Glycogen Synthesis and Breakdown and PDC Flashcards
What do glycogen granules contain?
all the enzymes required for glycogen synthesis and breakdown
Why is it useful to store glycogen?
- glycogen catabolism is faster than Fatty acids
- can be used under anaerobic conditions in skeletal muscle
- doesn’t disturb osmotic pressures like glucose
- breakdown of glycogen in muscle provides G1P faster than glucose can be taken into the blood
- liver’s store of glycogen can supply glucose to blood for 12 hours
where is G6P phosphatase located?
- in the ER tissue (facing inwards)
- nearly absent in muscle
- some selective expression in liver
what is the use of glycogen in the muscle, liver, and other tissue?
- muscle: local energy production for muscle contraction
- liver: used to maintain blood glucose levels
- other tissue: have small glycogen store for their own use
what is the use of branching of glycogen polymers?
to provide a large number of ends to allow multiple sites for synthesis/degradation
how many reducing and non-reducing ends are there in glycogen polymers? on which ends can new glucose be added in?
- only 1 reducing end and many non-reducing ends
- new glucose residues are added to non-reducing ends
how are glycosyl residues linked? how are branched points linked (and how often)?
- glycosyl residues are linked by alpha-1-4 glycosidic bonds
- branches linked by alpha-1-6 linkages every 8-14 residues
what are the steps of glycogen synthesis?
- synthesis of UDP-glucose from G1P and UTP
- elongation of pre-existing glycogen chain using UDP-glucose
- creation of new 1,6-glycosyl branch points
Explain how glucose gets transformed into UDP glucose. how is energy provided? is it reversible or irreversible?
- glucose converted G6P by hexokinase then converted to G1P by phosphoglucomutase
- G1P converted to UDP-glucose by UDP-glucose pyrophosphorylase
- energy comes from inorganic pyrophosphatase activity
- overall reaction is irreversible
what enzyme elongates a pre-existing glycogen chain?
glycogen synthase
how is UDP restored?
nucleoside diphosphate kinase
what is the role of glycogenin and how many glycogenin are there per glycogen molecule?
primes the synthesis of a new glycogen molecule and there is only 1 per glycogen
what is the mechanism of glycogenin?
- glycogenin attaches a glucose residue donated by UDPG to the OH group of its Tyr
- It then extends the glucose chain by up to 7 additional UDPG-donated glucose residues to form a glycogen primer
- glycogen synthase can then add more glucose
What is the role of the branching enzyme and what are the three rules it must follow?
- it creates new branch points
rules:
1. transfers ~7 glycosyl residues to the C6-OH
2. each transferred segment must come from a chain of at least 11 residues
3. the new branch point must be at least 4 residues away from other branch points
what is the ATP cost per glucose residue?
2 ATP
what are the steps of glycogen breakdown?
- generation of G1P
- debranching
- conversion of G1P to G6P
how is G1P generated from a glycosyl residue? What rule must be followed?
- glycogen phosphorylase adds a phosphate to the residue, which separates it from the glycogen (phosphorolysis)
- this enzyme will release glucose units within 4 residues of a branch point
What is the role of the debranching enzyme? How does it work?
- it is an enzyme which removes the 4-residue branch left by the glycogen phosphorylase
It has 2 activities:
1. glucosyltransferase transfers a alpha-(1-4) linked trisaccharide to the non-reducing end of another branch
2. glucosidase hydrolyzes the remaining residue to yield a glucose directly (occurs in ~8% of glycogen)
what is the role of phosphoglucomutase?
converts G1P to G6P (glycogen breakdown)
what happens to the G6P from broken down glycogen in the muscle and the liver?
- muscle: continues in glycolysis to generate ATP
- liver: converted to glucose and goes into the circulation
how many ATP are consumed/generated in glycogen synthesis, glycogen breakdown, and what is the net production of ATP from 1 glucose stored as glycogen?
- synthesis: consumes 2 ATP
- breakdown: generates 33 ATP
- net: 31 ATP (compared to 32 ATP)
what is a monocyclic enzyme cascade?
an enzyme cascade with the aim to covalently modify a single target enzyme
what is a bicyclic enzyme cascade?
an enzyme cascade with the aim to modify one of the modifying enzyme along with the target enzyme
what are the 2 regulatory mechanisms for glycogen metabolism?
