By: Oman Ph.

Glycogen Metabolism
2
By: Oman Ph.
3/30/2023

Structure of glycogen
Glycogen is homopolysaccaharides formed of branched α-D
glucose.
The main glycosidic bond is α1-4 linkage. Only at the branching
point, the chain is attached by α1-6 linkage.
Unlike starch the branching is frequent.
Location of Glycogen: is present mainly in cytoplasm of liver
and muscles.
Although almost 6-10% of the liver by weight is composed of
glycogen, its contribution in muscle by weight never exceeds
1%. Still, owing to the huge total muscle mass, the total
amount of glycogen stored in muscles is much higher than that
in liver.
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By: Oman Ph.
3/30/2023

Functions of Glycogen
Glucose cannot be stored as such within the cells
because it is osmotically active and at equivalent
amounts to glycogen, it will induce osmotic lysis of
the cell due to the uptake of considerable amounts of
water.
Liver glycogen maintain normal blood glucose
concentration especially during the early stage of
fasting (between meals). Liver glycogen is depleted
after 12-18 hours fasting.
Muscle glycogen serves as emergency store for its own
usage during exercise.
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By: Oman Ph.
3/30/2023

Dynamic nature of glycogen stores in liver. Glycogen molecule is regularly
build and degraded to store or supply glucose that furnishes body energy.
5
PPi
UTP
UDP
Glycogen (Glucose)n+1
UDP-Glucose
Glucose-1-P
Pi
Glucose-6-P
2 Pi
Glycogen
(Glucose)n
Glycogen
(Glucose)n
Glycogen
Synthase
Glycogen
Phosphorylase
Phosphoglucomutase
UDP-Glucose
Pyrophosphorylase
Pyrophosphatase
By: Oman Ph.
3/30/2023

1. Synthesis of glycogen (glycogenesis)
It is the formation of glycogen in liver and muscle
Substrates for glycogen synthesis:
1. In liver: a) Blood glucose
b) Other hexoses; fructose & galactose
c) Non-carbohydrate sources; glycerol and lactate.
These compounds (non-carbohydrate sources) first converted
in to glucose by gluconeogenesis.
2. In muscle: Blood glucose only.
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By: Oman Ph.
3/30/2023

Glycogenesis…
Glycogen(n residues) + UDP-glucose glycogen(n +1 residues) + UDP
A branching enzyme transfers a segment from the end of a
glycogen chain to the C6 hydroxyl of a glucose residue of
glycogen to yield a branch with an α(1→6) gycosidic linkage.
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By: Oman Ph.
3/30/2023

Glycogen catabolism (breakdown): glycogenolysis
Enzymes of glycogenolysis
Glycogen Phosphorylase
Glycogen Debranching Enzyme
Phosphoglucomutase
Glycogen Phosphorylase catalyzes phosphorolytic cleavage of
the α (1→4) glycosidic linkages of glycogen, releasing
glucose-1-phosphate as reaction product.
Glycogen(n residues) + Pi → glycogen (n–1 residues) +
glucose-1- phosphate
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By: Oman Ph.
3/30/2023

Cont…
Liver Glucose-6-phosphatase catalyzes the following, essential to
the liver's role in maintaining blood glucose:
Glucose-6-phosphate + H2O glucose + Pi
Muscle tissues lack this enzyme that is why muscles do not
contribute glucose for circulation . Glucose -6-phosphate in
muscle enters glycolysis.
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By: Oman Ph.
3/30/2023

10
By: Oman Ph.
3/30/2023

Fates of glycogen
In liver – The synthesis and breakdown of
glycogen is regulated to maintain blood
glucose levels.
In muscle - The synthesis and breakdown of
glycogen is regulated to meet the energy
requirements of the muscle cell.
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By: Oman Ph.
3/30/2023

