Glycogen Flashcards
What are glycogen granules?
- Each glycogen granule = several glycogen molecules + proteins
- Contain all the enzymes required for glycogen synthesis and breakdown
- 1 Glycogen granule ~ 50,000 glucose units
Why is glycogen stored?
- Glycogen catabolism is faster than FAs
- Can be used under anaerobic contitions in skeletal muscles
- Doesn’t disturb osmotic pressure → same osmotic pressure for 1 glucose monomer than for 1 glycogen
- Breakdown of glycogen in muscle provides G1P, faster than glucose can be taken up from the blood
*EFFICIENCY
What is the difference between Glycogen in Muscles vs in the Liver?
Muscle:
- Available for local energy production for muscle contration (Selfish)
- Absence of G6P phosphatase → can’t release glucse to blood
~2% of glycogen by weight
Liver:
- Used to maintain blood glucose levels
- Expression of G6P phosphatase ONLY in liver
~10% glycogen by weight
Other tissues:
- Have small glycogen stores for their own use
- Energy suring fasting or anaerobic glycolysis (brief hypoxia)
Where is Glucose-6-phosphatase located in the cell? Why?
G-6-Phosphatase is in the ER membrane (TM) with active site inside the ER lumen
Important for COMPARTMENTALIZATION of metabolic reactions. If it was in cytosol, would change all G6P → glucose and glycolysis would not be possible. Now, Glycolysis can occur in the cytosol and Glycose formation from G6P can occur in the ER lumen an be transported outside the cell to the blood stream
What is the structure of Glycogen?
Polymer of glucose residues:
- 1 Reducing end (start point of synthesis)
- Multiple non-reducing ends due to branching (allows for multiple sites of synthesis/degradation) → lack C1-OH group
- a(1→4) linkage between glucose
- a(1→6) linkage at branch points (every 8-14 residues)
What are the main steps of glycogen synthesis?
Synthesis: G1P → {UDP-glucose pyrophosphorylase + inorganic pyrophosphatase} → UDP-glucose → {glycogen synthase makes UDP + branching enzyme} → Glycogen
- Synthesis of UDP-glucose
- Elongation of a pre-existing glycogen chain using UDP-glucose
- Creation of new 1,6-glucosyl branch points
Explain the 1st step of glycogen synthesis.
Synthesis of UDP-glucose from G1P and UTP = phosphoanhydride exchange
G1P + UTP → {UDP-glucose pyrophosphorylase} → PPi + UDP-glucose
*∆G˚ ~ 0kJ/mol
Then, PPi → {inorganic pyrophosphate} → 2Pi
*∆G˚= -19.2kJ/mol → drives the whole reaction by getting rid of PPi very quickly → overall irreversible
*PPi is from UTP → UMP and binds to G1P to form UDP-Glucuose
Explain the 2nd step of glycogen synthesis.
Glycogen Synthase elongation of the pre-existin glycogen chain
UDP-glucose + Glycogen’s nonreducing end (n) → {Glycogen synthase} → Glycogen(n+1) + UDP
∆G˚= -13.4kJ/mol
UDP + ATP ⭤ {nucleoside diphosphate kinase} ⭤ UTP + ADP
What is the role of glycogenin?
It is responsible for making the glycogen primer (8-12 glucoses) → Glycosyltransferase
UDP-Glucose → Glycogenin-Tyr194-Glucose + UDP
1) Glycogenin attaches glucose residue to -OH on its Tyr194
2) Glycogenin extends the glucose chain up to 7 more residues (primer)
3) Glycogen synthase takes over for the rest
What is the structure of glycogen synthase and of glycogenin?
Glycogenin acts as a homodimer bound to the reducing ends of 2 glycogen molecules (1 each). Glycogen synthase is bound to 1 glycogenin and and synthesizes the rest of that molecule
What are the rules for glycogen branching?
