GLYCOGEN METABOLISM
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Transcript of GLYCOGEN METABOLISM
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
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.
….. ….
O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2
H
H H O
H OH
OH H
CH2OH
H
H H
O O OO
O
α-1,4 α-1,4 α-1,4
α-1,6
α-1,4
α-1,4
O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2
H
H H O
H OH
OH H
CH2OH
H
H H
O O OO
O
H OH
OH H
CH2 OH
H
H
H
O
H OH
OH H
CH2 OH
H
H
H
O
H OH
OH H
CH2 OH
H
H
H
O
H OH
OH H
CH2 OH
H
H
H
O
O
O
O
Glycogen
….. ….
O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2
H
H H O
H OH
OH H
CH2OH
H
H H
O O OO
O
α-1,4 α-1,4 α-1,4
α-1,6
α-1,4
α-1,4
O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2
H
H H O
H OH
OH H
CH2OH
H
H H
O O OO
O
H OH
OH H
CH2 OH
H
H
H
O
H OH
OH H
CH2 OH
H
H
H
O
H OH
OH H
CH2 OH
H
H
H
O
H OH
OH H
CH2 OH
H
H
H
O
O
O
O
A single molecule can have a molecular mass of up to 108 Da withmore 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 anon-reducing end
Glycosyl units are attachedand mobilized from thereducing 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 weightMuscle - Approximately 400 g or 1-2% of the fresh weight
Most other cells have small amounts of glycogen stored
Glycogen
Glucose 6-P
Glucose
Blood glucose
Glycogen
Glucose 6-PG6Pase
GLYCOLYSIS
LIVER MUSCLE
Sources of blood glucose after a meal
Meal
Glycogen
Gluconeogenesis
8 16 24 2 7 30
Hours Days
mM glucose
8
4
Glycogen synthesisGlycogenesis
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
Glucose
Glucose 6-phosphate
Glucose 1-phosphate
UDP-glucose
Glycogenn+1
Glycogenn+1 with an additional branch
Hexokinase/Glucokinase
Phosphoglucomutase
UDP-glucose pyrophosphorylase
Glycogen synthase
Branching enzyme
ATP
ADP
UTP
PPi
Glycogenn
2 Pi + H2O
Pyrophosphatase
Glycogen synthesis - Glycogenesis
O
H OH
OH H
CH2OH
H
OH
H H
OH
+ ATP + ADP
GlucokinaseHexokinase
Same reaction, same enzymes, and same regulation as in glycolysisIrreversible
Glucose Glucose 6-phosphate
O
H OH
OH H
CH2OPO32-
H
OH
H H
OH
Hexokinase
Glucose 6-phosphate (low phosphofructokinase activity)
Glucokinase
High blood glucose (release from GKRP, High Km)
Insulin stimulates gene transcription (only in liver)
-
++
O
H OH
OH H
CH2OPO32-
H
OH
H H
OH
O
H OH
OH H
CH2OH
H
OH
H H
OPO32-
O
H OH
OH H
CH2OPO32-
H
OH
H H
OPO32-
Glucose 6-phosphate
Glucose 1-phosphate
Glucose 1,6-bisphosphate
OHSer
OPO32-
Ser
OPO32-
Ser
Phosphoglucomutase
O
H OH
OH H
CH2OH
H
OH
H H
OPO32-
Glucose 1-phosphate
O
H OH
OH H
CH2OH
H
OH
H H
O – P – O – P – O - uridine
UDP-glucose
O O
O- O-
+ O- – P - O – P – O – P – O - uridine
O O O
O- O- O-
UTP
O- – P – O – P – O-
O O
O- O-
Pyrophosphate (PPi)
+
UDP-glucose pyrophosphorylase
O- – P – O – P – O-
O O
O- O-
Pyrophosphate (PPi)
+ H2O 2 Pi
Pyrophosphatase
NB: Irreversible reaction
Glucose 1-phosphate + UTP UDP-glucose + PPi
PPi + H2O 2 Pi
Glucose 1-phosphate + UTP + H2O UDP-glucose + 2 Pi
The irreversible hydrolysis of pyrophosphate drives the synthesis ofUDP-glucose
O
H OH
OH H
CH2OH
H
OH
H H
O – P – O – P – O - uridine
UDP-glucose
O O
O- O-
+
O
H OH
OH H
CH2OH
H
H H
HOO - R
α-1,4
Glycogen (n residues)
O
H OH
OH H
CH2
H
H H O
H OH
OH H
CH2OH
H
H H
O
α-1,4
O - Rα-1,4 HO
O- – P – O – P – O - uridine
O O
O- O-
+
Glycogen (n+1 residues)UDP
Glycogen synthase
