Glycogen Metabolism

Question 1

What is glycogen

Answer 1

The storage form of glucose

Question 2

Where is glycogen found

Answer 2

Liver and muscles

Question 3

How is glycogen stored differently in the liver vs. the muscles?

Answer 3

Glycogen = stored in granules
- Muscles: glycogen is stored in beta granules
- Liver: beta granules combine to form alpha granules (10-15 beta granules)

Question 4

Why is having glycogen beneficial to vertebrate organisms?

Answer 4

Some tissues (ex: brain) are in need of a constant supply of glucose. The compact structure of glycogen granules allows for glucose to easily be stored when there is excess and made available at a short notice.

Question 5

Describe the structure of glycogen

Answer 5

• In the center there is a glycogenin homodimer surrounded by tiers of chains of about 13 glucose residues
• Inner B-chains contain two branch points
• Outer A-chains are unbranched (make up the majority of the granule)
• Reducing end: anomeric carbon (number 1 position)
• Non-reducing end (number 4 carbon)

Question 6

What creates the branched structure of glycogen?

Answer 6

(α1→6) linkages
These linkages contribute to the branched structure of B chains

Question 7

What linkages are responsible for conecting the A and B chains of glycogen?

Answer 7

(α1→4) linkages

Question 8

What 3 enzymes are important for the breakdown of glycogen (glycogenolysis)?

Answer 8

1. Glycogen phosphorylase
2. Glycogen debranching enzyme
3. Phosphoglucomutase

Question 9

Describe the first step of glycogenolysis

Answer 9

Glycogen phosphorylase uses phosphate to convert glucose to glucose 1-phosphate breaking bonds between glucose residues at the (α1→4) linkage leaving behind another non-reducing end

Question 10

When will glycogen phosphorylase stop?

Answer 10

When there are 4 glucose molecules left on the glycogen chain

Question 11

Describe the regulation of glycogen phosphorylase

Answer 11

• 2 forms
• Phosphorylase B: dephosphorylated (inactive)
• Phosphorylase A: phosphorylated (active)

Question 12

What is the role of phosphorylase kinase B

Answer 12

Regulates the activation of glycogen phosphorylase into its A form

Question 13

What is the role of phosphoprotein phosphatase 1 (PP1)

Answer 13

Regulates the deactivation of glycogen phosphorylase into its B form

Question 14

Describe the hormonal regulation of Phosphorylase B kinase

Answer 14

• Glucagon (liver) and epinephrin (muscle) act on GPCR’s located on the surface of the cell
• Once the receptor is activated GS-alpha subunit is activated
• GS-alpha subunit converts ATP into cyclic AMP thus raising the concentration

Question 15

What role does cyclic AMP play in the hormonal regulation of phosphorylase B kinase

Answer 15

• Cyclic AMP activates protein kinase A which allosterically activates phosphorylase B kinase
• Phosphorylase kinase B activates glycogen phosphorylase into its A form (active)
• Glycogen breakdown is stimulated

Question 16

The hormonal regulation of phosphorylase B kinase is an example of what?

Answer 16

Ań enzymatic cascade
Initiated by epinephrin or glucagon

Question 17

Describe the allosteric regulation of glycogen phosphorylase by glucose

Answer 17

• Glycogen phosphorylase is a glucose sensor
• 2 allosteric sites separate from the active site will bind to glucose molecules at certain concentrations
• Glycogen phosphorylase undergoes a conformational change when glucose binds to its allosteric sites (serine residues stick out making it easier to remove phosphate groups)
• When blood glucose is low glucagon / epinephrin initiate cascade mechanism that activates glycogen phosphorylase to A form (glucose = released into the blood)
• When blood glucose is high glucose binds to inhibitory allosteric sites on glycogen phosphorylase A resulting in PP1 catalyzing the dephosphorylation of glycogen phosphorylase A into its inactive B form