- allosteric control of glycogen phosphorylase and glycogen synthase
- covalent modification by cascade phosphorylation
what does glycogen phosphorylase do?
it induces breakdown of glycogen
how is glycogen phosphorylase regulated?
has two conformations: T (inactive) and R (active)
- allosteric control: switches between T and R form and controlled by amount of substrate/product available – more ATP/G6P causes inactivation and AMP causes activation
- covalent control: when in T form, can be modified in order to get to the most active form: phosphorylase kinase brings to most active and phosphoprotein phosphatase brings to less active form
what are the 4 states of glycogen phosphorylase? and how are the related?
- T form most inactive: unphosphorylated
- T-form: phosphorylated
- R-form: unphosphorylated
- R-form most active: phosphorylated
when in T-form can be either phosphorylated or unphosphorylated, where it can then go to it’s corresponding R-state
what does phosphorylase kinase do?
- activates glycogen phosphorylase by phosphorylation
- inactivated glycogen synthase by phosphorylation
what are the two forms of phosphorylase kinase?
- B: inactive but active if calcium is elevated
- A: active even at low calcium
what are the 4 types of subunits and what do they do/how are they regulated?
- alpha and beta: regulatory subunits (phosphorylated by PKA and dephosphorylated by PP1)
- gamma: catalytic subunit (phosphorylates glycogen phosphorylase and glycogen synthase )
- delta: calmodulin (confers calcium sensitivity)
how is phosphorylase kinase modulated?
- hormonally (epinephrine) via cAMP
- neuronally through release of Ca++ for muscle contraction and glycogen degradation
what is the role of glycogen synthase?
enzyme that builds up glycogen from UDP-glucose
how is glycogen synthase regulated?
covalent:
- phosphorylated: less active
- occurs when phosphorylase kinase is active, cAMP-stimulated PKA is active, glycogen synthase kinase is active, and PP1c is inactive
- non-phosphorylated: active
- occurs when PP1c is active, phosphorylase kinase is inactive, low [cAMP], glycogen synthase is inactive
allosteric
- only glycogen synthase b (aka phosphorylated form)
- G6P facilitates dephosphorylation
glycogen synthase activity depends on the fraction of the enzyme in the ___ form.
unmodified
what are the target proteins of PKA?
- phosphorylase kinase
- PP1 inhibitor
- glycogen synthase
what does the fraction of active PKA to inactive depend on?
the intracellular concentration of cAMP
how does PP1c inhibit glycogen breakdown?
- dephosphorylates glycogen phosphorylase
- dephosphorylates phosphorylase kinase a
- dephosphorylates glycogen synthase to activate glycogen synthesis
dephosphorylates its own inhibitory peptide PP1-inhibitor)
what is the role of PP1c?
to inhibit glycogen breakdown
how is PP1c regulated?
- inhibited by PP1-inhibitor (when it is in its phosphorylated form)
- phosphorylation of PP1-inhibitor is controlled by PKA and PP1c
how is PP1c hormonally regulated?
- when bound to glycogen, there is more synthesis due to decreased phosphorylation, which is enhanced by insulin
- when not bound to glycogen, there is breakdown due to increased phosphorylation, which is enhanced by epinephrine (which will increase PKA)
What is the role of insulin (receptor tyrosine kinase) in glycogen metabolism?
Causes the phosphorylation of glycogen synthase kinase, which turns it on and causes glycogen synthesis
What is the role of the G-alpha-s coupled receptor in glycogen metabolism? What is it’s mechanism?
- Hormone bonds to its receptor
- Alpha g-protein binds GTP
- It dissociates from the gamma and beta subunits
- Activates adenylate cyclase
- Adenylate cyclase produces cAMP
- cAMP activates PKA
- PKA triggers response (activates glycogen phosphorylase — glycogen breakdown)
What hormones can bind to G-alpha-s receptor? Which tissue is it in?
- epinephrine in muscle
- glucagon in liver
What is the role and mechanism of the G-alpha-q coupled receptor?
- Bonding of ligand activates phospholipase C, which hydrolysis PIP2 to IP3 and DAG
- IP3 stimulates the release of calcium in the ER, so activates various cellular processes through calmodulin
- DAG activates protein kinase C, which also modulated many processes (inhibits glycogen synthesis)
What happens in the muscle at stress or upon contraction in relation to glycogen metabolism?
