GLYCOGEN METABOLISM
Learning objectives:
Describe composition and glycosidic bonds in
glycogen
Describe the biochemical pathway of glycogen
synthesis
Describe the biochemical pathway of
glycogenolysis
Discuss regulation of glycogen metabolism
Glycogen
…
..
CH
2O
H
H
H
O
O
OH
H
H
CH
2O
H
H
H
O
O
OH
OH
H
H
O
OH
CH
2O
H
H
H
α-1,4
H
H
O
OH
H
H
O
OH
CH
2O
H
H
α-1,4
H
H
O
OH
H
H
H
O α-1,6
CH2OH
CH2OH
O
H
…..
O
H
OH
H
H
OH
O
H
H
O
H
H
OH
H
H
OH
O
H
O
H
OH
H
H
OH
O
H
H
O
CH2OH
CH2OH
CH2OH
CH2
H
H
H
OH
H
H
OH
O
O
H
OH
H
H
OH
O
H
H
O
OH
H
H
OH
H
H
OH
α-1,4
CH2OH
CH2
O
H
O
H
OH
H
H
OH
α-1,4
O
H
H
O
H
H
OH
H
H
OH
α-1,4
Glycogen is a branched homopolysaccharide composed of α-D-glucose units
bound by α-1,4 and (at branch points) α-1,6 glycosidic bonds.
On average, there are branches for every 8-10 glycosyl residues.
….
Glycogen
…
..
CH
2O
H
H
H
O
O
OH
H
H
CH
2O
H
H
H
O
O
OH
OH
H
H
O
OH
CH
2O
H
H
H
α-1,4
H
H
O
OH
H
H
O
OH
CH
2O
H
H
α-1,4
H
H
O
OH
H
H
H
O α-1,6
CH2OH
CH2OH
O
H
…..
O
H
OH
H
H
OH
O
H
H
O
H
H
OH
H
H
OH
O
H
O
H
OH
H
H
OH
O
H
H
O
CH2OH
CH2OH
CH2OH
CH2
H
H
H
OH
H
H
OH
O
O
H
OH
H
H
OH
O
H
H
O
OH
H
H
OH
H
H
OH
α-1,4
CH2OH
CH2
O
H
O
H
OH
H
H
OH
α-1,4
O
H
H
O
H
H
OH
H
H
OH
α-1,4
A single molecule can have a molecular mass of up to 108 Da with
more than 500,000 glucosyl residues.
Glycogen forms intracellular glycogen granules in the cytoplasm.
….
Electron micrograph of a section of a liver cell showing glycogen deposits
as accumulations of electron dense particles (arrows).
Glycosyl residue attached by
an α-1,6 glycosidic bond
Glycosyl residue at a
non-reducing end
Glycosyl units are attached
and mobilized from the
reducing ends
Glycogen is an intracellular storage form of readily available glucose
Main stores of glycogen in the human body:
Liver - Approximately 100 g or 10% of the fresh weight
Muscle - Approximately 400 g or 1-2% of the fresh weight
Most other cells have small amounts of glycogen stored
LIVER
Glycogen
Glucose 6-P
G6Pase
MUSCLE
Glycogen
Glucose 6-P
Glucose
GLYCOLYSIS
Blood glucose
Sources of blood glucose after a meal
mM glucose
8
4
Meal
Glycogen
Gluconeogenesis
8
16
24
Hours
2
7
Days
30
Glycogen synthesis
Glycogenesis
Glycogen is synthesized from molecules of α-D-glucose.