Question 18

Describe the second step of glycogenolysis

Answer 18

• The debranching enzyme (oligo (α1→6) to (α1→4) glucantransferase) catalyzes 2 successive reactions
• Transferase activity shifts three glucose residues from the branch of one chain to the nonreducing end of another chain (elongation)
• There is one single glucose residue remaining at the branch point
• The (α1→6) glucosidase activity of the debranching enzyme releases the remaining glucose residue
• The remaining unbranched (α1→4) polymer is now ready for further phosphorylation by glycogen phosphorylase

Question 19

Describe the third step of glucogenolysis

Answer 19

Phosphoglucomutase converts glucose 1-phosphate to glucose 6-phosphate

Question 20

What role does serine play in the third step of glucogenolysis?

Answer 20

• The serine residue located at the active site of phosphoglucomutase is what is initially phosphorylated
• The phosphate group is transferred from the number 1 position to the number 6 position leaving you with glucose 6-phosphate

Question 21

How does the use of glucose 6-phosphate differ between the muscle and liver?

Answer 21

Muscle: glucose 6-phosphate enters glycolysis
Liver: glucose 6-phosphate enters the ER reticulum where it can be released into the bloodstream when blood glucose levels drop

Question 22

Describe how glucose 6-phosphate is released into the blood stream by the liver when glucose levels drop

Answer 22

• Requires glucose 6-phosphatase (only present in the liver and kidney)
• Glucose 6-phosphatase = located on the endoplasmic reticulum with its active site on the lumen side of the ER
• Glucose 6-phosphate formed in the cytosol enters the ER lumen through G6P transporter (T1)
• In the lumen, the active site of glucose 6-phosphatase will remove the phosphate group from glucose
• Glucose is then transported from the lumen back into the cytosol (by transporters T2 and T3)
• Glucose can then enter the bloodstream via the plasma membrane transporter GLUT2

Question 23

Describe glycogenesis by UDP glucose

Answer 23

• The generation of glycogen from individual glucose molecules requires UDP glucose (a sugar nucleotide)
• Anomeric carbon of the sugar is activated by attachment to a nucleotide through a phosphate ester linkage
• UDP glucose and other sugar nucleotides = important in glycogen synthesis and other carbohydrate derivatives (ex: monosaccharides, disaccharides etc)

Question 24

Why is UDP glucose important?

Answer 24

• Easy to make (high -ΔG = irreversible pathway)
• Uracil has many groups that can undergo noncovalent interactions with enzymes; the free energy of binding can contribute to the catalytic activity of the enzyme
• Good leaving group (picks up / gives off glucose easily)
• Acts as a tag setting some hexoses with nucleotidyl groups aside for a particular purpose

Question 25

Describe the formation of UDP glucose

Answer 25

- Generated by glucose 1-phosphate - Start with uridine try-phosphate + glucose 1-phosphate - Sugar enzyme will take up sugar via its phosphate group and make a phosphor-ester bond between two phosphate molecules - The product = molecule of UDP glucose + molecule of pyrophosphate - Pyrophosphate (2 phosphate groups) breaks down easily into other phosphate groups - The breakdown of a pyrophosphate group into phosphate is what drives the reaction forward (large -ΔG)

Question 26

What is the starting point for glycogen synthesis?

Answer 26

Glucose 6-phosphate

Question 27

How is glucose 6-phosphate derived?

