1. GLYCOGEN METABOLISM
Dr Vivek Joshi, MD

2. CONTENTS Introduction Biomedical importance
Glycogenesis (Glycogen Synthesis)
Glycogenolysis (Glycogen Breakdown)
Regulation of Glycogen Metabolism
Glycogen Storage Disease (GSD)

3. Glycogen Metabolism Glycogenesis- Synthesis of Glycogen
Glycogenolysis- Breakdown of Glycogen
Glycogen
Carbohydrate
Polysaccharide
Homopolysaccharide
Polymer of Glucose
Chain of Glucosyl units linked by α ( ) with α (1 - 6) branch every 8-10 residues.

4. Glycogen-Ideal storage form
Glucose residues in a linear sequence are linked to each other by a
1→4 glycosidic linkage
Branched hompolysaccharide of D-glucose
Branches every 8-12 residues along the chain, linked as
1→6 linkages
Branch

5. Single reducing end bound to Glycogenin
Glycogen –Storage Form
Single reducing end bound to Glycogenin
Nonreducing ends Gn
Single reducing end bound to Glycogenin 5

6. Glycogen breakdown yields Glucose
Brain &RBC’S-Dependent on Glucose
Replenish Glucose in between meals, sleep, Fasting and
Early Starvation
Muscle Glycogen-Energy
Liver Glycogen-Maintains Blood Glucose(12-14 hrs of
Fasting)
Source of Glucose in the body:
Diet-Inconsistent
Gluconeogenesis-SLOW
3. Glycogenolysis-Rapid release of Glucose
Liver Weight g
Muscle mass-35 Kg

7. LIVER Glycogen maintenance of blood glucose levels
Stores about gms of glycogen. Amounts to gms with water of hydration
Glycogen used for maintenance of blood glucose levels
Lasts about hrs
MUSCLE Glycogen as a body energy store
Stores about gm in an average adult
Not available for blood glucose maintenance due to absent Glucose 6-phosphatase
Provides energy during bursts of muscle activity

8. Source of Glucose in the body
Carbohydrates in diet
Sporadic
Frequency unreliable
Glycogen
Significant storage
Rapidly mobilized - fast response
Total hepatic storage-enough to maintain blood
glucose levels during a hr fast
Gluconeogenesis
Slow in reacting to a fall in blood glucose levels
Advantage of glycogen – it is there and can be quickly used. Enough to last many hours.

9. Glycogen synthesis OR Glycogenesis
Sites of Synthesis:
Most cells of the body significantly in the liver and the muscle
Sub cellular site: Cytosol
Steps:
Glucose uptake -GLUT2 and GLUT4
Conversion of Glucose to Glucose -6-P
Conversion of Glucose-6-P to UDP-Glucose
Elongation of chain: Glycogen Synthase (Glycogen Fragment/Glycogenin to Initiate Glycogen synthesis)
Introduction of branches: Branching enzyme
High Carbohydrate meal- Glucose uptake by Glucose transporters

10. GLUCOSE TRANSPORTERS (GLUT)
Name
Tissues
Km,Glucose
Functions
GLUT I
Most tissues
(Brain, Red cells)
~1 mM
Basal uptake of glucose
GLUT 2
Liver
Pancreatic beta cells
~15 mM
Uptake and release of glucose by the liver
Beta –cell Glucose sensor
GLUT 3
Basal uptake
GLUT 4
Skeletal muscle
Adipose tissue
~5mM
Insulin-stimulated glucose uptake ;
Stimulated by exercise in skeletal muscle
Normal Blood Glucose Level : mg/dl
4-5.5 mmol/L

11. Glycogen synthesis OR Glycogenesis
Branching enzyme [ Amylo -( ) (1 6) transglucosidase] transfers 6-8 residues to the neighbouring branch to create a branch point[ α ( ) linkage]
The above processes continues till a highly branched Glycogen is formed.
Rate limiting enzyme for synthesis of Glycogen- Glycogen Synthase

12. Glycogen synthesis OR Glycogenesis
Glucose uptake (GLUT2 in the Liver/ GLUT4 in the Muscle)
Formation of UDP-Glucose
Glucose
ATP
Hexokinase /Glucokinase
ADP
Glucose-6-P
Phosphoglucomutase
Glucose-1-P
UDP glucose transfers glucose to the nonreducing end of the growing chain of glycogen
UDP Glucose is essential for the synthesis of Glycoconjugates and other sugars
GLUT- Glucose Transporters,Facilitative bidirectional transport,
Five types
- GLUT-1:Brain,Kidney,Placenta-Uptake of Glucose
- GLUT-2: Liver,Kidney,Small Intestine,Pancreas- Rapid uptake and release of Glucose
- GLUT-3: Brain,Kidney ,placenta- Uptake of Glucose
- GLUT-4: Heart,Skeletal muscle,adipose tissue- Insulin stimulated uptake of Glucose
-GLUT-5: Small Intestine- Absorption of Glucose
Sodium dependent unidirectional transporter
-SGLT1&2:Small intestine &kidney-Active uptake of glucose from lumen of intestine and reabsorption of glucose in proximal tubule of kidney
against the concentration gradient
Glucokinase-High Km, Not Inhibited by Glucose-6-phosphate,Hepatic Tissues
Hexokinase-Low Km, Inhibited by Glucose-6-phosphate,Extrahepatic tissues
UTP
2Pi
PPi
Hydrolysis of PPi drives the reaction forwards
UDP-Glucose 12