Regulation of glycogenesis and glycogenolysis
There is coordinated regulation of glycogenesis and
glycogenolysis i.e. conditions leading to stimulation of
glycogenolysis inhibitng at the same time glycogenesis and
vise versa.
During fasting glycogenolysis is stimulated and glycogenesis
is inhibited. This provides blood glucose from liver glycogen.
After meal part of absorbed glucose (40%) goes to general
circulation to be utilized. The remaining (60%) is converted
into glycogen in liver.
So after meal, glycogenesis is stimulated and glycogenolysis is
inhibited.
12
By: Oman Ph.
3/30/2023

Cont…
After meal→ ↑ blood glucose →insulin secretion =
stimulates glycogen synthase and inhibit glycogen
phosphorylase. Hence, glycogenesis is favored.
During fasting→ ↓blood glucose→ Glucagon ,
Epinephrine,… → Stimulate glycogen phosphorylase
and inhibit glycogen synthase.
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By: Oman Ph.
3/30/2023

14
By: Oman Ph.
3/30/2023

Infusion of glucose into mouse liver
15
By: Oman Ph.
3/30/2023

Covalent modification of glycogen metabolism
enzymes…???
3/30/2023 By: Oman Ph. 16

Glycogen storage disease
“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 the tissues.
It is also called glycogenosis.
They are mainly due to deficiency of Enzymes of
glycogen metabolism.
There are 8 types of glycogen storage diseases (read).
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By: Oman Ph.
3/30/2023

Glycolysis
 Catabolism of glucose to provide ATPs and NADH
molecules
 Also provides building blocks for anabolic pathways.
 Glycolytic cleavage of glucose or other hexoses into
pyruvate/lactate forms the backbone of the carbohydrate
metabolism.
 However, because it can run independent of
mitochondria/O2, it is mainly called anaerobic oxidation of
glucose.
3/30/2023 18
By: Oman Ph.

Cont…
Can catabolize sugars other than glucose:
Fructose ----> 2 glyceraldehyde- 3-phosphate
Galactose --> glucose 1-phosphate --> glucose - 6-
phosphate
mannose ---> mannose 6-phosphate --> fructose -6-
phosphate
The enzymes of glycolysis pathway are also utilized in the
opposite direction to synthesize glucose in gluconeogenesis.
Therefore, glycolysis is an amphipathic pathway.
3/30/2023 19
By: Oman Ph.

Intracellular site & tissue distribution
• Glycolysis occurs in the cell cytoplasm of all tissues of the body. It is
especially important in the tissues with unutilizability or short supply
of oxygen.
• Mammalian erythrocyte (RBCs): The red cells are devoid of
mitochondria and depend on glycolysis as the main source of energy.
RBCs are unique in that about 90% of its total energy requirement is
provided by glycolysis.
• Contracting muscles: Due to partial occlusion of blood vessels by the
muscular contraction, there is a decrease in oxygen availability that
creates temporary anaerobic conditions. Most of the energy of the
rapidly contracting muscles comes from the anaerobic oxidation of
glucose through glycolysis.
3/30/2023 20
By: Oman Ph.

Cont…
Cornea, lens and some parts of retina: They have a
limited blood supply and lack of mitochondria.
Therefore, needed energy is derived from glycolysis.
Stages of glycolysis
1. Energy requiring stage
- One molecule of glucose is converted in to 2
molecules of gyceraldehyde-3-phosphate
-These steps require 2 molecules of ATP.
2. Energy producing stages
-These steps produce ATP molecules
3/30/2023 21
By: Oman Ph.

Activation of Glucose
Circulating blood glucose metabolically inert unless it is activated to
glucose-6-phosphate (Glu-6-P) inside the cells. Once
phosphorylated, glucose is trapped inside the cells because cell
membrane is impermeable to it. The activation takes place with the
help of tissue-specific isoenzymes.
3/30/2023 22
By: Oman Ph.

Hexokinase and Glucokinase
Hexokinase
• Found in the cytosol of most tissues
• Low specificity: it’s a common for hexoses
• Low Km: high affinity for glucose
• Inhibited by Glucose-6-phosphate
Glucokinase
• Found in the Liver and pancreatic β-cells
• Also a ‘hexokinase’
• High specificity for glucose
• High Km and not inhibited by glucose-6-phosphate
3/30/2023 23
By: Oman Ph.

