MB - Glycogen Metabolism

Question 1

Describe the structure of Glycogen?

Answer 1

A branched polymer of glucose

• Main chain a(1–>4) linkage
• Side chain a(1–>6) linkage

Forms Glycosidic bonds

Question 2

Where is Glycogen found?

Answer 2

Liver and Muscles

Question 3

What is the first step in Glycogenolysis?

Answer 3

Glycogen –> Glucose-1-Phosphate
Enzyme: Glycogen Phosphorylase
Breaks a glycosidic bond (like a hydrolysis reaction)

Question 4

What is the second step in Glycogenolysis?

Answer 4

Glucose-1-Phosphate –> Glucose-6-Phosphate
Enzyme: Phosphoglucomutase
Reaaranges the phosphate group to C-6

Question 5

Why is phosphorylis more efficient than hydrolysis for glycolysis?

Answer 5

Produces G-6-P which goes straight into glycolysis

Hydrolysis produces glucose which requires ATP to be converted into G-6-P

Question 6

What 3 isozymes is Glycogen Phosphorylase known as?

Answer 6

• Muscle (mGP)
• Brain (bGP)
• Liver (lGP)

Question 7

What prosthetic group is present in Glycogen Phosphorylase?

Answer 7

Pyridoxal Phosphate (PLP)

Question 8

What feature of glycogen and of phosphorylase allow for rapid catabolism?

Answer 8

• Glycogen is a branched structure
• Phosphorylase is processive

Question 9

How is Phosphorylase processive?

Answer 9

Remains attatched to its substrate and performs multiple rounds of catalysis before dissociating

Question 10

What can’t Phosphorylase’s break down

Answer 10

1–>6 bonds
1–>4 bonds within 4 units of a branch point

Question 11

What is the use of glycogenolysis in the muscle and liver respectively?

Answer 11

Muscle: Glycolysis –> ATP

Liver: Glucogenesis –> ^ Blood Glucose Lvl

Question 12

How does Glycogenesis occur? (4 steps)

Answer 12

A) Glucose –> G-6-P

• Hexokinase

B) G-6-P –> G-1-P

• Phosphoglucomutase

C) G-1-P –> UDP-Glucose

• UDP-Glucose Pyrophosphorylase

D) UDP-Glucose –> Glycogen

• Glycogen Synthase

Question 13

How does glycogen synthase work? (2)

Answer 13

• Adds to existing chains
• Creates a(1–>4) glycosidic bonds

Question 14

What are some problems with Glycogen Synthase? (2)

Answer 14

• Can’t start chains
• Can’t make a(1–>6) glycosidic bonds

Question 15

What are the 2 important enzymes involved in glycogenesis?

Answer 15

a) Glycogenin
- Builds initial 8-unit primer chain (extended by glycogen synthase)

b) Branching Enzyme
- Cuts off 7 units and reattaches via a(1–>6) glycosidic bonds