Glycogenolysis

Question 1

Glycogenolysis steps in the liver

Answer 1

1. Glucagon receptor receives glucagon, and beta-adrenergic receptor receives catecholamines from adrenal gland
2. Receptors stimulate adenylate cyclase converting ATP to cAMP
3. cAMP activates protein kinase A
4. Protein kinase A stimulates the phosphorylation of phosphorylase kinase which simultaneously inhibits glycogen synthase from making more glycogen AND activates glycogen phosphorylase (which breaks down 1,4 linkage of glycogen into glucose-1 phosphate)
5. alpha-1,6 linkage breakdown (by alpha 1,6-glucosidase (debranching enzyme))

Question 2

alpha 1,6 linkage breakdown

Answer 2

Uses debranching enzyme (alpha-1,6 glucosidase)
1. Transfers a block of 3 residues to the non-reducing end
2. Cleaves single remaining alpha-1,6-linkage to directly yields a glucose

Question 3

Liver glycogen breakdown routes

Answer 3

1. Glucose released into blood (main function). Able to do this because GLUT 2 transporter is bidirectional. Both types of glycogen can go into the blood. alpha 1,4 glycogen can get into blood from glucose-6-phosphatase.
2. Fuel for glycolysis. alpha-1,4 glycogen is used for this.

Question 4

Where does untrapping of glycogen occur?

Answer 4

Endoplasmic reticulum

As untrapping occurs, and accumulates, the concentration gradient will cause the transport of it back into the blood through GLUT 2

Question 5

Glycogenolysis in muscle

Answer 5

1. beta-adrenergic receptor receives catecholamines from the adrenal gland (as a response to stress of exercise). There is no glucagon receptor
2. Receptor stimulate adenylate cyclase that converts ATP to cAMP
3. cAMP used to produce Protein Kinase A (intercellular cascade)
4. Protein kinase A (OR muscle contraction releasing Ca2+; skips previous steps) stimulates the phosphorylation of phosphorylase kinase which simultaneously inhibits glycogen synthase from making more glycogen AND activated glycogen phosphorylase (which breaks down 1,4 linkage of glycogen)
5. All glycogen that is broken down is used for glycolysis in the liver