quite exacting. The block of 7 or so residues must include the nonreducing terminus and come from a chain at least 11 residues long. In addition, the new branch point must be at least 4 residues away from a preexisting one.


4) Glycogen branching(Contd.)

Branching is important because it increases the solubility of glycogen. Furthermore, branching creates a large number of terminal residues, the sites of action of glycogen phosphorylase and synthase. Thus, branching increases the rate of glycogen synthesis and degradation.


Glycogenolysis (Degradation of glycogen)

Glycogen degradation consists of three steps: (1) The release of glucose 1-phosphate from

glycogen, (2) The remodeling of the glycogen substrate

to permit further degradation, and(3) The conversion of glucose 1-phosphate

into glucose 6-phosphate for further metabolism.


Enzymes required for glycogen degradation

The efficient breakdown of glycogen to provide glucose 6-phosphate for further metabolism requires four enzyme activities:one to degrade glycogen, two to remodel glycogen so that it remains a substrate for degradation, and one to convert the product of glycogen breakdown into a form suitable for further metabolism.


Enzymes required for glycogen degradation (contd.)

a) Phosphorylase- Glycogen phosphorylase, the key enzyme in glycogen breakdown, cleaves its substrate by the addition of orthophosphate (Pi) to yield glucose 1-phosphate. The cleavage of a bond by the addition of orthophosphate is referred to as phosphorolysis.



b) Transferase and Debranching enzyme- The Transferase shifts a block of three glucosyl residues from one outer branch to the other.

This transfer exposes a single glucose residue joined by an α -1,6-glycosidic linkage.

α-1,6-Glucosidase, also known as the debranching enzyme, hydrolyzes the α -1, 6-glycosidic bond, resulting in the release of a free glucose molecule.



c) Phosphoglucomutase- Glucose 1-phosphate formed in the phosphoroylytic cleavage of glycogen must be converted into glucose 6-phosphate to enter the metabolic mainstream. This shift of a phosphoryl group is catalyzed by Phosphoglucomutase.


Steps of degradation

Phosphorylase catalyzes the sequential removal of glucosyl residues from the nonreducing ends of the glycogen molecule (the ends with a free 4-OH group. Orthophosphate splits the glycosidic linkage between C-1 of the terminal residue and C-4 of the adjacent one.

1) Release of Glucose-1-P


Advantages of phosphoroylytic cleavageThe phosphoroylytic cleavage of glycogen is energetically advantageous because the released sugar is already phosphorylated. In contrast, a hydrolytic cleavage would yield glucose, which would then have to be phosphorylated at the expense of the hydrolysis of a molecule of ATP to enter the glycolytic pathway. An additional advantage of phosphoroylytic cleavage for muscle cells is that glucose 1-phosphate, negatively charged under physiological conditions, cannot diffuse out of the cell.


2) Remodeling of the glycogen substrate to permit further degradation

The α-1,6-glycosidic bonds at the branch points are not susceptible to cleavage by phosphorylase. Glycogen phosphorylase stops cleaving α -1,4 linkages when it reaches a terminal residue four residues away from a branch point.Because about 1 in 10 residues is branched, glycogen degradation by the phosphorylase alone would come to a halt after the release of six glucose molecules per branch.


2) Remodeling of the glycogen substrate to permit further degradation(Contd.)

Transferase shifts a block of three glycosyl residues from one outer branch to the other. This transfer exposes a single glucose residue joined by an α-1,6-glycosidic linkage.Debranching enzyme, hydrolyzes the α -1, 6-glycosidic bond, resulting in the release of a free glucose molecule.


2) Remodeling of the glycogen substrate to permit further degradation (contd.)

Transferase and α-1,6-glucosidase, remodel the glycogen for continued degradation by the phosphorylase. The free glucose molecule released by the action of debranching enzyme is phosphorylated by the glycolytic enzyme hexokinase. Thus, the Transferase and α-1,6-glucosidase convert the branched structure into a linear one, which paves the way for further cleavage by phosphorylase.


3) Conversion of glucose 1-phosphate to glucose 6-phosphate

Phosphoglucomutase converts glucose 1-phosphate into glucose 6-phosphate in a reversible reaction.Glucose 6-phosphate derived from glycogen can (1) be used as a fuel for muscle; (2) be converted into free glucose in the liver and subsequently released into the blood; (3) be processed by the pentose phosphate pathway.