Priming of glycogen synthesis
Glycogen synthase can NOT add glucosyl residues to free glucoseor 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 linkageto 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
TyrHO
Glycogenin
Tyr O
Glycogenin
8 UDP-glucose +
Glycogenin
…
Non-reducing end Cleaveage of α-1,4 bond
…
α-1,6bondNon-reducing ends
“Branching enzyme”Amylo-α(1,4) → α(1,6)-transglucosidase
Stoichiometry
Glucose + ATP + UTP + H2O + Glycogenn →
Glycogenn+1 + ADP + UDP + 2 Pi
Degradation of glycogenGlycogenolysis
Occurs in cytoplasm
Major product is glucose 1-phosphate from breaking α-1,4 bondsMinor product is glucose from breaking α-1,6 bonds
Glucose 1-phosphate : Glucose ≈ 10:1
Glycogenn
Glycogenn-1
Glycogen with branch
Glucose Glycogen with one less branch
Pi
Glycogen synthesis - Glycogenesis
Glucose 1-phosphate
Glucose 6-phosphate
Glucose
Phosphoglucomutase
G6PaseH2O
Glycolysis
Glycogen phosphorylase
…
“Debranching enzyme”
H2OPi
O
H OH
OH H
CH2OH
H
H H O
H OH
OH H
CH2
H
H H O
H OH
OH H
CH2OH
H
H H
O O
α-1,4 α-1,4
O - RHO
O- – P – OH
O
O-
+
+
O
H OH
OH H
CH2OH
H
OH
H H
OPO32-
Glucose 1-phosphate Glycogen with n-1 residues
O
H OH
OH H
CH2
H
H H O
H OH
OH H
CH2OH
H
H H
O
α-1,4
O - RHO
Phosphate Glycogen with n residues
Glycogen phosphorylase
NH
C
NH Lys
OH
CH3
2-O3PO-CH2
Pyridoxal phosphate is a coenzyme for the phosphorylasereaction.
Pyridoxal phosphate is bound to a nitrogen of a lysylresidue of glycogen phosphorylase
The phosphate of pyridoxal phosphate exchanges protons with the phosphate reactant, which allows the reactant to donate aproton to the oxygen atom on carbon 4.
+
O
H OH
OH H
CH2OH
H
OH
H H
OPO32-
O
H OH
OH H
CH2OPO32-
H
OH
H H
OPO32-
Glucose 1-phosphate
Glucose 1,6-bisphosphate
OHSer
OPO32-
Ser
OPO32-
Ser
Phosphoglucomutase
O
H OH
OH H
CH2OPO32-
H
OH
H H
OH
Glucose 6-phosphate
O
H OH
OH H
CH2OPO32-
H
OH
H H
OH
Glucose 6-phosphate
O
H OH
OH H
CH2OH
H
OH
H H
OH
Glucose
+ H2O + Pi
Glucose-6-phosphatase
(G6Pase)
Same reaction as in gluconeogenesisOccurs in endoplasmic reticulum and involves a glucose 6-phosphatase transporter and a catalytic subunit
The catalytic subunit is regulated at the level of transcription
…
α-1,6bond
Glycogen phosphorylase stops when 4 glucosyl units remain on each chain from a branch point
b c
a’
b’
a
c’
d’
d e
…b ca d eb’ c’a’
α-1,6bond
d’
Oligo-α(1,4)→α(1,4)-glucan transferase(debranching enzyme)
Amylo-α(1,6)-glucosidase(debranching enzyme)
…b ca d eb’ c’a’d’ +Glucose
H2O
Approximate Stoichiometry
Glycogenn+11 + 10 Pi + H2O →
Glycogenn + 10 Glucose 6-phosphate + Glucose
Glycogen
Glucose 6-P
Glucose
Blood glucose
Glycogen
Glucose 6-PG6Pase
GLYCOLYSIS
LIVER MUSCLE
Regulation of glycogen metabolism
Skeletal muscleGlycogen 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 areresting.
Resting state: Low [AMP], High [ATP]
Hormonal regulation of metabolismHormone Type Secreted by Secreted in response to
Insulin Protein Pancreatic beta cells High blood [glucose]
Glucagon Polypeptide Pancreatic alpha cells Low blood [glucose]
Epinephrine Catecholamine Adrenal medulla Stress(adrenalin) Nervous system Low blood [glucose]
Glucocorticoids Steroid hormone Adrenal cortex StressLow blood [glucose]
Glucagon is the most important hormone signalinglow blood glucose concentration, while epinephrine andglucocorticoids play secondary roles.
