Glycogen Metabolism

Question 1

Describe the structure of Glycogen

Answer 1

A branched chain homopolymers made up of glucose residues by alpha 1,4 and alpha 1,6 bonds

the alpha 1,6 bonds form branches

Question 2

What is the name of the protein that binds to glucose at the reducing end of glycogen?

Answer 2

Glycogenin

Question 3

What is the main use of glycogen in the liver?

Answer 3

Maintaining blood glucose

Question 4

What is the main use of glycogen in skeletal muscle?

Answer 4

Supplies glucose for ATP generation

Question 5

T or F: Glucose 6-phosphotase is not found in skeletal muscle cells

Answer 5

True

Glucose 6-phosphotase catalyzes the conversion of glucose 6-phosphate to glucose.

Skeletal muscle cells do not need the glucose molecules. They need ATP, so Glucose 6-phosphotase will just enter glycolysis.

Question 6

What two hormones stimulate glucose 6-phosphotase in the liver?

Answer 6

Glucagon and epinephrine

Question 7

Glycogenesis requires energy in the form of what molecule?

Answer 7

UTP

Question 8

What intermediate is glycogen formed from and degraded to?

Answer 8

Glucose 1 -phosphate

Degradation and synthesis require different enzymes

Question 9

What molecule donates glucose during glycogen synthesis (glycogenesis)?

Answer 9

Uridine diphosphate-glucose (UDP-glucose)

this molecule is NOT an intermediate in glycogen degradation

Question 10

enzyme that adds glucosyl residue from UDP-glucose to a preexisting chain of glycogen via a glycogen primer?

Answer 10

Glycogen Synthase

• Forms alpha 1,4 bonds
• This is a regulated step of glycogenesis

Question 11

What is the glycogen primer? How does it work?

Answer 11

Glycogenin

Works by autoglycosylation
- the protein adds a glucosyl residue from UDP-glucose to a tyrosine residue within itself. It continues to extend the chain until it is long enough to serve as a primer

Question 12

How are the branches of glycogen made?

Answer 12

Amylo-4,6-transferase

enzyme cleaves a small chain of glucose residues off glycogen and reattaches them to another glycosly unit via an alpha 1,6 bond

Question 13

What enzymes catalyzes the degradation of glycogen?

Answer 13

1. Glycogen phosphorylase

• enzyme cleaves glycogen at non-reducing end to form glucose 1-phosphate
• this enzyme is regulated
• enzyme CANNOT cleave the alpha 1,6 bond (branch bond)

2. Debranching Enzyme
   - has two activities: Transfers glucosyl residues, and hydrolysis of alpha 1,6 bonds (branching)

Question 14

What regulates the glycogenesis and glycogenolysis?

Answer 14

• Insulin:Glucagon ratio

- blood glucose levels

Question 15

After a carbohydrate meal, what processes are stimulated in the liver?

Answer 15

-Glycogenesis
(fatty acid synthesis, triglyceride synthesis, and glycolysis also occur)

Due to high insulin, low glucagon, and high blood glucose

Question 16

How does glucagon regulate glycogen metabolism in the liver?

Answer 16

Glucagon causes the phosphorylation of Glycogen phosphotase, and Glycogen snythase

Glycogen Phosphorylase = active when phosphorylated
Glycogen snythase = inactibe when phosphylated

Glucagon promotes glycogenolysis (degradation)

Question 17

Describe the process of Glucagon regulation in the liver

Answer 17

1. Glucagon binds to cell-surface GPCR that activates adenylate cyclase
2. Adenylate cyclase produces cAMP from ATP
3. cAMP binds to and activates PKA
4. Activated PKA initiates a phosphorylation cascade leading to the phosphorylation of Glycogen Phosphorylase
5. Glycogen Phosphorylase coverts glycogen to glucose 1-phosphate

Question 18

What type of enzyme removes the phosphate from Glycogen synthase and Glycogen Phosphorylase? What is the name of this enzyme in the Liver?

Answer 18

Protein phosphotases

Called Hepatic Protein phosphotase-1 in the liver

• Inactive during fasting state

Question 19

How does insulin regulate glycogen metabolism in the liver?

Answer 19

Activates Hepatic Protein phosphotase-1

- which removes the phosphate on Glycogen Synthase allowing it to make glycogen

Question 20

How does epinephrine affect glycogen metabolism in the liver?

Answer 20

Epinephrine stimulates glycogenolysis

• when bound to Beta-receptors, it has a mechanism of action as glucagon (GPCR mechanism)
• when bound to alpha-receptors, epinephrine stimulates the release of Ca2+ (Calmodulin mechanism)

Question 21

How does glucagon regulate glycogen metabolism in skeletal muscle?

Answer 21

It has NO EFFECT on skeletal muscle

- muscle glycogen levels for not very during fasting/feeding states

Question 22

How is glycogen degradation regulated in skeletal muscle?

Answer 22

Occurs when there is a high demand for ATP
aka high levels of AMP

Epinephrine and high Ca2+ also stimulates degradation