Regulation of glycogen breakdown and synthesis Flashcards
ATP is made from ?
Glucose breakdown, but the cell cannot store glucose – as it is soluble, it will create a high osmotic potential and draw water into the cell
Therefore what is used as an energy store for the cell ?
Instead, the cell uses a glucose polymer called glycogen which is insoluble and inert
In muscle, what is the Glycogen for ?
Glycogen is a glucose reserve for the muscle only
In the liver, what is the Glycogen for ?
Glycogen is a glucose reserve for the maintenance of blood glucose concentration
Explain the structure of glycogen ?
- Highly branched polysaccharide of glucose consisting of (alpha-1,4) linked glucose molecules with an (alpha-1,6) branch about every 12 glucose residues
- Formed joined to a tyrosine residue on the protein glycogenin
- Provides large number of ends at which enzymes can act
All the enzymes involved in glycogen degradation and synthesis are associated with?
The glycogenin particle
The primary enzymes in this process allosterically respond to ?
Metabolites that signal the energy needs of the cell (e.g.AMP): intracellular homeostasis
How is glycogen metabolism regulated ?
They are also regulated by hormones triggering signal cascades in the cell: extracellular homeostasis
Give an overview of glycogen degradation ?
- Glycogen phosphorylase = works on nonreducing ends breaking α 1,4 bonds to release glucose-1-phosphate until it reaches four residues from an (α 1,6) branch point
- Transferase = transfers a block of three residues to the nonreducing end of the chain in an α 1,4 linkage
- Debranching = enzyme cleaves the single remaining (α 1,6)-linked glucose, which becomes a free glucose unit (i.e., NOT glucose-1-phosphate)
The alpha-1,4 linkages are broken down by ?
Phosphorolysis, catalysed by the enzyme glycogen phosphorylase
What does Phosphorolysis remove and form ?
This removes single units from the non-reducing end to form glucose-1-phosphate: already phosphorylated to enter glycolysis
Is phosphorylase energy efficient ?
It is energy efficient because it consumes no ATP
What is Phosphorolysis analogous to ?
Phosphorolysis is analogous to hydrolysis (with phosphate acting to split the bond like water in hydrolysis reactions)
What cannot leave the muscle cell ?
Also, G1P cannot leave the muscle cell (no transporters)
What does Phosphoglucomutase isomerise?
Phosphoglucomutase isomerises it to G6P, and it can then enter glycolysis
Glycogen phosphorylase is the key ?
Regulatory enzyme: regulated by the energy charge, and reversible phosphorylation
What AMP (high during contraction) activate ?
Phosphorylase
ATP and glucose-6-phosphate (sign of high energy) which both compete with ?
AMP binding, inhibit phosphorylase
Thus glycogen breakdown is inhibited when ?
ATP and glucose-6-phosphate are plentiful and activated during periods of energy shortage
Allosteric enzyme structure ?
Homodimer of two identical subunits
Why is the active site buried in deep groove in protein ?
Don’t want water getting in as will end up with hydrolysis instead of phosphorolysis
What different conformation does the active site have ?
Has tense (inactive) and relaxed (active) conformation
Muscle phosphorylase is regulated by ?
The intracellular energy charge
What is the T conformation stabilised by ?
- ATP
- G-6-P
- Glucose (Liver)
What is the R conformation stabilised by ?
- AMP
- Pi
- Phosphorylation
The default state of muscle phosphorylase is the less ?
Active b form in the tense state: only make energy when needed
But what would this mean happens during contraction ?
But this would mean during contraction it would cycle between R and T states as ATP and AMP levels rise and fall
What does Phosphorylase b kinase convert ?
Phosphorylase from b to a form: it is allosterically activated by calcium
What does Protein Phosphatase-1 (PP1) do ?
Dephosphorylates it back to b form
What are both Phosphorylase b kinase and PP1 regulated by?
Extracellular homeostatic mechanisms to link glycogen breakdown with muscle contraction
Phosphorylase a is active regardless of allosteric regulators: however, if AMP is also bound, the ?
Phosphorylated N-terminus is buried inside the protein, inaccessible to the phosphatase
Phosphorylase b is usually inactive because ?
The equilibrium favours the tense state
Phosphorylase a is usually active because ?
The equilibrium favours the relaxed state
What do Allosteric modulators shift ?
Allosteric modulators shift the R-T equilibrium
What does Phosphorylation overrides ?
Allosteric regulation
Phosphorylase b kinase b (less active) can be phosphorylated via ?
AMP dependent kinase to give phosphorylase b kinase a (active form)
The active phosphorylated kinase has a ?
Reduced requirement for calcium
Phosphorylation on a α and β-subunits decreases requirement for ?
Ca2+ on δ-subunit leading to activation, although Ca2+ is still required for full activation
PKA, in the cytoplasm, consists of ?
Two regulatory and two catalytic subunits: the regulatory units keep it switched off
What happens when cAMP binds to the regulatory units?
They dissociate and the catalytic subunits can therefore become active and phosphorylate targets on serine/threonine residues
What is Protein Phosphatase-1 (PP1) ?
Protein phosphatase-1 (PP1) is a phosphatase that reverses the effects of protein kinase A
What happens when PKA is active ?
It phosphorylates and activates protein inhibitor 1 (I1): this binds PP1 and keeps it inactive
What happens when there is no PKA signal ?
I1 is inactive, and PP1 can dephosphorylate I1 itself and then dephosphorylate target proteins in the cytoplasm including glycogen phosphorylase and phosphorylase kinase
What does Phosphorylation involve ?
Transfer of a phosphate group from ATP to an amino acid side chain (usually serine, threonine or tyrosine) of the target protein by a protein kinase
Removal of the phosphate group is catalysed by ?
A second enzyme, a protein phosphatase
What does PP1 dephosphorylate ?
Glycogen synthase, activating it, and phosphorylase kinase and glycogen phosphorylase, deactivating them
Explain Glycogenesis (glycogen synthesis) ?
- Glycogen synthase can only add glucose units to a pre-existing chain of more than 4 glucosyl residues
- UDP donates the first glucosyl residue and attaches it to a tyrosine in the glycogenin priming protein
- A branching enzyme generates branches by cleaving an α-1,4-linkage, moving a block of approximately seven glucoses and synthesizing an α-1,6-linkage.
- Glycogen synthase can then extend the branched polymer
- The use of different pathways for synthesis and breakdown offers control
What tetrameric allosteric enzyme increases Vmax ?
Glucose-6-phosphate
What does Insulin inactivate during glycogen synthesis ?
GSK3 and activates PP1
In Glycogen Synthase, Glucose 6-phosphate acts allosterically to make ?
Glycogen synthase b a better substrate for PP1
What is important to note about phosphorylation and dephosphorylation in Glycogen
Phosphorylation inactivates the enzyme and dephosphorylation activates it (reverse of phosphorylase)
What kind of reaction is Glycogen synthase ?
This is a synthetic reaction, not a degradative reaction
What is Extracellular homeostasis ?
Regulation by hormones
What is Intracellular homeostasis ?
Regulation by metabolites in the cell
What is Allosteric regulation?
Binding of factors changes enzyme activity
What does the use of second messengers and enzymes amplify ?
The response