13. Glycogen synthesis OR Glycogenesis
Glycogen Fragment/Glycogenin to Initiate Glycogen synthesis
Glycogenin
Glucose
Glucose
Glucose
Tyrosine
The core sequence is bound to a protein, Glycogenin 13

14. Elongation- Glycogen Synthase
Glycogenin/Pre Existing Glycogen Fragment ?
Glycogen Synthase
Branching Enzyme
UDP
UDP -Glucose
Branching Enzyme (Glucosyl 4:6 Transferase)
Elongation- Glycogen Synthase

15. Summary of Glycogen Synthesis

16. Glycogen Breakdown OR Glycogenolysis
NOT a reverse of Glycogenesis but a separate pathway
Site-Liver, Muscle
Glycogen degradation requires the activity of two
enzymes
Glycogen Phosphorylase
Debranching enzyme

17. Glycogenolysis Glycogen Phosphorylase
Removes Glucose residues from the non reducing end of Glycogen .
It utilises the cytosolic phosphate for the above process and releases glucose from glycogen as Glucose-1-PO4
The above process continues till 3-4 residues of Glucose remain from the branch point.
Debranching enzyme has dual activity- α (1 - 4) , α ( ) glucan transferase activity AND α ( ) transglucosidase activity.

18. Glycogenolysis Debranching Enzyme Dual Enzyme activity
[-(1 4) -(1 4) glucan transferase activity- Transfers 3 glucose residues to the neighbouring branch to expose the branch point
[Amylo (1 6) transglucosidase activity breaks the branch point to release free Glucose.
90% of Glucose released from Glycogen as Glucose-1-PO4 and 10% as free Glucose
Rate limiting enzyme for Glycogen Degradation- Glycogen Phosphorylase

19. Glycogenolysis ? GLUCOSE GLYCOGEN PHOSPHORYLASE DEBRANCHING ENZYMEPi
Glucose-1-PO4
Glycogenolysis

21. Regulation of Glycogen Metabolism
Allosteric regulation
Glycogen Synthesis stimulated at high levels of energy and substrate
Glycogen Degradation increased when energy and glucose supplies are low

22. Allosteric regulation- Glycogen Metabolism in the LIVER
Short Term Regulation
Allosteric regulation
Covalent modification
Long Term Regulation:
Induction and Repression

23. Allosteric regulation- Glycogen Metabolism in the MUSCLE

24. MAJOR HORMONES IN METABOLISM

25. INSULIN-ANABOLIC HORMONE

26. Glycogen Phosphorylase Glycogen Phosphorylase
GLUCAGON
Glycogen Phosphorylase
(Glycogen Breakdown)
ACTIVE
Glycogen Synthase
(Glycogen synthesis)
LESS ACTIVE
Glycogen Phosphorylase
(Glycogen Breakdown)
LESS ACTIVE
Glycogen Synthase
(Glycogen synthesis)
ACTIVE
INSULIN

27. INSULIN- Dephosphorylates key enzyme
Liver and Muscle
GLUCAGON
Liver
GLYCOGEN SYNTHASE
INSULIN- Dephosphorylates key enzyme
GLUCAGON/EPINEPHRINE- Phosphorylates key enzyme
GLYCOGEN PHOSPHORYLASE
Glucagon(Glucagon Receptors) and Epinephrine(Beta Adrenergic receptors) Phosphorylates key enzyme
GLUCAGON
Liver
EPINEPHRINE
Liver and Muscle
Ca++ and AMP
Muscle

28. The muscle does not have receptors for Glucagon
Covalent Modification- Glycogen Metabolism
The muscle does not have receptors for Glucagon

29. Covalent Modification- Glycogen Metabolism

30. Hormone independent Glycogen Phosphorylase activation in the Muscle
Ca+2 by nerve stimulation (short bursts of exercise)
AMP as a result of rapid ATP consumption

32. Glycogen Storage Diseases

33. Glycogen Storage Diseases (GSD)
Group of inherited disorders characterized by defective mobilization of NORMAL GLYCOGEN /Deposition of ABNORMAL GLYCOGEN
Classification based on the enzyme deficiency and affected tissue.
GSD can affect the liver, the muscles or both
About Eleven known types of GSD
In general, GSDs - Autosomal-recessive conditions EXCEPT, liver phosphorylase kinase deficiency (GSD IX).