Synthesis occurs in the cytosol
Synthesis requires energy
ATP for phosphorylation of glucose
UTP for generating an activated form of glucose: UDP-glucose
Glycogen synthesis - Glycogenesis
ATP
ADP
UTP
Pyrophosphatase
PPi
2 Pi + H2O
Glycogenn
Glucose
Hexokinase/Glucokinase
Glucose 6-phosphate
Phosphoglucomutase
Glucose 1-phosphate
UDP-glucose pyrophosphorylase
UDP-glucose
Glycogen synthase
Glycogenn+1
Branching enzyme
Glycogenn+1 with an additional branch
CH2OH
O
H
H
OH
H
OH
H
CH2OPO32-
OH
Glucose
O
H
H
OH
+ ATP
H
OH
Glucokinase
Hexokinase
H
+ ADP
H
OH
OH
H
OH
Glucose 6-phosphate
Same reaction, same enzymes, and same regulation as in glycolysis
Irreversible
Hexokinase
-
Glucose 6-phosphate (low phosphofructokinase activity)
Glucokinase
+
+
High blood glucose (release from GKRP, High Km)
Insulin stimulates gene transcription (only in liver)
Phosphoglucomutase
Ser
OPO32-
CH2OPO32O
H
H
OH
CH2OPO32O
H
Ser
OH
H
OH
H
H
OH
H
OH
H
OH
Glucose 6-phosphate
H
OPO32-
OH
H
CH2OH
OH
O
H
Glucose 1,6-bisphosphate
H
OH
Ser
H
OPO32-
OH
OPO32-
H
H
OH
Glucose 1-phosphate
CH2OH
O
H
H
OH
O
H
OPO32-
OH
H
O-
UDP-glucose pyrophosphorylase
O
H
O
H
H
OH
O-
UDP-glucose
O
O
O – P – O – P – O - uridine
OH
O-
UTP
CH2OH
H
OH
O-
OH
Glucose 1-phosphate
H
O
O- – P - O – P – O – P – O - uridine
+
H
O
O-
+
O
O- – P – O – P – OO-
O-
Pyrophosphate (PPi)
O
O
Pyrophosphatase
O- – P – O – P – O O-
+ H2O
2 Pi
O-
Pyrophosphate (PPi)
Glucose 1-phosphate + UTP
PPi + H2O
Glucose 1-phosphate + UTP + H2O
NB: Irreversible reaction
UDP-glucose + PPi
2 Pi
UDP-glucose + 2 Pi
The irreversible hydrolysis of pyrophosphate drives the synthesis of
UDP-glucose
CH2OH
CH2OH
O
H
H
OH
H
O
H
H
OH
O-
H
OH
O
O – P – O – P – O - uridine
OH
O
H
+
H
H
O-R
HO
O-
α-1,4
H
UDP-glucose
OH
Glycogen (n residues)
Glycogen synthase
CH2OH
CH2
O
O
O- – P – O – P – O - uridine
O
H
H
OH
+
H
HO
O-
O
H
H
OH
H
H
OH
α-1,4
OH
UDP
O
H
H
OH
Glycogen (n+1 residues)
O-R
α-1,4
Priming of glycogen synthesis
Glycogen synthase can NOT add glucosyl residues to free glucose
or to oligosaccharides of less than 8 glucosyl residues
Priming is catalyzed by the protein GLYCOGENIN
The first glucosyl residue is attached in an O-glycosidic linkage
to the hydroxyl group of tyrosine of Glycogenin itself
7 additional residues are attached by glycogenin
Glycogenin remains attached to the reducing end of the glycogen
molecule
8 UDP-glucose
+
HO
Tyr
Glycogenin
Glycogenin
O
Tyr
Glycogenin
…
Non-reducing end
Cleaveage of α-1,4 bond
“Branching enzyme”
Amylo-α(1,4) → α(1,6)-transglucosidase
Non-reducing ends
α-1,6
bond
…
Stoichiometry
Glucose + ATP + UTP + H2O + Glycogenn
Glycogenn+1 + ADP + UDP + 2 Pi
→
Degradation of glycogen
Glycogenolysis
Occurs in cytoplasm
Major product is glucose 1-phosphate from breaking α-1,4 bonds
Minor product is glucose from breaking α-1,6 bonds
Glucose 1-phosphate : Glucose ≈ 10:1
Glycogen synthesis - Glycogenesis
Pi
Glycogenn
Glycogen phosphorylase
Glucose 1-phosphate
Glycogenn-1
Phosphoglucomutase
…
Glucose 6-phosphate
H2O
G6Pase
Glucose
Pi
Glycolysis
Glycogen with branch
H2O
Glucose
“Debranching
enzyme”
Glycogen with
one less branch
CH2OH
O
O
H
O- – P – OH
O-
H
OH
+
CH2OH
CH2
H
H
H
OH
O
HO
O
H
H
O
α-1,4
H
OH
O
H
H
H
H
OH
H
H
OH
O-R
α-1,4
H
OH
Phosphate
Glycogen with n residues
Glycogen phosphorylase
O
H
H
OH
H
H
OH
+
H
OH
Glucose 1-phosphate
O
H
H
OPO32-
OH
CH2OH
CH2
CH2OH
H
HO
O
H
H
O
H
H
OH
H
H
OH
α-1,4
H
OH
Glycogen with n-1 residues
O-R
Lys
N
H
C
2-O
OH
3PO-CH2
N+
H
CH3
Pyridoxal phosphate is a coenzyme for the phosphorylase
reaction.