- Transfers ~ 7 glucosyl residues at the time to the C6-OH position
- Each transferred segment must come from a chain of 11 residues (have 4 residues left on the old branch)
- The new branch point must be at least 4 residues away from the other branch points
*Branching enzyme takes part of an existing branch and transfers it to another spot making a new branching point
What is the complete name of the branching enzyme?
amylo-(1,4→1,6)-transglycosylase
What is the general equation for glycogen synthesis (balance sheet)?
What is the cost of glycogen synthesis?
Glucose + 2ATP + (Glycogen, n) + H2O → (Glycogen, n+1) + 2ADP + 2Pi
- 1 ATP from hexokinase/glucokinase
- 1 ATP for UTP regenration
- H2O for PPi hydrolysis → 2Pi
→ Synthesis of glycogen from glucose costs 2 ATP/residue
What are the 3 main steps of glycogen breakdown?
1) Generation of glucose-1-phosphate
2) Debranching
3) Conversion of glucose-1-phosphate to glucose-6-phosphate
Glycogen → {debranching enzyme} → + Pi → {glycogen phosphorylase} → G1P
What are the 2 possible ways to cleave a glycosidic bond?
- By Hydrolysis
Glycosidic bond (HC-O-CH) + H2O → HC-OH + OH-CH
*The H2O is used, 1H on the one that leaves with the O and the OH on the other - By phosphorolysis
Glycosidic bond + Phosphate → Glucose-O-P + Glucose-OH
No need to invest any ATP to trap the glucose inside the cell with phosphorolysis, get G1P directly from Glycogen (no UDP)
Explain the phosphorolysis of glycogen to produce G1P (step 1).
What is ∆G˚?
Glycogen + Pi → {glycogen phosphorylase} → Glucose-1-P + glycogen(n-1)
∆G˚ = +3.1kJ/mol → high intracellular Pi/G1P ratio (~100) makes the reaction exergonic in vivo
→ Non-reducing end only
→ Glycogen Phosphorylase stops cleaving at 4 glucose from a branch point (doesn’t fit in site anymore)
What is the full name of the debranching enzyme? What does it mean?
*For breakdown of glycogen
a(1→4) glucosyltransferase and a(1→6) glucosidase
a(1→4) glucosyltransferase:
- Transfers a(1→4) linked trisaccharide (3 glucose) to the nonreducing end of another branch (new a(→4) linkage)
- 3 units are then available to be cleaved from the branch by phosphorylase
*Because glycogen phosphorylase stops when 4 units away form branch point
a(1→6) glucosidase:
- Remaining glucosyl (1 glucose branch) residue is hydrolyzed (not phosphorylated) to yield glucose and debranched glycogen
→ ~92% of glycogen’s glucose converted to G1P and ~8% direclty converted to glucose
By what reaction is G1P converted to G6P to be metabolized after glycogen breakdown?
G1P → {phosphoglucomutase} → G6P
Phosphoglucomutase-Ser-P phosphorylates G1P → G1,6P
Phosphoglucomutase-Ser-OH takes P from C1 → G6P
*Enzyme starts and end phosphorylated on Ser residue
Muscles: G6P → glycolysis → ATP
Liver: G6P → glucose → circulation
What is the balance sheet of full glucose metabolism, when it is first stored as glycogen and then broken down?
- Glycogen synthesis consumes 2 ATP/glucose
- Glycogen breakdown generate 33 ATP/glucose
*Becuase glycogen breakdown generates G6P already trapped into cell - Balance = 31ATP/glucose → 97% efficiency for storage of glucose as glycogen
Give an example of a non-covalent modification of enzymatic activity.
Allosteric regulation → through protein-protein interactions
Give an example of reversible and irriversible covalent modification of a protein which modulates its activity.
Reversible: phosphorylation/dephosphorylation
Irreversible: Pro-insulin (inactive) → {protease} Insulin (active, only keep both ends connected by disulfide bonds)
What is the difference between monocyclic enzyme cascades and bicyclic enzyme cascades?
