Question

Provide ạn exạmple of eạch for control of glycogen metạbolism:

ạ. Hormone control

b. Ạllosteric control

c. Control by covạlent modificạtion

Answer 1

a) Hormonal Control

The Pancreatic Hormone Glucagon is a hormonal regulator of Glycogen metabolism. Glucagon promotes Glycogen catabolism by upregulating the activity of Adenylate-cyclase. Adenylate cyclase, in turn, produces the secondary messenger cyclic-AMP, which acts as a downstream effector of Glucagon and activates Protein Kinase A, which deactivates Glucagon Synthetase and activates Phosphorylase Kinase.

Finally, activated Phosphorylase Kinase activates Glycogen Phosphorylase, an enzyme that breaks down Glycogen to yield Glucose-6-phosphate.

Another example of hormonal control of Glycogen Metabolism is the activity of Insulin. Insulin, when sensed by the cell has the opposite effect as that of Glucagon as Insulin promotes the conversion of cAMP into AMP, preventing Glycogen degradation by Glycogen Phosphorylase and promoting Glycogen anabolism.

b)

An example of allosteric regulation of Glycogen metabolism is the regulation of Glycogen synthase by Glucose-6-phosphate. When Glucose-6-Phosphate (G-6-P) is abundant in the cell, G-6-P binds to Glycogen synthase and activates it, promoting the conversion of UDP-Glucose to Glycogen.

c)

An exampe of control of Glycogen metabolism by covalent modification is the phosphorylation of Glycogen Phosphorylase by Phosphorylase Kinase (GPK). Glycogen Phosphorylase is an enzyme that releases Glucose-1-phosphate from Glycogen.

The dephosphorylated form (without Phosphae group attached) of Glycogen Phosphorylase is inactive and is unable to break the apha-1,4-glycosidic bonds of Glycogen. However, when Glycogen Phosphorylase is phosphorylated (Phosphate group covalently attached) by GPK, it becomes active and begins generating Glucose-1-phosphate.