Dr Vivek Joshi, MD

Introduction

Biomedical importance

Glycogenesis (Glycogen Synthesis)

Glycogenolysis (Glycogen Breakdown)

Regulation of Glycogen Metabolism

Glycogen Storage Disease (GSD)

Branch

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

Glycogen-Ideal storage form

NO OSMOTIC PRESSURE HIGHLY BRANCHED-READILY BROKEN DOWN-HORMONES

&ENZYMES WATER SOLUBLE MORE FREE GLUCOSE RESIDUES-NON REDUCING END

Gn

Nonreducing ends

Single reducing end bound to Glycogenin

Single reducing end bound to Glycogenin

Glycogen –Storage Form

Glycogen breakdown yields Glucose

Stores about 50-100gms of glycogen. Amounts to 200-300gms with water of hydration

Glycogen used for maintenance of blood glucose levels

Lasts about 12-14 hrs

MUSCLE Glycogen as a body energy store

Stores about 400-500gm in an average adult Not available for blood glucose maintenance due to

absent Glucose 6-phosphatase Provides energy during bursts of muscle activity

Carbohydrates in dietSporadicFrequency unreliable

GlycogenSignificant storageRapidly mobilized - fast responseTotal hepatic storage-enough to maintain blood glucose levels during a 12-14 hr fast

GluconeogenesisSlow in reacting to a fall in blood glucose levels

Source of Glucose in the body

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

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 Most tissues ~1 mM Basal uptake

GLUT 4 Skeletal muscle Adipose tissue

~5mM Insulin-stimulated glucose uptake ; Stimulated by exercise in skeletal muscleNormal Blood Glucose Level : 70-100 mg/dl

4-5.5 mmol/L

GLUCOSE TRANSPORTERS (GLUT)GLUCOSE TRANSPORTERS (GLUT)

Branching enzyme [ Amylo -(1 4) (1 6) transglucosidase] transfers 6-8 residues to the neighbouring branch to create a branch point[ α (1 6) linkage]

The above processes continues till a highly branched Glycogen is formed.

Rate limiting enzyme for synthesis of Glycogen- Glycogen Synthase

Glucose uptake (GLUT2 in the Liver/ GLUT4 in the Muscle)Formation of UDP-Glucose

Hydrolysis of PPi drives the reaction forwards

Glucose

Glucose-6-P

Hexokinase /GlucokinaseATP

ADP

Glucose-1-P

Phosphoglucomutase

PPi

UTP

UDP-Glucose

2Pi

Glycogen synthesis OR Glycogenesis

Glycogenin Tyrosine Glucose Glucose Glucose

Glycogen Fragment/Glycogenin to Initiate Glycogen synthesis

Glycogen synthesis OR Glycogenesis

UDP -Glucose UDP

Glycogen Synthase

Branching Enzyme

Glycogenin/Pre Existing Glycogen Fragment

Elongation- Glycogen Synthase

?

Summary of Glycogen Synthesis

NOT a reverse of Glycogenesis but a separate pathway

Site-Liver, Muscle Glycogen degradation requires the activity

of two enzymes

Glycogen PhosphorylaseDebranching enzyme

Glycogen Breakdown OR 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 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

Glycogenolysis

Pi Glucose-1-PO4

GLYCOGEN PHOSPHORYLASE

DEBRANCHING ENZYME

DEBRANCHING ENZYME

GLUCOSE

Glycogenolysis

?

Allosteric regulation Glycogen Synthesis stimulated at high

levels of energy and substrateGlycogen Degradation increased when

energy and glucose supplies are low

Regulation of Glycogen Metabolism

MAJOR HORMONES IN METABOLISM

Liver

WATER SOLUBLE HORMONES IN METABOLISM

INSULIN-ANABOLIC HORMONE

GLUCAGON

INSULIN

Glycogen Phosphorylase

(Glycogen Breakdown)LESS ACTIVE

Glycogen Synthase(Glycogen synthesis)

ACTIVE

Glycogen Phosphorylase

(Glycogen Breakdown)ACTIVE

Glycogen Synthase(Glycogen synthesis)