Pyridoxal phosphate is bound to a nitrogen of a lysyl
residue of glycogen phosphorylase
The phosphate of pyridoxal phosphate exchanges protons with
the phosphate reactant, which allows the reactant to donate a
proton to the oxygen atom on carbon 4.
CH2OH
Phosphoglucomutase
Ser
OPO32-
O
H
H
OH
H
OPO32-
OH
CH2OPO32O
H
Ser
OH
H
OH
H
H
H
OH
Glucose 1-phosphate
H
OPO32-
OH
H
OH
CH2OPO32-
Glucose 1,6-bisphosphate
O
H
H
OH
Ser
OPO32-
OH
H
H
OH
H
OH
Glucose 6-phosphate
CH2OPO3
O
H
H
OH
OH
2-
H
H
Glucose-6phosphatase
(G6Pase)
+ H2O
OH
H
OH
Glucose 6-phosphate
CH2OH
O
H
H
OH
OH
H
+ Pi
H
OH
H
OH
Glucose
Same reaction as in gluconeogenesis
Occurs in endoplasmic reticulum and involves a
glucose 6-phosphatase transporter and a catalytic subunit
The catalytic subunit is regulated at the level of transcription
Glycogen phosphorylase stops when 4 glucosyl units
remain on each chain from a branch point
a’
b’
c’
α-1,6
bond
d’
a
Oligo-α(1,4)→α(1,4)-glucan
transferase
(debranching enzyme)
a’
b’
d’
Glucose
+
a’
c
d
e
d’
c’
Amylo-α(1,6)-glucosidase
(debranching enzyme)
b
a
b
c
…
α-1,6
bond
d
e
…
H2O
b’
c’
a
b
c
d
e
…
Approximate Stoichiometry
Glycogenn+11 + 10 Pi + H2O →
Glycogenn + 10 Glucose 6-phosphate + Glucose
LIVER
Glycogen
Glucose 6-P
G6Pase
MUSCLE
Glycogen
Glucose 6-P
Glucose
GLYCOLYSIS
Blood glucose
Regulation of glycogen metabolism
Skeletal muscle
Glycogen must be broken down to provide ATP for
contraction, when the muscle is rapidly contracting,
or in anticipation of contractions in stress situations
like fear or excitement.
In rapidly contracting muscle: Low [ATP], High [AMP]
High [Ca++]
Stress:
High [Epinephrine]
Glycogen stores are replenished when muscles are
resting.
Resting state: Low [AMP], High [ATP]
Hormonal regulation of metabolism
Hormone
Type
Secreted by
Secreted in response to
Insulin
Protein
Pancreatic beta cells
High blood [glucose]
Glucagon
Polypeptide
Pancreatic alpha cells
Low blood [glucose]
Epinephrine
(adrenalin)
Catecholamine
Adrenal medulla
Nervous system
Stress
Low blood [glucose]
Glucocorticoids
Steroid hormone
Adrenal cortex
Stress
Low blood [glucose]
Glucagon is the most important hormone signaling
low blood glucose concentration, while epinephrine and
glucocorticoids play secondary roles.
Regulation of glycogen metabolism
Liver
Glycogen must be broken down to provide glucose
for maintaining blood glucose in fasting or for providing
additional glucose for skeletal muscles in stress situations.
Fasting:
High [Glucagon]
Stress:
High [Epinephrine]
Glycogen stores must be replenished in the fed state
Fed state:
High [Insulin]
High [Glucose]
Muscle
Glycogen
Glycogen
Glucose 6-phosphate
Glucose 6-phosphate
Rapidly contracting state
Stress
Resting state and with
abundant energy
Liver
Glycogen
Glycogen
Glucose 6-phosphate
Glucose 6-phosphate
Fasting state
Stress
Fed state
Key regulatory enzyme of glycogen breakdown:
Glycogen phosphorylase
Key regulatory enzyme of glycogen synthesis:
Glycogen synthase
Glycogen phosphorylase is a dimer of identical subunits.
Glycogen phosphorylase can exist in an active R (relaxed)
and an inactive T (tense) state.
In the T state, the catalytic site is partly blocked
Red: active site
Yellow:
Glycogen binding site
Red site:
Allosteric site for
AMP binding
Blue/green sites:
Phosphorylation sites
Allosteric regulation of glycogen phosphorylase
Regulation by energy state.
+
AMP (binding favors the active R state)
-
ATP (binding favors the inactive T state)
Regulation by feedback inhibition.
