M2: Glycogen Synthesis & Breakdown L6

Question 1

What is glycogen?

Answer 1

Branched polymer of glucose.

Question 2

When does glycogen storage increase and decrease?

Answer 2

Glycogen levels increase after meals and are utilized (decreased) during fasting/exercise.

Question 3

What are glycogen granules and what do they contain?

Answer 3

~120,000 glucose units/granule

Contain all the enzymes required for glycogen synthesis and breakdown

Question 4

What is the purpose of storing glycogen?

Answer 4

• Glycogen catabolism is faster than FA’s at generating ATP.
• Can be used under anaerobic conditions in sk. muscle
• Doesn’t disturb osmotic pressure as would an
  equivalent amount of glucose monomers
  (glucose would disturb the osmotic pressure).
• Breakdown of glycogen in muscle provides G1P, faster than glucose can be taken up from the blood.
• Liver’s capacity to store glycogen is sufficient to supply the brain with glucose for only about 12 h.

Question 5

What is glycogen in muscle used for?

Answer 5

It’s available for local energy production for muscle contraction.

Question 6

What is G6P phosphatase? Where is it located?

Answer 6

An enzyme that removes the phosphate group from G6P which allows glucose to leave a cell and go into the blood. It is located in the membrane of the ER, whose catalytic site faces the ER lumen.

Question 7

How does having a low G6P phosphatase in muscle tissues affect them?

Answer 7

It causes the muscle to use the glucose generated from glycogen for itself because it can’t release the glucose to the blood.

Question 8

What is glycogen in the liver used for?

Answer 8

To maintain blood glucose levels.

Question 9

How does the liver express G6P phosphatase?

Answer 9

Selectively.

Question 10

How does G6P phosphatase work?

Answer 10

G6P from the cytosol enters the ER lumen through a G6P transporter (T1) . Once inside, G6P interacts with the catalytic site of G6P phosphatase to be turned into glucose + Phosphate. Then glucose is transported out of the ER lumen into the cytosol through a glucose transporter (T2). Once in the cytosol, glucose is transported through a GLUT 2 transporter in the plasma membrane and into the capillaries.

Question 11

Why is the catalytic site of G6P phosphatase facing the ER lumen?

Answer 11

For compartmentalization. You don’t wan the cell to compete for the G6P pool. One G6P pool is in the cytosol for glycolysis, and theres another one in the ER lumen for export.

Question 12

What is a non-reducing end and how do you identify it?

Answer 12

Identify: lack of a C1-OH group.

What is it: It is the only part of the glycogen chain that you can add or remove glucoses from.

Question 13

How many reducing ends are there per glycogen granule?

Answer 13

One.

Question 14

What are the different ways to link glucosyl residues?

Answer 14

1. C1-C4 (alpha 1-4 linkages)

2. C1-C6 (alpha 1-6 linkage)

Question 15

What kind of link makes a branching point in glycogen and why are they useful?

Answer 15

Link: C1-C6 linkage

useful because branches provide a large number of non-reducing ends to allow multiple sites for synthesis/degradation.

Question 16

How often in a glycogen molecule do you find branching points?

Answer 16

Every 8-14 residues.

Question 17

What are the rate limiting enzymes for glycogen synthesis and breakdown?

Answer 17

Synthesis: Glycogen synthase
Breakdown: Glycogen phosphorylase

Question 18

What is the glycolytic intermediate that branches off into glycogen synthesis? Through which enzyme?

Answer 18

intermediate: G6P

Enzyme that turns G6P to G1P is phosphoglucomutase.

Question 19

What are the 3 steps of glycogen synthesis?

Answer 19

1. Synthesis of UDP-glucose from G1P and UTP.
2. Elongation of a pre-existing glycogen chain using UDP-glucose and glycogen synthase.
3. Creation of new 1,6-glucosyl branch points using the branching enzyme.

Question 20

Memorize the glycogen synthesis cycle.

Answer 20

See L6 slide 9-15

Question 21

Why is the production of UDP-glucose in vivo highly favourable and irreversible?

Answer 21

In vivo, the Ppi that come from UTP is hydrolized right away by pyrophosphatase. This leads to a low concentration of ppi in vivo, which makes the overall reaction highly exergonic and favourable. G˚ < 0.

Question 22

What is glycogen synthase’s role?

Answer 22

It elongates a PRE-EXISTING glycogen chain by removing UDP from UDP-glucose when attaching it to the non-reducing end of glycogen (ALPHA 1-4 LINKAGE).

Question 23

What happens to the UDP released when UDP-Glucose is broken to bind to glycogen? Why?

Answer 23

UDP added to ATP to form UTP and ADP. This is catalyzed by nucleoside diphosphate kinase. This is done in order to restore UTP which is used for the synthesis of UDP-glucose (step 1).

Question 24

When is ATP invested during glycogen synthesis? Through which enzymes?