Cont…
3/30/2023 24
By: Oman Ph.

Cont….
3/30/2023 25
By: Oman Ph.

Energy from Glycolysis
Under anaerobic conditions:
ATP invested in the activation phase
• One ATP in the activation of glucose to Glu-6-P.
• One ATP in the activation of Fru-6-P to Fru-1,6-DiP.
Total ATP invested = 2 ATP
ATP Gained:
• 2 ATP by substrate level phosphorylation from 1,3-
diphosphoglycerate.
• 2 ATP from substrate level phosphorylation from
phosphoenol pyruvate.
Net ATP gained = 4 ATP gained - 2 ATP lost = 2 ATP
3/30/2023 26
By: Oman Ph.

Cont…
During rapid contraction (exertion), muscle cells require a
very fast supply of energy in the form of ATP. The extra
demand of ATP is met by glycolysis because substrate level
phosphorylation is 100-times faster than oxidative
phosphorylation and oxygen is low because of vessels
occlusion by exertion.
3/30/2023 27
By: Oman Ph.

Under aerobic conditions:
Total ATP invested in the activation phase = 2 ATP, as
under anaerobic conditions.
Total ATP gained = 8 or 10 ATP:
a) 4 ATP (obtained by substrate level phosphorylation)
b) 2NADH, H+ (produced from oxidation of glyceraldehyde-3-
phosphate); One NADH can give rise to either two or three
ATP through respiratory chain depending upon which shuttle
system is used for its transfer across the mitochondrial
membrane ,therefore, 2 X 2 or 3 ATP = 4 or 6 ATP.
Net ATP gained : 8 ATP – 2 ATP = 6 ATP, Or, 10
ATP – 2 ATP = 8 ATP
3/30/2023 28
By: Oman Ph.

Special features of Glycolysis in RBCs
Mature RBCs contains no mitochondria, thus they depend
up on only glycolysis for energy production.
Lactate is always the end product.
The RBCs have ability to form 2,3-bisphosphogycerate
(2,3-BPG) through what is called Rapoport-Luebering
cycle.
2,3-BPG decrease affinity of hemoglobin to oxygen, thus
good tissue oxygenation.
3/30/2023 29
By: Oman Ph.

Fig. Rapoport-Luebering cycle
3/30/2023 30
By: Oman Ph.

Significance of 2,3-BPG
It decreases the affinity of hemoglobin (Hb) to O2 to
help dissociation and unloading of O2 in tissue capillaries.
Therefore, the level of 2,3-BPG increases markedly under
hypoxic conditions (high altitudes, stagnant hypoxia and
anemic hypoxia).
Red blood cells have a high concentration of 2,3-BPG (4-5
mM, equimolar to Hb) in contrast to presence in trace
amounts in other cells.
3/30/2023 31
By: Oman Ph.

Biological functions of Glycolysis
Energy production
Oxygenation of tissue: through 2,3-BPG formation
that decreases the affinity of hemoglobin to oxygen
Provides important intermediates for synthesis of
molecules
3/30/2023 32
By: Oman Ph.

Fig. Function of Glycolysis
3/30/2023 33
By: Oman Ph.

Fate of pyruvate
1. lactate production in the absence of oxygen/mitochondria
This reaction helps for regeneration of NAD+.
3/30/2023 34
By: Oman Ph.

Conversion of lactate back to glucose
The Cori cycle involves the utilization of lactate,
produced by glycolysis in non-hepatic tissues,
(such as muscle and erythrocytes) as a carbon
source for hepatic gluconeogenesis. In this way the
liver can convert the anaerobic byproduct of
glycolysis, lactate, back into more glucose for
reuse by non-hepatic tissues.
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By: Oman Ph.

3/30/2023 36
By: Oman Ph.

2. Conversion to ethanol
It is fermentation by yeast enzymes
Fig. general fate of pyruvate
3/30/2023 37
By: Oman Ph.

3) entry into citric acid cycle in the mitochondria
3/30/2023 38
By: Oman Ph.