Monocyclic → covalent modification of the target enzyme E
Bicyclic enzyme cascade → covalent modification of one of the modifying enzyme F in addition to the target enzyme E
*Signal transduction signal amplifies signal in a cell
*Often phosphorylation cycles/cascades
Which 2 general regulatory mechanisms are responsible for regulation of the glycogen metabolism?
- Allosteric control of glycogen phosphorylase and glycogen synthase
- Covalent modification by cascade phosphorylation (interconversion of 2 forms of the enzymes with different properties)
What enzymes are involved in regulation of the glycogen metabolism?
Glycogen breakdown:
1. Glycogen phosphorylase (Gycogen(n) → Glycogen(n-1) + G1P)
2. Phosphorylase kinase
3. PKA
Glycogen synthesis:
4. Glycogen synthase
5. Phosphoprotein phosphatase (PP1)
By what mechanisms (no pathways) is glycogen phosphorylase activity regulated?
2-way regulation → for fine-tunning
Covalent control:
- Phosphorylation/dephosphorylation (2ATP → 2ADP)
- Phosphorylation activates (less than allosteric control)
- Faster (often occurs 1st)
Allosteric control:
- R/T transition
- Inhibited by ATP/G6P
- Inhibited by glucose in the liver
- Activated by AMP (in muscles when exercise)
- Driven by metabolites accumulation in the cell → slower
What is glycogen phosphorylase b vs a?
glycogen phosphorylase b → non-phosphorylated
glycogen phosphorylase a → phosphorylated
*R (active) vs T (inactive) is due to allosteric control and additional to a/b
What is the proportion of active glycogen phosphorylase determined by, under most physiological conditions?
Determined by the rate of covalent modifications
Covalent modifications determine a/b isotypes (active vs not)
What are the modulators of phosphrylase kinase in the muscles?
Modulated through 2 inputs:
- Hormonal (epinephrine) via cAMP
- neural through release of Ca for muscle contraction and glycogen degradation
Phosphorylase kinase-b → inactive, active if [Ca] elevated
Phosphorylase kinase-a → active even at low [Ca]
*a = active, b = inactive
What pathway is phosphorylase kinase involved in? What is its role?
Involved in regulation of glycogen breakdown, when active:
- Activates glycogen phosphorylase by phosphorylation
- Inactivates glycose synthase by phosphorylation
*It is phosphorylated by protein kinase A to be activated
*It is dephosphorylated by active PP1 (phosphoprotein-phosphatase-1)
How does the structure of phosphorylase kinase allow its regulation?
- Found as tetramer
- 4 types of subunits → a, b, y, g
a and b subunits → phosphorylated by PKA; dephosphorylated by PP1
*Can have up to 8 phosphorylations/time (x4) so fine-tune, more phosphorylation = more active
y → catalytic subunit → phosphorylates both glycogen phosphorylase (activates) and glycogen synthase (inactivates)
g → calmodulin (CaM) → confers calcium sensitivity
How does the structure of PKA allow its regulation?
PKA = heterotetramer
4x cAMP binds to 2 regulatory subunits (2 each) → 2 catalytic subunits dissociate and act as kinases on target proteins
[cAMP] intracellular concentration determined the fraction of PKA in the active form
*Activation of PKA does not depend on convalent modifications
There is a phosphatase that takes phosphates off PKA → no activation of PKA → no activation of phosphorylase kinase → no activation of glycogen phosphatase → promotes glycogen synthesis
What mechanism allows glycogen synthase regulation?
Covalent:
Phosphorylated → less active
- When active phosphorylase kinase
- When cAMP-stimulated active PKA
- When glycogen synthase kinase is active/inactive PP1c (ex: presence of epinephrine)
Non-phosphorylated → active
- When PP1c is active
- low [cAMP]
- ex: presence of insulin → inactive GSK3b
Allosteric:
→ only on glycogen synthase b (not a)
- G6P facilitates dephosphorylation of synthase b
*Glycogne synthase activity depends on the fraction of the enzyme in the unmodified form
What are the target proteins of the 2 catalytic subunits of Protein Kinase A?
