Lecture 19: Glycogen

Question 1

What is glycogen?

Answer 1

Glycogen is a branched glucose polymer and is the storage form for glucose, especially in muscle/liver

Question 2

How are glucoses linked in glycogen?

Answer 2

Linearly by alpha-1,4-glycosidic links and every ~10 residues branched by an alpha-1,6-glycosidic link

Question 3

What is the advantage of glycogen branching?

Answer 3

More efficient packing of energy stores. More branches also means more substrate ends to remove glucose from, enabling more efficient breakdown.

Question 4

How are glycogen bonds broken down?

Answer 4

Phosphorylase cleaves 1,4 links
Transferase and 1,6 glucosidase deal with the last 4 glucose residues of a branch

Question 5

How is phosphorylase b allosterically regulated?

Answer 5

Phosphorylase b requires AMP (not just active) and is inhibited by ATP and G6P

Question 6

Hormonal control of glycogen

Answer 6

Epi mobilizes glycogen to glucose (glycolysis or for blood)
Glucagon stimulates liver glycogenolysis/gluconeogen.
Insulin antagonizes epi and glucagon, has opposite effect

Question 7

How does insulin promote glycogen storage?

Answer 7

1. Stim. glycogen synthase by inhib. GSK3
2. Activate phosphodiesterase -> reduce cAMP -> inhib. glycogenloysis
3. Activate GLUT4 in muscle

Question 8

How does AMP binding to phosphorylase a keep it activated?

Answer 8

AMP binding tucks away the phosphate on phosphorylase a, preventing protein Pase from reaching it; reversed by glucose.

Question 9

Glycogen storage diseases and their associated enzyme deficiencies

Answer 9

Von Gierke’s (Type I, G6Pase)
Pompe’s (Type II, lysosome 1,6-glucosidase)
Cori’s (Type III, debranching enzyme)
McArdle’s (Type V, muscle phosphorylase)
Hers’ (Type VI, liver phosphorylase)
Tarui’s (Type VII, muscle PFK)