Reversibility of glycolysis
 Reversible reactions means that the same enzymes
catalyze the reaction in both direction.
 All reactions of glycolysis except three are reversible.
 Irreversible reactions are sites of regulation of glycolysis.
Pasteur effect –is the inhibition of glycolysis in the
presence of oxygen. Aerobic condition of glucose produce
increase amount of ATP and citrate-those inhibit PFK-
1=inhibition of glycolysis.
3/30/2023 39
By: Oman Ph.

Regulation of glycolysis
• The rate of glycolysis is regulated by controlling the
three irreversible enzymes.
• Glucokinase (hexokinase), Phosphofructokinase-1, and
pyruvate kinase.
• These enzymes catalyze what is called committed
reactions of the pathway.
1. Hormonal regulation
a. Insulin: stimulates synthesis of all key enzymes of
glycolysis
b. Glucagon: inhibits the activity of all key enzyme of
glycolysis.
3/30/2023 40
By: Oman Ph.

Cont..
2. Energy regulation
a) High level of ATP inhibits PFK-1 and pyruvate
kinase
b) High level of ADP and AMP stimulate PFK-1.
3. Substrate and product regulation
a) Glucose-6-phosphate inhibits hexokinase (and not
glucokinase)
b) Citrate inhibits PFK-1
c) Fructose 1,6-bisphosphate stimulates pyruvate kinase
d) Fructose 2,6-bisphosphate stimulate PFK-1.
3/30/2023 41
By: Oman Ph.

Fig. Regulation of Glycolysis
3/30/2023 42
By: Oman Ph.

In vitro inhibition of glycolysis
Arsenate and iodoacetate: inhibit glycolysis by inhibiting
glyceraldeyde-3-phosphate dehydrogenase enzyme.
Fluoride-inhibits enolase enzyme-clinical laboratories use
fluoride to inhibit glycolysis by adding it to the blood
before measuring blood glucose.
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By: Oman Ph.

Clinical aspects of glycolysis
• There are many diseases associated with impaired
glycolysis. They include
1. Pyruvate kinase (PK) deficiency
• This leads of excessive hemolysis of RBCs
leading to hemolytic anemia.
•Genetic deficiency of PK enzyme causes decrease
the rate of glycolysis and decrease production of
ATP.
2. Hexokinase deficiency
It leads to hemolytic anemia due to decrease ATP
production.
3/30/2023 44
By: Oman Ph.

Lactic acidosis
• It is the lowered blood PH and bicarbonate levels due to
increase blood lactate above normal level.
• Causes- increase formation as in severe muscular
exercise ( increased glycolysis) or decrease utilization in
case of lack of oxygen.
3/30/2023 45
By: Oman Ph.

The TCA cycle

Pyruvate dehydrogenase reaction & control
mechanism
• Glycolysis occurs in the cytosol of cells and ends up with pyruvate in
aerobic conditions.
• Pyruvate should enter the mitochondrion to be metabolized further.
• The inner membrane is the major permeability barrier of the mitochondria.
• How does the pyruvate get into the mitochondrion from the cytosol?
• To get from intermembrane space to matrix involves pyruvate translocase
(symporter that also moves H+ into matrix).
• The matrix contains Pyruvate Dehydrogenase, enzymes of Krebs Cycle,
and other pathways, e.g., fatty acid oxidation & amino acid metabolism.
3/30/2023 By: Oman Ph. 47

Cont…
Pyruvate Dehydrogenase catalyzes oxidative decarboxylation of
pyruvate, to form acetyl-CoA.
Pyruvate Dehydrogenase: a large complex containing
multienzyme .
Oxidative decarboxylation of pyruvate is a preparatory stage for
Krebs' cycle and involves removal of CO2 and hydrogen from
pyruvate to be oxidized into acetyl-CoA
3/30/2023 By: Oman Ph. 48

Fig. Oxidative decarboxylation of pyruvate
3/30/2023 By: Oman Ph. 49

Cont…
Acetyl CoA, a product of the Pyruvate Dehydrogenase reaction,
is a central compound in metabolism.
The "high energy" thioester linkage makes it an excellent donor
of the acetate moiety.
Oxidative decarboxylation of pyruvate to acetyl COA produces
one molecule of NADH. This produces 3 ATP molecule
through respiratory chain Phosphorylation.
3/30/2023 By: Oman Ph. 50