Answer 24

1. Glucose to G6P through hexokinase.

2. UDP to UTP through nucleoside diphosphate kinase.

Question 25

What is glycogenin’s role?

Answer 25

To link together two glucose residues (can make a brand new glycogen chain).

Question 26

How many glycogenin’s are present for a glycogen chain?

Answer 26

One.

Question 27

What is the mechanism of glycogenin?

Answer 27

1) Glycogenin attaches a glucose residue donated by UDPG to the OH group of its Tyr 194.
2) Glycogenin then extends the glucose chain by up to 7 additional UDPG- donated glucose residues to form a glycogen “primer.” (8 glucose residues total).
3) Then glycogen synthase (bound to glycogenin) commences glycogen synthesis by extending the primer.

Question 28

What are the consequences of needing glycogen synthase to be bound to glycogenin in order to be efficient?

Answer 28

1. The number of glycogen granules is determined by the number of glycogenin molecules.
2. The size of the glycogen molecule stops when glycogen synthase is no longer in contact with glycogenin (limits size).

Question 29

What are the domains of glycogen synthase? When does glycogen synthase detach from glycogenin?

Answer 29

Glycogen synthase has 3 domains: binding domain (binds to glycogenin), a catalytic domain (adds glucose units to glycogen), a linker region (variable in length). The bigger the linker the longer the glycogen molecule can get. Once the size of the glycogen molecule gets passed the size of the linker, it can no longer increase in size.

Question 30

When does glycogen synthase take over the elongation of glycogen?

Answer 30

Once glycogenin has made its 8 unit chain (primer).

Question 31

What are the rules the branching enzyme follows when making new branches of glycogen?

Answer 31

1. Transfers ~7 glucosyl residues to the C6-OH.
2. Each transferred segment must come from a chain of 11 residues minimum.
3. The new branch point must be at least 4 residues away from other branch points.

Question 32

How many ATPs/glucose residue is needed for glycogen synthesis?

Answer 32

2 ATP/glucose residue.

Question 33

What are the steps of glycogen breakdown?

Answer 33

1. Generation of glucose-1-phosphate using glycogen phosphorylase
2. Debranching
3. Conversion of glucose-1-phosphate to glucose-6-phosphate using phosphoglucomutase

Question 34

Memorize the glycogen breakdown.

Answer 34

L6 Slides 17-21.

Question 35

What are the two ways to cleave a glycosidic bond?

Answer 35

1. Hydrolysis

2. Phosphorolysis

Question 36

What is the advantage of using phosphorolysis over hydrolysis when cleaving a glycosidic bond?

Answer 36

Using hydrolysis creates a glucose molecule. The glucose molecule would then need to be converted to G6P by hexokinase which consumes an ATP.

Using phosphorolysis creates a G1P directly. G1P can then be converted into G6P by phosphoglucomutase. This saves the use of an ATP.

Question 37

What enzyme is used for phosohorolysis of glycogen?

Answer 37

Glycogen phosphorylase.

Question 38

What are the rules of glycogen breakdown using glycogen phosphorylase?

Answer 38

• Phosphates are only added to the non-reducing ends.

- Glycogen phosphorylase will release glucose units within 4 residues of a branch point.

Question 39

Why is the glycogen phosphorylase reaction favoured in vivo when it is endergonic in vitro?

Answer 39

The levels of inorganic phopshate are much greater than the levels of G1P in vivo. Reactants > Products. This drives the rxn forward. The reaction is therefore exergonic in vivo.

Question 40

When do you need the debranching enzyme?

Answer 40

When the branch gets down to 4 residues, glycogen phosphorylase no longer works, so you need the de-branching enzyme.

Question 41

What 2 enzymes are part of the debranching enzyme?

Answer 41

1. alpha(1-4) glucosyltransferase

2. alpha(1-6) glucosidase

Question 42

What is the role of alpha (1-4) glucosyltransferase?

Answer 42

• Transfers an α(1-4) linked trisaccharide to the nonreducing end of another branch (new α(1-4) linkage).
• The 3 units are then available for glycogen phosphorylase reaction.

Question 43

What is the role of alpha(1-6) glucosidase?

Answer 43

• The remaining glycosyl residue is hydrolyzed to yield glucose and debranched glycogen

Question 44

What’s the difference between the muscle and the liver once G6P is made by phosphoglucomutase from G1P?

Answer 44

Muscle: G6P continues into glycolysis to generate ATP

Liver: G6P is converted to glucose (by G6P phosphatase) and goes to circulation

Question 45

What is the difference in net ATP synthesis using glycolysis vs Glycogen breakdown?

Answer 45

Glycolysis: net 32 ATP

Glycogen breakdown: net 31 ATP bc synthesis of glycogen costs 2 ATP and breakdown generates 33 ATP… 33-2 = 31 ATP).

Question 46

Is the storage of glucose as glycogen an efficient process?

Answer 46

Yes. 31/32 = 97% efficiency of energy extraction from glycogen in the form of glucose.