34. GSD-Diagnosis Patient History (Individual's symptoms)
Physical Examination
Biochemical tests (CK-Level).
Occasionally, a muscle or liver biopsy is required to confirm the actual enzyme defect.
Keep an eye on what you will biopsy
i.e. Mc Ardle’s deficiency – Liver biopsy liver is normal – but needed muscle biopsy.

35. All recessive
Single enzyme deficiencies

36. GSD-TYPES Primarily Liver involvement- I,IV,VI
Primarily Muscle involvement- - II,V
Both liver and muscle-III
Adult onset-Mc Ardle’s(V)
Most common, Clinically significant end-organ GSD with significant morbidity-Type –I*
GSD with significant mortality-Type –II* 36

37. Liver, Skeletal and Cardiac muscle
GSD-TYPES
Clinicopathologic Category
Specific Type
Enzyme Deficiency
Hepatic Type
Type I
Von Gierke disease
Liver
Glucose-6-phosphatase
Generalized Type
Type II
Pompe disease
Liver, Skeletal and Cardiac muscle
Lysosomal glucosidase (acid maltase)
Myopathic Type
Type V
McArdle Syndrome
Skeletal muscle
Muscle phosphoylase

39. Glucose-6-P accumulates- Glycolysis in the muscle - Increase Lactate
Glucose uses up phosphate –AMP rise-Breakdown to uric acid-Increase uric acid
Hepatomegaly-Glucose-6-P and Glycogen accumulates-Highly osmotic
Lactate competes with uric acid for excretion-Increase uric acid
Hyperlipidemia

40. Glycogen Storage Disease Type I (Von Gierke’s disease)
Children with GSD I are unable to release glucose from liver glycogen- “FASTING HYPOGLYCEMIA”
If untreated this results in prolonged periods when their blood sugar level is too low
They present in early childhood with sweating, irritability, poor growth and muscle weakness
Liver enlargement -“HEPATOMEGALY” occurs due to excessive accumulation of Glucose-6-P AND glycogen (Cannot be broken down normally).

41. Treatment
Primarily consists of giving glucose drinks frequently during the day and, in most cases, continuously overnight through a tube passed down the nose into the stomach (a nasogastric tube)
As children get older, treatment with cornstarch, which releases glucose slowly into the gut, may be very effective.
With such intensive treatment most children do well and their symptoms improve as they reach adulthood.
Give regulated amount of glucose, not excess
Also give something that releases slowly

42. Can you correlate……..
A 4-year old girl is brought to the hospital OPD with the complaints of sweating,headache,fatigue,nausea and loss of weight with symptoms more severe in the morning before breakfast. On Clinical examination , the child had hepatomegaly with severe fasting hypoglycemia.Further studies reveal inherited enzyme deficiency.
Q1.What is your probable diagnosis ?
Q2. Which enzyme deficiency is responsible for the above clinical condition?
Q3. What is the cause of fasting hypoglycemia?
Von Gierke’s disease
Glucose -6-phosphatase
Glucose -6-phosphatase ,source of Liver Glucose in the body, so its deficiency will reduce glucose levels specially during the sleep .

43. Glycogen Storage Disease Type II (Alpha Glucosidase -“Acid maltase” deficiency, Pompe's Disease)
Some amount of glycogen is continuously degraded by the Lysosomal enzyme, acid maltase
Deficiency of the enzyme results in accumulation of glycogen vacuoles in the cytosol (liver, heart, muscle)
GSD II usually presents within the first months of life with severe muscle weakness and heart muscle involvement -“CARDIOMEGALY”.

44. No treatment has been found to prevent the progression of the most severe (infantile) form of this disorder
Affected children die from “HEART FAILURE ”, usually before the age of 18 months
There are however, milder forms of GSD II in which the heart is not affected and where symptoms do not develop until later in childhood or in adult life and the progression of the illness is slower

45. Q Which enzyme deficiency leads to the above condition?
Correlate….
A 12 month old girl shows slowly progressing muscle weakness involving her arms and legs and developed difficulty in breathing .Liver was enlarged and CT scan revealed CARDIOMEGALY.A muscle biopsy showed muscle degeneration with many enlarged prominent Lysosome filled with clusters of electron dense granules. Her parents were told that without treatment ,the child’s symptom would continue to worsen and likely result in death in 1-2 year. Enzyme replacement therapy was initiated.
Acid Maltase (Alpha1-4 Glucosidase) in the Lysosome that degrade Glycogen
Q Which enzyme deficiency leads to the above condition?