Regulation of glycogen breakdown and synthesis

Question 1

ATP is made from ?

Answer 1

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

Question 2

Therefore what is used as an energy store for the cell ?

Answer 2

Instead, the cell uses a glucose polymer called glycogen which is insoluble and inert

Question 3

In muscle, what is the Glycogen for ?

Answer 3

Glycogen is a glucose reserve for the muscle only

Question 4

In the liver, what is the Glycogen for ?

Answer 4

Glycogen is a glucose reserve for the maintenance of blood glucose concentration

Question 5

Explain the structure of glycogen ?

Answer 5

• 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

Question 6

All the enzymes involved in glycogen degradation and synthesis are associated with?

Answer 6

The glycogenin particle

Question 7

The primary enzymes in this process allosterically respond to ?

Answer 7

Metabolites that signal the energy needs of the cell (e.g.AMP): intracellular homeostasis

Question 8

How is glycogen metabolism regulated ?

Answer 8

They are also regulated by hormones triggering signal cascades in the cell: extracellular homeostasis

Question 9

Give an overview of glycogen degradation ?

Answer 9

• 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)

Question 10

The alpha-1,4 linkages are broken down by ?

Answer 10

Phosphorolysis, catalysed by the enzyme glycogen phosphorylase

Question 11

What does Phosphorolysis remove and form ?

Answer 11

This removes single units from the non-reducing end to form glucose-1-phosphate: already phosphorylated to enter glycolysis

Question 12

Is phosphorylase energy efficient ?

Answer 12

It is energy efficient because it consumes no ATP

Question 13

What is Phosphorolysis analogous to ?

Answer 13

Phosphorolysis is analogous to hydrolysis (with phosphate acting to split the bond like water in hydrolysis reactions)

Question 14

What cannot leave the muscle cell ?

Answer 14

Also, G1P cannot leave the muscle cell (no transporters)

Question 15

What does Phosphoglucomutase isomerise?

Answer 15

Phosphoglucomutase isomerises it to G6P, and it can then enter glycolysis

Question 16

Glycogen phosphorylase is the key ?

Answer 16

Regulatory enzyme: regulated by the energy charge, and reversible phosphorylation

Question 17

What AMP (high during contraction) activate ?

Answer 17

Phosphorylase

Question 18

ATP and glucose-6-phosphate (sign of high energy) which both compete with ?

Answer 18

AMP binding, inhibit phosphorylase

Question 19

Thus glycogen breakdown is inhibited when ?

Answer 19

ATP and glucose-6-phosphate are plentiful and activated during periods of energy shortage

Question 20

Allosteric enzyme structure ?

Answer 20

Homodimer of two identical subunits

Question 21

Why is the active site buried in deep groove in protein ?

Answer 21

Don’t want water getting in as will end up with hydrolysis instead of phosphorolysis

Question 22

What different conformation does the active site have ?

Answer 22

Has tense (inactive) and relaxed (active) conformation

Question 23

Muscle phosphorylase is regulated by ?

Answer 23

The intracellular energy charge

Question 24

What is the T conformation stabilised by ?

Answer 24

• ATP
• G-6-P
• Glucose (Liver)

Question 25

What is the R conformation stabilised by ?

Answer 25

- AMP - Pi - Phosphorylation

Question 26

The default state of muscle phosphorylase is the less ?

Answer 26

Active b form in the tense state: only make energy when needed

Question 27

But what would this mean happens during contraction ?

Answer 27

But this would mean during contraction it would cycle between R and T states as ATP and AMP levels rise and fall

Question 28

What does Phosphorylase b kinase convert ?

Answer 28

Phosphorylase from b to a form: it is allosterically activated by calcium

Question 29

What does Protein Phosphatase-1 (PP1) do ?

Answer 29

Dephosphorylates it back to b form

Question 30

What are both Phosphorylase b kinase and PP1 regulated by?

Answer 30

Extracellular homeostatic mechanisms to link glycogen breakdown with muscle contraction

Question 31

Phosphorylase a is active regardless of allosteric regulators: however, if AMP is also bound, the ?

Answer 31

Phosphorylated N-terminus is buried inside the protein, inaccessible to the phosphatase

Question 32

Phosphorylase b is usually inactive because ?

Answer 32

The equilibrium favours the tense state

Question 33

Phosphorylase a is usually active because ?

Answer 33

The equilibrium favours the relaxed state

Question 34

What do Allosteric modulators shift ?

Answer 34

Allosteric modulators shift the R-T equilibrium

Question 35

What does Phosphorylation overrides ?

Answer 35

Allosteric regulation

Question 36

Phosphorylase b kinase b (less active) can be phosphorylated via ?

Answer 36

AMP dependent kinase to give phosphorylase b kinase a (active form)

Question 37

The active phosphorylated kinase has a ?

Answer 37

Reduced requirement for calcium

Question 38

Phosphorylation on a α and β-subunits decreases requirement for ?

Answer 38

Ca2+ on δ-subunit leading to activation, although Ca2+ is still required for full activation

Question 39

PKA, in the cytoplasm, consists of ?

Answer 39

Two regulatory and two catalytic subunits: the regulatory units keep it switched off

Question 40

What happens when cAMP binds to the regulatory units?

Answer 40

They dissociate and the catalytic subunits can therefore become active and phosphorylate targets on serine/threonine residues

Question 41

What is Protein Phosphatase-1 (PP1) ?

Answer 41

Protein phosphatase-1 (PP1) is a phosphatase that reverses the effects of protein kinase A

Question 42

What happens when PKA is active ?

Answer 42

It phosphorylates and activates protein inhibitor 1 (I1): this binds PP1 and keeps it inactive

Question 43

What happens when there is no PKA signal ?

Answer 43

I1 is inactive, and PP1 can dephosphorylate I1 itself and then dephosphorylate target proteins in the cytoplasm including glycogen phosphorylase and phosphorylase kinase

Question 44

What does Phosphorylation involve ?

Answer 44

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

Question 45

Removal of the phosphate group is catalysed by ?

Answer 45

A second enzyme, a protein phosphatase

Question 46

What does PP1 dephosphorylate ?

Answer 46

Glycogen synthase, activating it, and phosphorylase kinase and glycogen phosphorylase, deactivating them

Question 47

Explain Glycogenesis (glycogen synthesis) ?

Answer 47

- 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

Question 48

What tetrameric allosteric enzyme increases Vmax ?

Answer 48

Glucose-6-phosphate

Question 49

What does Insulin inactivate during glycogen synthesis ?

Answer 49

GSK3 and activates PP1

Question 50

In Glycogen Synthase, Glucose 6-phosphate acts allosterically to make ?

Answer 50

Glycogen synthase b a better substrate for PP1

Question 51

What is important to note about phosphorylation and dephosphorylation in Glycogen

Answer 51

Phosphorylation inactivates the enzyme and dephosphorylation activates it (reverse of phosphorylase)

Question 52

What kind of reaction is Glycogen synthase ?

Answer 52

This is a synthetic reaction, not a degradative reaction

Question 53

What is Extracellular homeostasis ?

Answer 53

Regulation by hormones

Question 54

What is Intracellular homeostasis ?

Answer 54

Regulation by metabolites in the cell

Question 55

What is Allosteric regulation?

Answer 55

Binding of factors changes enzyme activity

Question 56

What does the use of second messengers and enzymes amplify ?

Answer 56

The response