- Phosphorylase kinase
- PP1 inhibitor
- Glycogen synthase
What is GSK3b?
Glycogen synthase kinase → will phosphorylate glycogen kinase → inactivate it → inhibit glycogen synthesis
Which enzymes modulate glycogen synthase activation? (covalent modifications by phosphorylation)
- Protein Kinase A (active) → phosphorylate → inactive glycogen synthase b
- GSK3b → phosphorylate → inactive glycogen synthase b
- active PP1 → dephosphorylate → active glycogen synthase a
How does PP1c inhibit glycogen breakdown?
*PP1 is a phosphatase
- Dephosphorylating glycogen phosphorylase
- Dephosphorylating phosphorylase kinase a
- Dephosphorylating glycogen synthase to activate glycogen synthesis
- Dephosphorylating it’s own inhibitory peptide: PP1-inhibitor
What enzyme is reponsible for the catalytic (not hormonal) regulation of PP1c?
What regulates that enzyme?
PP1c is inhibited by the phosphorylated/active for of phosphoprotein phosphatase inhibitor-1-a
→ inhibitor binds to PP1 to inactivate it (not a kinase/phosphatase)
PP1c inhibitor is controlled by PKA → phosphorylates it to activates it
How is PP1c hormonally regulated?
Glycogen-Gm subunit-PP1ctalytic subunit (all bound)
GM = intermediate regulatory protein subunit than allow PP1c to bind to glycogen
Active PP1c is bound to GM-glycogen → promotes glycogen synthesis
Inactive PP1c when not bound to GM-glycogen → leads to increase glycogen breakdown
Fed state → Insulin → Insulin-stimulated protein kinase → 1x phosphorylation of GM → PP1c bound → glycogen synthesis
Running/fasting → epinephrine → PKA → 2nd phosphorylation of GM → dissociation of PPC1c → inactive
Which proteins are involved in regulation of degradation of glycogen?
- Protein kinase A (active bound to cAMP)
- Phosphorylase kinase (active when phosphorylated)
- Glycogen phosphorylase (active when phosphorylated)
Which proteins are involved in regulation of synthesis of glycogen?
- Glycogen synthase (active when not phosphorylated)
- Phoshoprotein phosphatase-1 (active whrn not phosphorylated)
- Phosphoprotein phosphatase inhibitor-1 (active when phosphorylated)
By what pathway is the insulin receptor involved in glycogen control?
Insulin receptor = receptor tyrosine kinase (phosphorylates itslef) → phosphorylate IRS → activate protein kinases → phosphorylate glycogen synthase kinase (inactivates GSK3b)
When not phosphorylated, glycogen syntase kinase will phosphorylate glycogen synthase (inactivate it)
*PP1 dephosphorylates it to reactivate it
*IRS = insulin receptor substrate
So insulin → glycogen synthesis bc inactivates the inhibitor of glycogen synthase
Which are the 3 main hormonal receptors involved in glycogen regulation?
- Insulin receptor
- Gαs - coupled receptors
- Gαq - coupled receptors
Explain the Gas-coupled receptors’ control in glycogen metabolism.
*beta-adregenic receptor
- Hormone binds to its receptor
- GDP bound Ga subunit binds GTP
- GTP-bound Ga dissociates from Gb and Gy subunits
- GTP-bount Ga activates adenylate cyclase
- Adenylate cyclase produces cAMP (2ndary messenger)
- cAMP activates Protein Kinase A
- Activated PKA triggers cellular response → phosphorylates phosphorylase kinase (activates it)
- Phosphorylase kinase phosphorylates Glycogen phosphorylase (b → a // activated) → chops off glucose form glycogen to make G1P
*Epinephrine (muscles) and Glucagon (liver) → glycogen breakdown
Which hormones bind to Gas-coupled receptor in which tissue?
Almost only glycogen synthesis in muscles and liver:
Liver → Glucagon (produced by pancreas) binds to Gas’ extracellular portion
Muscles → Epinephrine binds
*They both have the same effect on the pathway
Explain the Gaq-coupled receptors’ control in glycogen metabolism.