Answer 27

- Hexokinase I and II (muscle) - Hexokinase IV (liver) Glucose + ATP → glucose 6-phosphate + ADP OR... - Glucose = converted to lactate by glycolysis - Lactate is taken up by the liver and converted to glucose 6-phosphate by gluconeogenesis - Glucose 6-phosphate is converted to glucose 1-phosphate by phosphogluco-mutase Glucose 6-phosphate ⇌ Glucose 1-phosphate - Glucose 1-phosphate is converted to UDP-glucose by sugar enzyme Glucose 1-phosphate + UTP → UDP-glucose + PPi

Question 28

How does UDP glucose catalyze the addition of glucose to the non-reducing end of glycogen

Answer 28

- Catalyzed by glycogen synthase - UDP glucose donates glucose residues which promotes the transfer of the glucose residue from UDP-glucose to a nonreducing end of a branched glycogen molecule forming an (α1→4) linkage (elongation) *Glycogen synthase CANNOT form branches*

Question 29

Describe glycogen synthase regulation

Answer 29

- 2 forms - Phosphorylated: inactive (glycogen synthase B) - Dephosphorylated: active (glycogen synthase A)

Question 30

What is the role of GSK3 in glycogen synthase regulation?

Answer 30

GSK3 adds phosphoryl groups to three serine residues near the carboxyl terminus of glycogen synthase A converting it to its B form *GSK3 requires a primer*

Question 31

When can glycogen synthase B be activated without GSK3?

Answer 31

In the presence of its allosteric activator glucose 6-phosphate

Question 32

What is the purpose of Casein Kinase II (CKII)?

Answer 32

- Primer - Adds phosphate group to GSK3 GSK3 will then take over and add the three phosphate groups needed to inactivate glycogen synthase

Question 33

Describe priming by casein kinase II (CKII)

Answer 33

- Long chain of serine residues that get phosphorylated or dephosphorylated - First serine gets phosphorylated at the +4 position which is identified by casein kinase II - Phosphorylation creates a negative patch on the glycogen synthase enzyme - Negative patch is recognized by positive residues on GSK3 enzyme - Following serine to get phosphorylated is at the number 0 position - The result is a cascading effect the following serines to be phosphorylated are at the -4 and -8 positions

Question 34

Describe the hormonal regulation of GSK3

Answer 34

- Hormonally regulated by glucagon and epinephrin - Insulin converts glycogen synthase to its active form by blocking GSK3 activity and activating PP1

Question 35

What makes the (α1→6) linkages found in glycogen?

Answer 35

Glycogen branching enzyme

Question 36

Describe the branching of glycogen by the glycogen branching enzyme

Answer 36

- Acts on chain of glucose 11 monomers long - Transfers 6 or 7 residues from the nonreducing end of glycogen branch sticking them onto the first glucose residue at a more interior position of the same glycogen chain thus creating a new branch

Question 37

What is the biological effect of branching?

Answer 37

- Increase the number of nonreducing ends thus increasing the number of sites accessible to glycogen phosphorylase and glycogen synthase *Only act on nonreducing ends*

Question 38

How is the synthesis of a new glycogen molecule initiated?

Answer 38

- Glycogenin (homodimer) - Glycogenin is both the primer for glycogen and the enzyme that catalyzes the assembly of new glycogen chains

Question 39

Describe the synthesis of a new glycogen molecule

Answer 39

- Both monomers of glycogenin contain a tyrosine residue - Glucose residue is transfered from UDP-glucose to the hydroxyl group of Tyr of glycogenin (2 step process pg. 2054) - Glycogenin has the ability to add up to 8 glucose residues - Once 8 residues have been added glycogen synthase will take over and extend the chain

Question 40

What hormone favors the activation of glycogen synthase?

Answer 40

Insulin (lowers blood sugar)

Question 41

Describe how insulin favors the activation of glycogen synthase

Answer 41

- Favors active form of glycogen synthase by blocking GSK3 activity (inhibits glycogen synthase by converting it to its inactive form) - Activates PP1 (turns glycogen phosphorylase into its B form (no glycogen break down) *These two steps are required for activation of glycogen synthase*

Question 42

What hormone favors the deactivation of glycogen synthase?

Answer 42

Glucagon (raises blood sugar)

Question 43

Does glucose 6-phosphate favor the active or inactive form of glycogen synthase?

Answer 43

- Active - Binds to glycogen synthase promoting a conformation that is a good substrate for PP1