Conversion of Glucose-6-P to free Glucose

The liver contains a hydrolytic enzyme, glucose 6-phosphatase, which cleaves the phosphoryl group to form free glucose and orthophosphate.

Glucose 6-phosphatase is absent from most other tissues. Consequently, glucose 6-phosphate is retained for the generation of ATP. The liver releases glucose into the blood during muscular activity and between meals to be taken up primarily by the brain and skeletal muscle.


Role of Pyridoxal –P in Glycogen degradation

Pyridoxal phosphate (PLP), a derivative of vitamin B6, serves as prosthetic group for Glycogen Phosphorylase.

p yrid o x a l p h o sp h a te (P L P )

NH

CO

P

O O

O

O H

C H 3

CH O

H 2


Role of Pyridoxal –P in Glycogen degradation(contd.)Pyridoxal phosphate (PLP) is held at the active site by a Schiff base linkage, formed by reaction of the aldehyde of PLP with the e-amino group of a lysine residue.In contrast to its role in other enzymes, the phosphate of PLP is involved in acid/base catalysis by Phosphorylase.

NH

CO

P

O O

O

O

CH 3

HC

H 2

N

(C H 2)4

Enz

H

+

Enzym e (Lys)-PLP Schiff base


Energy Expenditure for Glycogenesis


Energetics of Glycogenesis and Glycogenolysisone ATP is hydrolyzed incorporating glucose 6-phosphate into glycogen. The energy yield from the breakdown of glycogen is highly efficient. About 90% of the residues are phosphorolytically cleaved to glucose 1-phosphate, which is converted at no cost into glucose 6-phosphate. The other 10% are branch residues, which are hydrolytically cleaved. One molecule of ATP is then used to phosphorylate each of these glucose molecules to glucose 6-phosphate.


Regulation of Glycogen Metabolism

The principal enzymes controlling glycogen metabolism—glycogen phosphorylase and glycogen synthase—are regulated by allosteric mechanisms and covalent modifications due to reversible phosphorylation and dephosphorylation of enzyme protein in response to hormone action


Regulation of Glycogenesis(Allosteric modification)

Glycogen Synthase is allosterically activated by glucose-6-P.Thus Glycogen Synthase is active when high blood glucose leads to elevated intracellular glucose-6-P. It is useful to a cell to store glucose as glycogen when the input to Glycolysis (glucose-6-P), and the main product of Glycolysis (ATP), are adequate.


Regulation of Glycogenesis by covalent modification (phosphorylation):

The hormones glucagon and epinephrine activate G-protein coupled receptors to trigger cAMP cascades. Both hormones are produced in response to low blood sugar. Glucagon, which is synthesized by a-cells of the pancreas, activates cAMP formation in liver.Muscle cells lack Glucagon receptorsEpinephrine activates cAMP formation in muscle



Glycogen synthase exists in both phosphorylated or nonphosphorylated statesActive glycogen synthase a is dephosphorylated and inactive glycogen synthase b is phosphorylatedThe cAMP cascade results in phosphorylation of a serine hydroxyl of Glycogen synthase, which promotes transition to the inactive state.



Phosphorylation of Glycogen Synthase promotes the "b" (less active) conformation. The cAMP cascade thus inhibits glycogen synthesis. Instead of being converted to glycogen, glucose-1-P in liver may be converted to glucose-6-P, and dephosphorylated for release to the blood.

Glycogen Glucose

Hexokinase or Glucokinase

Glucose-6-Pase Glucose-1-P Glucose-6-P Glucose + Pi Glycolysis Pathway

Pyruvate Glucose metabolism in liver.


Role of Insulin In Glycogenesis

Insulin, produced in response to high blood glucose, triggers a separate signal cascade that leads to activation of Phosphoprotein Phosphatase.

This phosphatase catalyzes removal of regulatory phosphate residues from Glycogen Synthase enzyme.

Thus insulin antagonizes effects of the cAMP cascade induced by glucagon & epinephrine.

cAMP is hydrolyzed by phosphodiesterase, so terminating hormone action; in liver insulin increases the activity of phosphodiesterase.