- Binding of ligand → qa subunit to bind GTP
- qa dissociates from y and b and activates Phospholipase C
- PLC hydrolyzes PIP2 → IP3 + DAG
- IP3 stimulates release of Ca at the ER (binds to Ca channel) → Calmodulin effects
- DAG activates protein kinase C → inhibits glycogen synthase, etc.
What are the 2ndary messenger in the G-protein coupled pathways ?
Gas pathway → cAMP
Gap pathway →DAP, IP3, Ca
What type of regulation do DAG and Ca induce?
Ca → allosteric regulation
DAG → covalent modification (phosphorylation)
What are some similarities and differences between the muscle and liver cells in glycogen metabolism?
Differences:
- Liver has G-6-phosphatase for glucose synthesis (altruistic)
- Liver has Glucagon receptor, not muscles
- Glut4 + Glut2 in the muscle vs Glut2 in the liver → when glycogen degradation, Glut2 allows export of glucose from the cell (concentration gradient favours efflux)
Similarities:
- Both cases, b-adregenic receptor → cAMP → glycogen degradation
- Both have insulin receptors
Where does the Calcium that signals to promote Glycogen degradation come from in the muscles vs in the liver?
In muscle → comes from muscle contractions
In liver → comes from a-adregenic receptor (Gaq)
*only in the liver, and not the same as glucagon receptor (Gas)
Which organ is responsible for secretion of epinephrine?
Adrenal glands
Which 2 stimuli promote glycogen breakdown in the muscles?
- Adrenaline signaling increases cAMP
- Muscle contraction increases Ca2+
Both activate PKA
Which 2 stimuli promote glycogen synthesis in the muscles?
- High blood sugar → more glucose available for glycogen synthesis
- High insulin → insulin receptor → increase Glut4 + promotes glycogen synthesis (inhibits the inhibitor of glycogen synthase)
What is the effect of stress on glycogen metabolite regulation?
Stress increases cAMP and Calcium
- Activates phosphorylase kinase → activates Glycogen phosphorylase
- Inhibits glycogen synthase
What is the effect of low blood glucose on cAMP?
Low blood sugar → Glucagon release → increase cAMP (not Calcium)
- Activates phosphorylase kinase → activates Glycogen phosphorylase
- Inhibits glycogen synthase
*Allows glucose to be released from the liver to the blood
What enzyme is deficient in von Gierke’s disease?
Glucose-6-phosphatase → catalyses final step to release glucose into bloodstream
Causes accumulation of glycogen of normal structure, but inability to increase blood glucose in response to glucagon or epinephrine
What are the symptoms of von Gierke’s Disease?
Glucose-6-phosphatase deficiency
- Massive liver enlargement (stored glycogen)
- Hypoglycemia (low blood sugar)
- Failure to thrive (liver can’t provide the other organs with glucose)
What enzyme is deficient in McArdle’s disease?
What does it cause?
Muscle Phosphorylase Deficiency → catalyzes glycogen breakdown to G1P (glycogen (n) + Pi → Glycogen (n-1) + G1P)
Glycogen breakdown is impaired → reduced fuel for glycolysis to keep up with the metabolic demand
*ONLY IN THE MUSCLES
What are the symptoms of McArdle’s disease?
- Painful cramps during exercise (because of lack of ATP)
*Because of the lack of glycogen breakdown in the muscles specifically
What disease shows very elevated levels ADP during light exercise?
McArdle’s disease → Can’t make G1P from glycogen in the muscles
ADP can’t be recycled to ATP because to do that you need substrate which can’t be produced in phosphorylase deficiency
*Specific to muscle glycogen breakdown
What enzyme is deficient in Hers’ disease?
What does it cause?
Liver phosphorylase deficiency → Can’t make G1P from glycogen in the liver
Inability to breakdoen glycogen in the LIVER
What are the symptoms of Hers’ disease?
Hypoglycemia because glycogen phosphorylase can’t respond to the need for glucose production by the liver
