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BB10004 > Glycogen structure, synthesis and breakdown > Flashcards

Glycogen structure, synthesis and breakdown Flashcards

1
Q

Describe the basic structure of glycogen

A
  1. α(1–>4) linked glucose
  2. α(1–>6) linked branch points
  3. Polydisperse
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2
Q

What does polydisperse mean

A
  1. No specific structure in every molecule, so where the branches are is different and length vary differently- and precise structures vary from molecule to molecule
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3
Q

What is the starting point of a glycogen molecule

A
  1. Glycogenin-Present as homodimer so there are two starting points where all glucose attaches
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4
Q

Describe the structure of glycogen

A
  1. Glycogenin homodimer
  2. Inner region – B-chains, two branch points
  3. Outer region – A-chains, not branched
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5
Q

How many glucose residues does a typical glycogen molecule contain and what is the theoretical maximum

A
  1. Typical glycogen molecules found in vivo contain around 1500 glucose residues
  2. (Theoretical maximum of ~55000)
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6
Q

Describe how branching occurs

A
  1. As glycogen grows
  2. when 13 residues have been added the branching enzyme recognises the target and makes a branch
  3. the new chain is then grown
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7
Q

Why are the outermost chains unbranched

A
  1. Makes the glucose easily accessible

2. Easier to get glucose off unbranched chain

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8
Q

What percentage of glucose does the outermost tier contain

A
  1. Outermost tier always contains 34.6% of the glucose of the glycogen molecule
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9
Q

What is glycogenin

A
  1. It is an enzyme
  2. Primer for formation is glycogenin
  3. Protein homodimer found at the core of a glycogen molecule
  4. Not necessarily in the centre
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10
Q

How are other proteins involved

A
  1. Other proteins are associated with each glycogen molecule
  2. These are proteins involved in synthesis and breakdown, including control
  3. No rigid stoichiometry (amounts)
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11
Q

How is glycogen synthesised at first

A
  1. Glycogenin is able to catalyse the addition of glucose to itself
  2. First glucose added to Tyr195 (amino acid can vary) but always to hydroxyl group of tyrosine side chain
  3. Subsequent glucose added to growing chain
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12
Q

When does the enzyme involved in the synthesis of glycogen change

A
  1. Once about 10-20 residues added the main synthetic enzyme takes over
  2. Glycogen synthase takes over
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13
Q

What are the activated precursors used in glycogen synthesis

A
  1. UDP-glucose – in eukaryotes (uracil diphosphate)

2. (ADP-glucose used in bacteria and plants [starch])

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14
Q

How is UDP-glucose formed

A
  1. UTP + glucose-1-phosphate –>UDP-glucose
  2. Formation of UDP-glucose releases a 2-phosphate group from UTP (Pyrophosphate)
  3. UDP is attached at carbon 1- alpha carbon
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15
Q

What drives the UDP-glucose formation

A
  1. Pyrophosphate is hydrolysed to form 2 inorganic phosphates catalysed by pyrophosphorylase
  2. Removal of product drives the reaction
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16
Q

How is glucose attached to the growing chain

A
  1. addition is a glycosyl transfer with release of UDP- only glycosyl part is attached
  2. Mechanism -double SN2? or SNi? – latest evidence is for SNi
  3. Whichever mechanism, the α conformation is retained
17
Q

What is the branching enzyme

A
  1. amylo-(1,4 –> 1,6)transglycosylase
18
Q

How does branching occur

A
  1. A terminal chain section of ~7 residues is transferred to the C6-OH of another glycogen chain
  2. Chain which has already been grown is taken from the end of one chain and put on to be a branch
  3. Enzyme cleaves alpha-1,4 linkage on growing chain to form 1,6 linkage on another chain- forms branch
  4. This new chain can be extended using glycogen synthase
  5. May not grow If glycogen synthase activity is blocked by number of other chains
19
Q

What are the 3 enzymes responsible for degradation

A
  1. Glycogen phosphorylase
  2. Glycogen debranching enzyme
  3. Phosphoglucomutase
20
Q

Describe the main function of glycogen phoshporylase

A
  1. Primary enzyme in degradation
21
Q

Describe the main function of glycogen debranching enzyme

A
  1. Removes branches
22
Q

Describe the main function of phosphoglucomutase

A
  1. Also has role in synthesis

2. Conversion of released units into glucose

23
Q

Describe the relationship between degradation and synthesis

A
  1. Occurs by a different pathway to synthesis
  2. Ensures that energetically this is possible
  3. Both can be controlled to allow very fine regulation
  4. Not at equilibrium- wouldn’t be effective
24
Q

What does glycogen phosphorylase do

A
  1. Takes inorganic phosphate present in cytosol and solution in cells
  2. Use addition of inorganic phosphate to position 1 to produce glucose-1-phosphate
25
Q

What does the structure of glycogen phosphorylase mean

A
  1. Because of structure can only get itself within 5 residues of a branch
  2. Can’t get active site to position 1
26
Q

What is the mechanism that glycogen phosphorylase uses

A
  1. Via a carbocation (SN1)
  2. Carbocation is stabilised by pyridoxal phosphate which is covalently linked to the enzyme
    (PLP is active form of Vit B6)
27
Q

What are the two roles of debranching enzyme

A
  1. Eukaryotic enzyme is bi-functional

2. Acts as a transferase and α-1,6 glucosidase

28
Q

What does debranching enzyme do

A
  1. The Phosphorylase gets to within 5 residues
  2. Debranching enzyme cleaves an alpha 1,4 linkage and moves chunk of chain onto adjacent branch
  3. Then hydrolyses 1,6 linkage to remove a single unit
  4. Forms longer chain which can be broken down by phosphorylase because there is no branch point
29
Q

What does phosphoglucomutase do

A
  1. converts G-1-P into G-6-P
  2. Phosphate attached to serine on enzyme
  3. Attaches to glucose to form bisphosphate intermediate
  4. Phosphate on 1 carbon goes onto the enzyme
  5. Equilibria, by changing concentration of 1-P or 6-P can change direction
30
Q

What happens to the G-6-P formed

A
  1. G-6-P enters glycolysis

2. OR in liver it can be dephosphorylated to form glucose for the blood

31
Q

What happens when glucose levels are plentiful

A
  1. G-6-P is formed by hexokinase
  2. Changes equilibrium position
  3. Phosphoglucomutase converts G-6-P in to G-1-P
  4. G-1-P is substrate to form UDP-glucose for glycogen synthesis
  5. Phosphoglucomutase is acting to help synthesis
32
Q

Describe Liver/muscle capacity for synthesis and degradation

A
  1. In liver – synthetic capacity and degradative capacity are about equal
  2. In muscle – degradation can happen about 300 times faster than synthesis
  3. Synthetic capacity of the muscle can increase with training

BB10004 (14 decks)

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