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Cell Regulation and Cancer > Regulation of glycogen breakdown and synthesis > Flashcards

Regulation of glycogen breakdown and synthesis Flashcards

1
Q

ATP is made from ?

A

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

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2
Q

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

A

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

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3
Q

In muscle, what is the Glycogen for ?

A

Glycogen is a glucose reserve for the muscle only

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4
Q

In the liver, what is the Glycogen for ?

A

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

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5
Q

Explain the structure of glycogen ?

A
  • 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
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6
Q

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

A

The glycogenin particle

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7
Q

The primary enzymes in this process allosterically respond to ?

A

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

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8
Q

How is glycogen metabolism regulated ?

A

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

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9
Q

Give an overview of glycogen degradation ?

A
  • 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)
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10
Q

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

A

Phosphorolysis, catalysed by the enzyme glycogen phosphorylase

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11
Q

What does Phosphorolysis remove and form ?

A

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

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12
Q

Is phosphorylase energy efficient ?

A

It is energy efficient because it consumes no ATP

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13
Q

What is Phosphorolysis analogous to ?

A

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

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14
Q

What cannot leave the muscle cell ?

A

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

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15
Q

What does Phosphoglucomutase isomerise?

A

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

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16
Q

Glycogen phosphorylase is the key ?

A

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

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17
Q

What AMP (high during contraction) activate ?

A

Phosphorylase

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18
Q

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

A

AMP binding, inhibit phosphorylase

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19
Q

Thus glycogen breakdown is inhibited when ?

A

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

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20
Q

Allosteric enzyme structure ?

A

Homodimer of two identical subunits

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21
Q

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

A

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

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22
Q

What different conformation does the active site have ?

A

Has tense (inactive) and relaxed (active) conformation

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23
Q

Muscle phosphorylase is regulated by ?

A

The intracellular energy charge

24
Q

What is the T conformation stabilised by ?

A
  • ATP
  • G-6-P
  • Glucose (Liver)
25
Q

What is the R conformation stabilised by ?

A
  • AMP
  • Pi
  • Phosphorylation
26
Q

The default state of muscle phosphorylase is the less ?

A

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

27
Q

But what would this mean happens during contraction ?

A

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

28
Q

What does Phosphorylase b kinase convert ?

A

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

29
Q

What does Protein Phosphatase-1 (PP1) do ?

A

Dephosphorylates it back to b form

30
Q

What are both Phosphorylase b kinase and PP1 regulated by?

A

Extracellular homeostatic mechanisms to link glycogen breakdown with muscle contraction

31
Q

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

A

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

32
Q

Phosphorylase b is usually inactive because ?

A

The equilibrium favours the tense state

33
Q

Phosphorylase a is usually active because ?

A

The equilibrium favours the relaxed state

34
Q

What do Allosteric modulators shift ?

A

Allosteric modulators shift the R-T equilibrium

35
Q

What does Phosphorylation overrides ?

A

Allosteric regulation

36
Q

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

A

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

37
Q

The active phosphorylated kinase has a ?

A

Reduced requirement for calcium

38
Q

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

A

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

39
Q

PKA, in the cytoplasm, consists of ?

A

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

40
Q

What happens when cAMP binds to the regulatory units?

A

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

41
Q

What is Protein Phosphatase-1 (PP1) ?

A

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

42
Q

What happens when PKA is active ?

A

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

43
Q

What happens when there is no PKA signal ?

A

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

44
Q

What does Phosphorylation involve ?

A

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

45
Q

Removal of the phosphate group is catalysed by ?

A

A second enzyme, a protein phosphatase

46
Q

What does PP1 dephosphorylate ?

A

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

47
Q

Explain Glycogenesis (glycogen synthesis) ?

A
  • 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
48
Q

What tetrameric allosteric enzyme increases Vmax ?

A

Glucose-6-phosphate

49
Q

What does Insulin inactivate during glycogen synthesis ?

A

GSK3 and activates PP1

50
Q

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

A

Glycogen synthase b a better substrate for PP1

51
Q

What is important to note about phosphorylation and dephosphorylation in Glycogen

A

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

52
Q

What kind of reaction is Glycogen synthase ?

A

This is a synthetic reaction, not a degradative reaction

53
Q

What is Extracellular homeostasis ?

A

Regulation by hormones

54
Q

What is Intracellular homeostasis ?

A

Regulation by metabolites in the cell

55
Q

What is Allosteric regulation?

A

Binding of factors changes enzyme activity

56
Q

What does the use of second messengers and enzymes amplify ?

A

The response

Cell Regulation and Cancer (13 decks)

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