Regulation of glycogen metabolism
LiverGlycogen must be broken down to provide glucosefor maintaining blood glucose in fasting or for providingadditional 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
Glucose 6-phosphate
Glycogen
Glucose 6-phosphate
Glycogen
Glucose 6-phosphate
Glycogen
Glucose 6-phosphate
Rapidly contracting stateStress
Resting state and withabundant energy
Fasting stateStress
Fed state
Liver
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 forAMP binding
Blue/green sites:Phosphorylation sites
Regulation by energy state.
-
+ AMP (binding favors the active R state)
ATP (binding favors the inactive T state)
Allosteric regulation of glycogen phosphorylase
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
P
PInactive ActiveT state R state
Glycogenphosphorylase b
Glycogenphosphorylase a
ATP ADP
Phosphorylase kinase
Protein phosphatase 1 (PP1)
H2OPi
Phosphorylation occurs in the fasted or stressed stateDephosphorylation is stimulated in the fed state
Ca++Ca++
Ca++ Ca++
Ca++Ca++
Ca++ Ca++P P
P P
P P
P P
PKA
PP1
PKA
PP1
Inactive Partly 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
Phosphorylase kinase is regulated by phosphorylation and Ca++ binding
One subunit is the Ca++ -binding calmodulin
Ca++
Ca++
Inactive Partly active Fully active
Cellmembrane
Glucagon(low blood glucose)
Glucagon receptor(liver) +
Adenylyl cyclase
Epinephrine(stress, fear)
Epinephrine receptor(muscle and liver)
ATP cAMP
+
+
Inactive ActiveProtein kinase A Protein kinase A (PKA)
Inactive ActivePhosphorylase kinase Phosphorylase kinase
P
Inactive glycogen Active glycogenphosphorylase b phosphorylase a
P
ATP ADP
ATP ADP
Glycogenn Glycogenn-1
Pi Glucose 1-phosphate
ATP cAMP + PPi
Adenylyl cyclase
cAMP
H2OPhosphodiesterase
AMP
Receptor
Adenylylcyclase
GDP
GTP
beta and gamma subunitof G-protein
alpha subunitof G-protein
Glucagon receptors and epinephrine receptors areG-protein-coupled receptor
GDP
When hormone is no longer present, intrinsic GTP hydrolase activity of the G-protein alpha subunithydrolyzes GTP to GDP, the alpha subunit re-associates with the beta and gamma subunits, andstimulation of adenylyl cyclase ends. cAMP is converted to AMP by phosphodiesterase. Thus, in the absence of hormone, the cAMP concentration rapidly falls.
β β
α αInsulin
Insulin receptorIt functions as atyrosine kinasewhen insulinis bound
β β
α α
P
P
P
P
Insulin receptor substrate
Autophosphorylation
β β
α α
P
P
P
P
P
Activation ofmultiple signalingpathways
Activationof proteinphosphatases
Activationof proteinkinases
In general, the protein kinases activated by insulin have opposite biological effects, the protein kinases activated by insulin have opposite biological effectsfrom those activated by glucagonfrom those activated by glucagon
In general, the protein phosphatases activated by insulin dephosphorylate proteins, the protein phosphatases activated by insulin dephosphorylate proteinsthat are phosphorylated by glucagon-stimulated protein kinases, such as PKAthat 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 synthaseand glycogen phosphorylase by glucose 6-phosphate
Regulation of glycogen synthase by phosphorylation
P
PActive Inactive
ATP ADP
PKA and Glycogen synthase kinase
Protein phosphatase 1 (PP1)
H2OPi
Phosphorylation occurs in the fasted or stressed stateDephosphorylation is stimulated in the fed state
Reciprocal regulation of glycogen phosphorylase andglycogen synthase by phosphorylation
P
PActive Inactive
ATP ADP
H2OPi
P
PInactive ActiveT state R state
ATP ADP
H2OPi
P P
P P
ATP ADP
H2OPi
Fasting/stress(glucagon/epinephrine)
PKA
Fed state (insulin) PP1
+
+
Phosphorylasekinase
Glycogenphosphorylase
Glycogensynthase
Active
And it is even more complex..
Scaffolding proteins of different subtypes in liver andmuscle can bind the glycogen particle, PP1, glycogen phosphorylase, and glycogen synthase
Binding brings participants of glycogen metabolismtogether.
Regulation of PP1 is itself complex with variousinhibitors responding to the metabolic state ofthe organism.