Regulation of GlycogenolysisGlycogen Synthase and Glycogen Phosphorylase are reciprocally regulated, by allosteric effectors and by phosphorylation.The control of phosphorylase differs between liver & muscleIn the liver the role of glycogen is to provide free glucose for export to maintain the blood concentration of glucose; In muscle the role of glycogen is to provide a source of glucose 6-phosphate for glycolysis in response to the need for ATP for muscle contraction.


Regulation of Glycogenolysis by Allosteric modification Glycogen Phosphorylase in muscle is subject to allosteric regulation by AMP, ATP, and glucose-6-phosphate. A separate isozyme of Phosphorylase expressed in liver is less sensitive to these allosteric controls.AMP (present significantly when ATP is depleted) activates Phosphorylase, promoting the relaxed conformation.ATP & glucose-6-phosphate, which both have binding sites that overlap that of AMP, inhibit Phosphorylase

Thus glycogen breakdown is inhibited when ATP and glucose-6-phosphate are plentiful.


Regulation of Glycogenolysis by Covalent Modification (phosphorylation):

The cAMP cascade results in phosphorylation of a serine hydroxyl of Glycogen Phosphorylase, which promotes transition to the active state. The phosphorylated enzyme is less sensitive to allosteric inhibitors. Thus, even if cellular ATP & glucose-6-phosphate are high, Phosphorylase will be active. The glucose-1-phosphate produced from glycogen in liver may be converted to free glucose for release to the blood.With this hormone-activated regulation, the needs of the organism take precedence over needs of the cell.



The enzyme phosphorylase is activated by phosphorylation catalyzed by phosphorylase kinase (to yield phosphorylase a) andInactivated by dephosphorylation catalyzed by phosphoprotein phosphatase (to yield phosphorylase b), in response to hormonal and other signals.



Hormone (epinephrine or glucagon)

via G Protein (G-GTP)

Adenylate cyclase Adenylate cyclase (inactive) (active) catalysis

ATP cyclic AMP + PPi

Activation Phosphodiesterase

AMP

Protein kinase A Protein kinase A (inactive) (active) ATP

ADP

Phosphorylase kinase Phosphorylase kinase (P) (b-inactive) (a-active) Phosphatase ATP

Pi ADP Phosphorylase Phosphorylase (P) (b-allosteric) (a-active)

Phosphatase

Pi


Role of cAMP In Glycogen degradationIncreasing the concentration of cAMP activates cAMP-dependent protein kinase, which catalyzes the phosphorylation by ATP of inactive phosphorylase kinase b to active phosphorylase kinase a, which in turn, phosphorylates phosphorylase b to phosphorylase a.

In the liver, cAMP is formed in response to glucagon, which is secreted in response to falling blood glucose; muscle is insensitive to glucagon.

In muscle, the signal for increased cAMP formation is the action of norepinephrine, which is secreted in response to fear or fright, when there is a need for increased glycogenolysis to permit rapid muscle activity.


Role of Ca++ in glycogen degradation

Ca++ also regulates glycogen breakdown in muscle. During activation of contraction in skeletal muscle, Ca++ is released from the sarcoplasmic reticulum to promote actin/myosin interactions. The released Ca++ also activates Phosphorylase Kinase, which in muscle includes calmodulin as its d subunit.Phosphorylase Kinase is partly activated by binding of Ca++ to this subunit.


Role of Ca++ in glycogen degradation

Muscle phosphorylase kinase, which activates glycogen phosphorylase, is a tetramer of four different subunits-α, β ,Υ and δ . The α and β subunits contain serine residues that are phosphorylated by cAMP-dependent protein kinase. The δ subunit is identical to the Ca2+-binding protein calmodulin. The binding of Ca2+ activates the catalytic site of the subunit even while the enzyme is in the dephosphorylated b state; the phosphorylated a form is only fully activated in the presence of Ca2+.

Phosphorylase Kinase inactive

Phosphorylase Kinase-Ca++ partly active

P-Phosphorylase Kinase-Ca++ fully active


Role of Insulin in Glycogen degradationBoth phosphorylase a and phosphorylase kinase a are dephosphorylated and inactivated by protein phosphatase-1. Protein phosphatase-1 is inhibited by a protein, inhibitor-1, which is active only after it has been phosphorylated by cAMP-dependent protein kinase. Thus, cAMP controls both the activation and inactivation of phosphorylase.Insulin reinforces this effect by inhibiting the activation of phosphorylase b. It does this indirectly by increasing uptake of glucose, leading to increased formation of glucose 6-phosphate, which is an inhibitor of phosphorylase kinase.