LESS ACTIVE

GLUCAGON

INSULIN

GLYCOGEN SYNTHASE

GLYCOGEN PHOSPHORYLASE

INSULINLiver and Muscle

GLUCAGONLiver

GLUCAGONLiver

EPINEPHRINELiver and Muscle

Ca++ and AMPMuscle

INSULIN- Dephosphorylates key enzyme GLUCAGON/EPINEPHRINE- Phosphorylates key enzyme

Covalent Modification- Glycogen Metabolism

The muscle does not have receptors for

Glucagon

Covalent Modification- Glycogen Metabolism

Ca+2 by nerve stimulation (short bursts of exercise)

AMP as a result of rapid ATP consumption

Hormone independent Glycogen Phosphorylase activation in the

Muscle

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).

Glycogen Storage Diseases Glycogen Storage Diseases (GSD)(GSD)

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.

GSD-DiagnosisGSD-Diagnosis

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*

GSD-TYPES

Clinicopathologic Category

Specific Type Enzyme Deficiency

Hepatic TypeType I

Von Gierke disease Liver

Glucose-6-phosphatase

Generalized TypeType II

Pompe diseaseLiver, Skeletal and Cardiac

muscle

Lysosomal glucosidase (acid

maltase)

Myopathic TypeType V

McArdle SyndromeSkeletal muscle

Muscle phosphoylase

GSD-TYPES

•Hyperlipidemia

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).

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.

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?

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”.

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

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.

Q Which enzyme deficiency leads to the above condition?

Children with GSD III are often first diagnosed with a swollen abdomen due to a very large liver with large stores of normal glycogen

“Limit Dextrin” type of Glycogen (Glycogen structure with short outer branch and single glucose residue at alpha 1-6 of outer branch) deposit in the liver &muscle cells

Some children have problems with low blood sugars on fasting but this is not as common as in GSD I.

Treatment consists of a high protein diet and prevention of prolonged periods of fasting

Patients affected by this rare myopathy are unable to produce Glycogen phosphorylase, the enzyme involved in the cleavage of glycogen to glucose-1- phosphate and glucose during anaerobic exercise

The consequence is exercise-induced myalgia and fatigue

The disorder affects all skeletal muscle, which results in significant disability.

People with Mc Ardle's disease experience symptoms during anaerobic and sustained exercise.

These include fatigue and then pain occurring within a few minutes of exercise, which if continued, leads to muscle spasm known as a contracture.

Following severe exercise, muscle damage may occur leading to myoglobinuria. This is a dark discolouration of the urine and may be a warning sign for acute renal failure, which results from excessive muscle breakdown

A simple blood test will usually reveal raised levels of muscle creatine kinase (CK)

The diagnosis is confirmed by muscle biopsy, which shows an excess of glycogen and absence of muscle phosphorylase

In up to 85% of patients from Northern Europe, an abnormality in the gene encoding for muscle phosphorylase, called the R49X mutation, can be detected on a blood test

Regular aerobic exercise would increase the ability to switch to fat as the fuel source for the muscle

At the start of exercise, when pain occurs, exercise should be slowed down or stopped until the pain subsides (usually just for 30 seconds), then the exercise tried again

Continuing to exercise with intense pain would result in muscle damage and should be avoided.

A sensible diet, rich in protein, to help replace damaged muscle and avoiding excessive weight gain is important

Tourniquets should not be used during operative procedures

Affected women do not seem to be disadvantaged by pregnancy or childbirth. A normal childbirth is possible in women with McArdle's disease

A 35 year old man came to the hospital OPD with a history of limited capability to perform strenous exercise .Patient gave a history of muscle cramps and Myoglobinuria after strenuous exercise. Clinically patient was well developed and had no abnormality at rest. On investigation the patient was not hypoglycemic but ECG abnormalities were recorded.This condition is due to the deficiency in the activity of which enzyme?

Characterized by Mild Fasting Hypoglycemia with Hepatomegaly

Von Gierke’s v/s Her’s Disease Breakdown of Glycogen

Gluconeogenesis

Glucose-1-P

Glucose-6-P GLUCOSE