Question

What are the effects of the following on the rates of glycogen synthesis and glycogen degradation:

(I put my thoughts and answers behind each one could you tell me if I'm correct and if my reasoning is correct/explain why)

1. Increasing Calcium Concentration (increase glycogen degradation and decrease synthesis because it activates phosphorylase kinase)
2. Increasing ATP concentration (Decrease glycogen degradation and increase glycogen synthesis)
3. Inhibiting adenyl cyclase (Decrease glycogen degradation...i dont know why)
4. Increasing Epinephrine (increase glycogen degradation because more energy is needed in muscles for "fight or flight" response) (act on G coupled receptor)
5. Increasing AMP concentration (increase glycogen degradation (more energy required))

Answer 1

1. Right, Increased calcium levels increases glycogen degradation. Glucagon in the liver cells activates a G-protein coupled receptor that triggers a cascade and activates phospholipase C (PLC). PLC ctivates release of calcium from ER into the cytoplasm where it binds with calmodulin. This complex activates glycogen phophorylase kinase which further activates glycoen phosphorylase. Glycogen phosphorylase is responsible for the breakdown of glycogen.

2. Right. Glycogen phosphorylase occurs in two states: T (tense state, less active) , R (relaxed state, more active).

During strenous excersice when AMP levels increases, AMP acts as allosteric activator of phosphorylase and thus increases the breakdown of glycogen to produce glucose for energy. Similaly ATP or glucose-6-phosphate acts allosteric inhibitors of phosphorylase.

3. Right. Hormones such as Glucagon binds to GPCR (G-protein coupled receptors) and activate adenylate cyclase. Adenylate cyclase activates cAMP. cAMP inturn activates PKA (Protein kinase A), PKA activates phosphorylase kinase. This kinase helps in the activation phophorylase which causes the breakdown of glycogen.

4. Right. Epinephrine also activates phosphorylase enzyme. Its action is seen in skeletal muscle cells. Muscle cells lack glucagon receptors.

5. Right. Explanation is same as mentioned above in second answer.