Allosteric regulation of glycogen synthesis and breakdown is done by regulation of enzymes glycogen synthase and glycogen phosphorylase. Hormonal regulation of glycogen synthesis and breakdown is done by hormones insulin and glucagon.

Glycogen synthase stimulates glycogen synthesis. Whenever the blood glucose level rises, the levels of glucose-6-phosphate rises. Glucose-6-phosphate stimulates glycogen synthase and thus glycogen synthesis occurs. When glycogen synthesis is not needed, the phosphorylation of glycogen synthase occurs that prevents the action of the enzyme by changing its structure.

Glycogen phosphorylase causes phosphorylation of glycogen that results in the formation of glucose-1-phosphate by the breakdown of glycogen. Phosphoglucomutase can then proceed to convert glucose-1-phosphate into glucose-6-phosphate. Glucose-6-phosphate can then either be fed into glycolysis, the pentose phosphate pathway or converted to glucose. Thus, allosteric regulation of glycogen synthesis and breakdown is done by glycogen synthase and the glycogen phosphorylase enzymes.

The insulin hormone stimulates the synthesis of glycogen. When the blood glucose level rises, insulin stimulates glycogen synthase to form glycogen from glucose. Glucagon acts opposite to the insulin and stimulates the breakdown of glycogen whenever blood glucose level falls.

 

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Key Points

• Glycogen synthase stimulates glycogen synthesis while glycogen phosphorylase stimulates glycogen breakdown by allosteric regulation.

• Insulin stimulates glycogen synthesis while glucagon stimulates glycogen breakdown by hormonal regulation.

Key Terms

Insulin: a hormone secreted from the pancreas to reduce blood sugar level

Glucagon: a hormone secreted from the pancreas to increase the blood sugar level

Allosteric: regulation of an enzyme by a molecule adding to a site other than the active site

Glycogen: a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria

Phosphorylation: the addition of a phosphate group to a protein