Causes glycogen breakdown:
- increase cAMP by adrenaline and increase Ca++ by muscle contraction
- activation of PKA (inhibits glycogen synthase, phosphorylation PP1c inhibitor, and activates phosphorylase kinase)
What happens in the muscle after a meal in relation to glycogen metabolism?
Glycogen synthesis:
- high blood glucose so high insulin
- insulin activates glycogen synthase through PP1c
- high G6P which activates glycogen synthase and inactivated glycogen phosphorylase
What happens in the liver after a meal in relation to glycogen metabolism?
Glycogen synthesis:
- insulin inhibits glycogen synthase kinase, which causes activation of glycogen synthase
- active PP1c which activates synthesis and inhibits breakdown
- G6P and glucose inhibits glycogen phosphorylase
- G6P activates glycogen synthase
What happens in the liver at stress in relation to glycogen metabolism?
Glycogen breakdown
- increases cAMP and calcium, activates phosphorylase kinase which activates glycogen phosphorylase, also inhibits glycogen synthase
- the glucose is then releases into blood
What happens in the liver when there is low blood glucose in relation to glycogen metabolism?
Glycogen breakdown
- increased cAMP, which activates phosphorylase kinase which activates glycogen phosphorylase, also inhibits glycogen synthase
- glucose then released in blood
what is Von Gierke’s Disease? What are the symptoms?
- glucose-6-phosphatase deficiency
- inability to increase blood glucose in response the glucagon or epinephrine
- symptoms: massive liver enlargement (G6P accumulation), hypoglycemia, failure to thrive
what is McArdle’s disease? What are the symptoms?
- muscle phosphorylase deficiency
- catalyzes glycogen breakdown to G1P, so glycogen breakdown is impaired and there is reduced fuel for glycolysis to keep up with the demand
- elevated ADP levels during exercise
- symptom: painful cramps during exercise, but subsides if continues to exercise
what is Hers’ disease? what are the symptoms?
- liver phosphorylase deficiency
- inability to breakdown glycogen in the liver
- symptoms: hypoglycemia
what is the role of pyruvate dehydrogenase?
it is the enzyme that catalyzes the reaction: pyruvate + CoA + NAD+ —> acetyl-CoA + CO2 + NADH
where does the PDC reaction occur?
in the mitochondrion
how does pyruvate get into the mitochondria?
by pyruvate translocase (H+ symport)
how many coenzymes are required for the PDC?
5
what are the 3 enzymes in the pyruvate dehydrogenase reaction? what are their locations relative to each other?
- E1, E2, E3
- their active sites are in close proximity of each other
What does E1 do? is this reaction reversible?
- does a decarboxylation reaction
- C1 of pyruvate released as CO2
- the C2 and C3 attach to TPP as a hydroxyethyl group
- is irreversible since CO2 diffuses out
what does E2 do? is this reaction reversible?
- generates acetyl-CoA and regenerates TPP for E1
- lipoamide is reduced by addition of acetyl group and regenerates TPP (creates a thioester bond)
- there is then a trans-esterification to CoA (creates acetyl-CoA)
- is a reversible reaction
what makes acetyl-CoA so special?
it has a high acyl group transfer potential and can donate the acetyl group to several acceptors
what does E3 do? is this reaction reversible?
- it resets E2 and E3 to their active state (it restores E2 and its own FAD for another round)
- redox reaction regenerates lipoamide
- FADH2 reoxidizes FAD by NAD, making NADH
- generates NADH
- is a reversible reaction
what are the mechanistic advantages of multienzyme complexes?
- minimized distance for substrates in between active sites (increases reaction rates without having to maintain large pools of intermediates)
- Metabolic intermediates are channeled between successive enzyme sites (this allows for minimized side reactions and protection for chemically labile intermediates)
- Coordinated control of reactions (shutting off one enzyme basically shuts off the whole system)
what are the two levels of control for PDC?
- product inhibition by acetyl-CoA and NADH
- covalent modification (phosphorylation of E1)
how is the phosphorylation and dephosphorylation of the PDC regulated?
Inactivation:
- PDK is allosterically activated by acetyl CoA and NADH (is inhibited by pyruvate, ADP, Ca++)
- PDK phosphorylates E1
- E1 becomes inactive
Activation:
- PDP is allosterically activated by Ca++
- PDP dephosphorylates E1
- E1 is active
