Metabolism 5: Glycogen

Question 1

What glucose transporter is used in the liver? In muscle?

Answer 1

Liver: GLUT-2 (insulin-independent, concentration-dependent)

Muscle: GLUT-4 (insulin-dependent)

Question 2

What hormone stimulate glycogen breakdown in liver? In muscle?

Answer 2

In liver, glucagon stimulates glycogen –> G-6-P

In muscle, Epi stimulates it

Question 3

Name the 3 steps and 2 enzymes needed for Glycogen synthesis from glucose

Answer 3

1. Glucose –> HK/GK –> G-6-P
2. (G-6-P –> G-1-P)
3. G-1-P –> Glycogen Synthase + Branching Enzyme –> Glycogen

Question 4

Name the 3 steps and 2 enzymes needed for Glycogen breakdown to glucose

Answer 4

1. Glycogen –> Glycogen Phosphorylase + De-branching Enzyme –> G-1-P
2. (G-1-P –> G-6-P)
3. G-6-P –> Glucose-6-Phosphatase –> Glucose

Question 5

How is Glycogen Phosphorylase regulated?

- What are the 2 forms of it?

Answer 5

• Glycogen Phosphorylase “a” is active form/phosphorylated
• Glycogen Phosphorylase “b” is inactive form
• cAMP, PI, Ca2+ activate Phosphorylase Kinase –> “a” form
• insulin activates Phosphoprotein Phosphatase –> “b” form
• cAMP inhibits Phosphatase –> “a” form
• AMP directly binds “b” form and causes it to become an active enzyme (P-independent)
• ATP/Glucose directly bind “a” form and cause it to become inactive enzyme (P-independent)

Question 6

How is Glycogen Synthase regulated?

- What are the 2 forms?

Answer 6

• Glycogen Synthase “a” is active/de-phosphorylated
• Glycogen Synthase “b” is inactive
• cAMP, DAG, Ca2+ activate Protein Kinase –> “b” form
• G-6-P binds to “b” form and makes it active enzyme
• Insulin activates Phosphoprotein Phosphatase –> “a”
• cAMP inactivates Phosphoprotein Phosphatase –> “b” promoted

Question 7

What is the rate-limiting enzyme in the Pentose Phosphate Pathway? And what necessary product does it make?

Answer 7

• Glucose-6-Phosphatase Dehydrogenase (G6PD)

- NADPH (key for RBCs for making NAPHD through PPP)

Question 8

What does a deficiency in Glucose-6-Phosphatase Dehydrogenase (G6PD) create?

Answer 8

• Hemolytic anemia (increased bilirubin, decreased Hb)

- Low NADPH –> increased H2O2/reactive oxygen species

Question 9

Why is NADPH required in RBCs?

Answer 9

Keeps Glutathione in reduced state which protects against oxidative damage.