-
Glucose 6-phosphate (G6P)
G6P concentration increases when G6P is generated
faster than it can be further metabolized, e.g. by glycolysis
Regulation by high blood glucose
-
Glucose (Only liver glycogen phosphorylase)
In the fed state with a high blood glucose concentration,
there is no need for the liver to secrete glucose
Regulation of glycogen phosphorylase by phosphorylation
Phosphorylase
kinase
ATP
ADP
P
Glycogen
phosphorylase b
Glycogen
phosphorylase a
P
Inactive
T state
Pi
H2O
Active
R state
Protein phosphatase 1
(PP1)
Phosphorylation occurs in the fasted or stressed state
Dephosphorylation is stimulated in the fed state
Phosphorylase kinase is regulated by phosphorylation and Ca++ binding
One subunit is the Ca++ -binding calmodulin
Ca++
PKA
Ca++
Ca++
PP1
P
P
Ca++
Ca++
Ca++
Ca++
P
P
Ca++
P
PKA
P
Ca++
Ca++
PP1
Inactive
Inactive
P
P
Partly active
Partly
active
Fully active
Fully
active
Phosphorylation occurs in the fasted or stressed state.
Dephosphorylation is stimulated in the fed state.
Ca++ binding occurs when the [Ca++] is high, e.g. during rapid muscle contractions
Glucagon
(low blood glucose)
Epinephrine
(stress, fear)
Adenylyl cyclase
Glucagon receptor
(liver)
+
Epinephrine receptor
(muscle and liver)
+
ATP
Cell
membrane
cAMP
+
Inactive
Protein kinase A
Active
Protein kinase A (PKA)
ATP
ADP
Inactive
Phosphorylase kinase
P
Active
Phosphorylase kinase
ATP
ADP
P
Inactive glycogen
phosphorylase b
Active glycogen
phosphorylase a
Glycogenn
Pi
Glycogenn-1
Glucose 1-phosphate
cAMP
Adenylyl cyclase
ATP
cAMP + PPi
H2O
Phosphodiesterase
AMP
Glucagon receptors and epinephrine receptors are
G-protein-coupled receptor
GDP
Adenylyl
cyclase
GDP
Receptor
beta and gamma subunit
of G-protein
alpha subunit
of G-protein
GTP
When hormone is no longer present, intrinsic GTP hydrolase activity of the G-protein alpha subunit
hydrolyzes GTP to GDP, the alpha subunit re-associates with the beta and gamma subunits, and
stimulation of adenylyl cyclase ends. cAMP is converted to AMP by phosphodiesterase.
Thus, in the absence of hormone, the cAMP concentration rapidly falls.
Insulin
Insulin receptor
It functions as a
tyrosine kinase
when insulin
is bound
α α
α α
α α
P
P
P
P
P
P
P
P
Autophosphorylation
P
Insulin receptor substrate
Activation
of protein
phosphatases
Activation of
multiple signaling
pathways
Activation
of protein
kinases
In general, the protein kinases activated by insulin have opposite biological effects
from those activated by glucagon
In general, the protein phosphatases activated by insulin dephosphorylate proteins
that are phosphorylated by glucagon-stimulated protein kinases, such as PKA
Regulation of glycogen synthase
Regulation by feed-forward mechanism.
+
Glucose 6-phosphate (G6P)
G6P concentration increases at high glucose concentrations
when G6P is generated faster than it can be further metabolized
NB: Reciprocal regulation of glycogen synthase
and glycogen phosphorylase by glucose 6-phosphate
Regulation of glycogen synthase by phosphorylation
PKA and Glycogen synthase kinase
ATP
ADP
P
P
Active
Inactive
Pi
H2O
Protein phosphatase 1
(PP1)
Phosphorylation occurs in the fasted or stressed state
Dephosphorylation is stimulated in the fed state
Reciprocal regulation of glycogen phosphorylase and
glycogen synthase by phosphorylation
Fasting/stress
(glucagon/epinephrine)
ATP
ADP
P
+
P
PKA
Phosphorylase
kinase
Pi
H2O
P
P
Active
ATP
ADP
ATP
ADP
P
P
Glycogen
phosphorylase
Glycogen
synthase
P
Inactive
T state
Pi
H2O
Active
R state
P
Active
Inactive
Pi
PP1
+
H2O
Fed state (insulin)
And it is even more complex..
Scaffolding proteins of different subtypes in liver and
muscle can bind the glycogen particle, PP1,
glycogen phosphorylase, and glycogen synthase
Binding brings participants of glycogen metabolism
together.
Regulation of PP1 is itself complex with various
inhibitors responding to the metabolic state of
the organism.