Regulation of Pyruvate dehydrogenase
• Pyruvate Dehydrogenase exist in two form. Phosphorylated
(inactive) and dephosphorylated (active) form.
• Factors stimulating Pyruvate Dehydrogenase
pyruvate, COASH, NAD+, insulin hormone
• Factors inhibiting Pyruvate Dehydrogenase
NADH, ATP, acetyl CoA
• In vitro inhibition of Pyruvate Dehydrogenase
a) Arsenic
b) Thiamin deficiency
3/30/2023 By: Oman Ph. 51

Origin of the acetyl group that converted in to acetyl-CoA
3/30/2023 By: Oman Ph. 52

Metabolic fates of acetyl COA
glucose-6-P
Glycolysis
pyruvate
fatty acids
acetyl CoA ketone bodies
cholesterol
oxaloacetate citrate
Krebs Cycle
3/30/2023 By: Oman Ph. 53

Location & purposes of TCA cycle
• TCA cycle also known Krebs’ cycle or citric acid cycle(CAC)
or catabolism of acetyl CoA.
• It is a serious of reactions in which acetyl CoA is oxidized into
Co2, H2O and energy.
• Location: matrix of mitochondria.
Purpose: Yields reduced coenzymes (NADH and FADH2) and
some ATP (2).
3/30/2023 By: Oman Ph. 54

Reactions of the TCA cycle
• Composed of 8 reactions
• 4 carbon intermediates are regenerated
• 2 molecules of CO2 released.
• Most of energy stored as NADH and FADH2.
3/30/2023 By: Oman Ph. 55

3/30/2023 By: Oman Ph. 56

Cont…
3/30/2023 By: Oman Ph. 57

Cont…
Net reaction for citric acid cycle:
acetyl CoA + 3NAD+ + FAD+ + GDP+ Pi ---> HS-CoA +
3NADH + FADH2 + GTP + 2CO2
Energy Budget so far from 1 molecule of glucose is given below
in the table
Glycolysis 2ATP 2NADH
Prep. step 2NADH
TCA cycle 2GTP/2ATP 6NADH &2
FADH2
Total 4ATP 10NADH &2
FADH2
3/30/2023 By: Oman Ph. 58

ATP production from a mole of glucose
Substrate level
phosphorylation
Oxidative phosphorylation
Glycolysis 2 ATP 6/4 ATP equivalents
Prep. step _ 6 ATP equivalents
TCA cycle 2 ATP 18 ATP equivalents + 4 ATP
equivalents
Total 4 ATP 34 ATP equivalents
Total ATP = 38/ 36
3/30/2023 By: Oman Ph. 59

3/30/2023 By: Oman Ph. 60

Anaplerotic reactions
 Removal of any of the intermediates from the TCA cycle
removes the 4 carbons that are used to regenerate oxaloacetate
during each turn of the cycle. With depletion of oxaloacetate, it
is impossible to continue oxidizing acetyl CoA.
 Reactions that serves to maintain the operation of the central
pathways of metabolism and which allows the operation and
coordination of other metabolic pathways that enter and leave
the central pathways.
3/30/2023 By: Oman Ph. 61

Fig: Efflux of intermediates from the TCA cycle
3/30/2023 By: Oman Ph. 62

Regulation of TCA cycle
TCA is regulated through the key enzymes (citrate synthase,
isocitrate dehydrogenase, α-ketoglutarate dehydrogenase) and
availability of oxygen.
1) Citrate synthase
Activated by ADP
Inhibited by high NADH, succinyl-CoA, citrate, ATP
2) Isocitrate dehydrogenase
Activated by high [Ca2+] and high [ADP]
Inhibited by high [NADH]
3) α-ketoglutarate dehydrogenase
Activated by high [Ca2+]
Inhibited by high [NADH] and high [succinyl CoA]
3/30/2023 By: Oman Ph. 63