Lecture 5: Gkycogen Metabolism

Question 1

Describe a brief overview of glycogen breakdown: Where is energy stored? where does it occur? what enzyme converts what substrate into what product? how does the enzyme work.

Answer 1

• energy stored in liver and muscle; occurs in cytosol.
• quick source of energy, 12-36 hour supply.
• glycogen phosphorylase (glycogen —> G1P)
• phosphorolysis reaction uses Pi to cleave an alpha(1—>4) branch point.

Question 2

Describe a brief overview of glycogen synthesis: What enzyme is used? Why does an intermediate need to be made first? which end is glucose attached to the glycogen chain?

Answer 2

-UDP pyrophosphatase
-Must make UDP glucose first!!
~activates glucose and is a good leaving group.
-glucose added to nonreducing end in glycogen.

Question 3

Structure of glycogenin. What is the purpose of glycogenin? what occurs after the glycogenin process is complete?

Answer 3

-Protein with Tyr 194 residue.
~add UDP glucose
~Add 6-7 more UDP glucose
-Now glycogen synthase (with branching enzyme) can now add alpha(1—>6) branches.

Question 4

Name the ON and OFF states of glycogen phosphorylase/synthase:

Answer 4

• Phosphorylation of glycogen phosphorylase = ON

- Phosphorylation of glycogen synthase = OFF

Question 5

What roles does insulin, glucagon, and epinephrine play in glycogen metabolism?

Answer 5

1. Insulin:
   
   • released if high blood [glucose]
   • make glycogen for storage, by activating glycogen synthase.
   • follows receptor tyrosine kinase pathway.
2. Glucagon:
   
   • released if low blood [glucose]
   • breakdown for fuel
   • effects liver only, not muscle
3. Epinephrine:
   
   • breakdown for fuel.