Reciprocal regulation of Glycogen synthesis and degradationGlycogen Synthase & Phosphorylase activity are reciprocally regulatedAt the same time as phosphorylase is activated by a rise in concentration of cAMP (via phosphorylase kinase), glycogen synthase is converted to the inactive form; both effects are mediated via cAMP-dependent protein kinase .Thus, inhibition of glycogenolysis enhances net glycogenesis, and inhibition of glycogenesis enhances net glycogenolysis


Clinical SignificanceGlycogen Storage Diseases"Glycogen storage disease" is a generic term to describe a group of inherited disorders characterized by deposition of an abnormal type or quantity of glycogen in tissues, or failure to mobilize glycogen.


Glycogen Storage Diseases

Symptoms in addition to excess glycogen storage:

When a genetic defect affects mainly an isoform of an enzyme expressed in liver, a common symptom is hypoglycemia, relating to impaired mobilization of glucose for release to the blood during fasting.

When the defect is in muscle tissue, weakness & difficulty with exercise result from inability to increase glucose entry into Glycolysis during exercise.

Additional symptoms depend on the particular enzyme that is deficient.



TypeName Enzyme

DeficiencyClinical Features

0— Glycogen

synthaseHypoglycemia; hyperketonemia; early death

I

Von Gierke's disease

Glucose 6-phosphatase

Glycogen accumulation in liver and renal tubule cells; hypoglycemia; lactic acidemia; ketosis; hyperlipemia

II

Pompe’s Disease Lysosomal 14 and 16 glucosidase (acid maltase)

Accumulation of glycogen in lysosomes: juvenile onset variant, muscle hypotonia, death from heart failure by age 2; adult onset variant, muscle dystrophy



Type Name Biochemical defect

Clinical Features

III Limit dextrinosis, Forbe's or Cori's disease

Debranching enzyme

Fasting hypoglycemia; hepatomegaly in infancy; accumulation of characteristic branched polysaccharide

IV Amylopectinosis, Andersen's disease

Branching enzyme

Hepatosplenomegaly; accumulation of polysaccharide with few branch points; death from heart or liver failure in first year of life


Glycogen Storage DiseasesType Name Biochemical

defectClinical Features

V Myophosphorylase deficiency, McArdle's syndrome

Muscle phosphorylase

Poor exercise tolerance; muscle glycogen abnormally high (2.5–4%); blood lactate very low after exercise

VI Hers' disease Liver phosphorylase

Hepatomegaly; accumulation of glycogen in liver; mild hypoglycemia; generally good prognosis




VII Tarui's disease Muscle and erythrocyte phosphofructokinase 1

Poor exercise tolerance; muscle glycogen abnormally high (2.5–4%); blood lactate very low after exercise; also hemolytic anemia

VIII Liver phosphorylase kinase

Hepatomegaly; accumulation of glycogen in liver; mild hypoglycemia; generally good prognosis




IX Liver and muscle phosphorylase kinase

Hepatomegaly; accumulation of glycogen in liver and muscle; mild hypoglycemia; generally good prognosis

X cAMP-dependent protein kinase A

Hepatomegaly; accumulation of glycogen in liver


Glycogen Metabolism(Summary)Glycogen represents the principal storage form of carbohydrate in the body, mainly in the liver and muscle. Glycogen is synthesized from glucose by the pathway of glycogenesis.It is broken down by a separate pathway, glycogenolysis. Glycogenolysis leads to glucose formation in liver and lactate formation in muscle owing to the respective presence or absence of glucose 6-phosphatase. Cyclic AMP integrates the regulation of glycogenolysis and glycogenesis by promoting the simultaneous activation of phosphorylase and inhibition of glycogen synthase.Insulin acts reciprocally by inhibiting glycogenolysis and stimulating glycogenesis. Inherited deficiencies in specific enzymes of glycogen metabolism in both liver and muscle are the causes of glycogen storage diseases.

Glycogen metabolism

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