Question

Regulation of Glycogen Metabolism (McKee page 317-323)

1. Compare activities of glycogen phosphorylase and glycogen synthase based on storing glucose energy in a cell
2. Describe the stimulus for the pancreas to release glucagon based on blood glucose concentration
3. Describe the target for the glucagon signal based on the cell type that has receptors for glucagon
4. Describe the activity of glycogen phosphorylase based on glucagon stimulation of the cell
5. Describe the activity of glycogen synthase based on glucagon stimulation of the cell
6. Describe the result of glucagon stimulation based on glucose release to the blood
7. Describe the activity of glycogen synthase based on insulin stimulation of the cell
8. Describe the activity of glycogen phosphorylase based on insulin stimulation of the cell

Answer 1

Ans: Glucose is stored in rhe cell in the form of Glycogen which is a polymer of glucose and its synthesis takes place in liver. Now glycogen phosphorylase catalyze tha rate limiting step of glycogenolysis i.e. breaking of glycogen into glucose molecules so that cell use glucose as energy source and glycogen synthase catalyzes the synthesis of glycogen from glucose when the body has excessive amount of glucose and it does not required by the body at that time.

So for storing glucose energy in the cell glycogen synthase is activated and glycogen phosphorylase is in inactivated form.

Glucagon is released by the pancreas to couterbalance the actions of insulin. Insulin is released to reduce the elevated level of blood sugar, so when the blood sugar level decreases below the optimum point than Glucagon is released by the pancreas which signals liver and muscle cells to change the stored glycogen back into glucose.

Target for glucagon signal is liver cells because liver cells store glucose in the form of glycogen and glucagon signals liver cells t change the stored glycogen back to glucose.

As I mentioned earlier that glycogen phosphorylase catalyzes the rate limiting step of glycogenolysis and glucagon signals liver cells to change glyogen back into glucose which means that activity of glycogen phosphorylase increases and lysis of glycogen into glucose takes place.

Because of glucagon, glycogen is breaking into glucose molecules so this means that the activity of glycogen synthase will be decreased as this enzyme is for glycogen synthesis and not for glycogenolysis.

Now after glucagon stimulation glycogen breaks down into glucose and the glucose goes into blood and blood glucose concentration rises to the optimal point and these glucose molecules will be used by the various cells for energy production.

Insulin regulates the blood glucose level when the blood glucose level elevated from the optimum level than pancreas releases insulin and insulin signals liver cells to store the elevated glucose into glycogen. So insulin triggers the glycogen syntheisis and hence activity of glycogen synthase will increase because it is the primary enzyme catalyzing the synthesis of glycogen.

Insulin stimulation decreases the activity of glycogen phosphorylase because this enzyme is the rate determining enzyme of glycogenolysis but insulin is stimulating glycogen synthesis and hence the activity of glycogen phosphorylase will decrease.