Glycogen phosphorylase

Definition

Glycogen phosphorylase is an enzyme that catalyzes the breakdown of glycogen into glucose-1-phosphate, playing a crucial role in glycogen metabolism. It is key in mobilizing glucose during periods of energy demand, such as exercise or fasting, and is tightly regulated by various mechanisms to ensure glucose levels remain stable in the body.

5 Must Know Facts For Your Next Test

1. Glycogen phosphorylase exists in two forms: the active phosphorylase a and the less active phosphorylase b, with interconversion regulated by phosphorylation and dephosphorylation.
2. The enzyme is activated by AMP and inhibited by ATP and glucose-6-phosphate, ensuring that energy production meets the cellular demands.
3. Glycogen phosphorylase is primarily found in liver and muscle tissues, where it plays distinct roles in regulating blood sugar levels and providing energy for muscle contractions.
4. Hormonal regulation via glucagon in the liver and epinephrine in muscles stimulates glycogen phosphorylase activity during low-energy states or stress.
5. The breakdown of glycogen via glycogen phosphorylase is essential during fasting or intense exercise to provide a quick source of glucose for energy.

Review Questions

• How does the structure of glycogen phosphorylase contribute to its function in glycogen metabolism?
  • The structure of glycogen phosphorylase includes an allosteric site where regulatory molecules can bind, altering its activity. The enzyme exists in two forms, phosphorylase a (active) and phosphorylase b (inactive), with the transition between these forms being regulated by phosphorylation. This structural versatility allows it to respond rapidly to changes in energy demand, facilitating effective glycogen breakdown when glucose levels are low.
• Discuss the regulatory mechanisms that control glycogen phosphorylase activity in response to cellular energy status.
  • Glycogen phosphorylase activity is tightly regulated by both allosteric effectors and covalent modifications. AMP acts as an allosteric activator, indicating low energy levels and promoting glycogen breakdown, while ATP and glucose-6-phosphate inhibit its activity when energy is plentiful. Additionally, hormonal signals such as glucagon and epinephrine trigger phosphorylation of the enzyme through signaling cascades, leading to an increase in its activity during periods of fasting or stress.
• Evaluate the implications of dysfunctional glycogen phosphorylase activity on metabolic health and disease.
  • Dysfunction in glycogen phosphorylase can lead to metabolic disorders such as Pompe disease or McArdle's disease, which are characterized by impaired glycogen breakdown. In Pompe disease, lack of proper enzyme function results in accumulation of glycogen within lysosomes, causing muscle weakness and respiratory issues. In McArdle's disease, individuals experience exercise intolerance due to inadequate glucose availability from muscle glycogen stores. These conditions highlight the importance of proper regulation and function of glycogen phosphorylase for maintaining metabolic homeostasis.

Related terms

glycogen: A multi-branched polysaccharide that serves as a form of energy storage in animals, primarily found in the liver and muscles.

glucose-1-phosphate:

A glucose molecule that has a phosphate group attached at the first carbon position, produced during the action of glycogen phosphorylase on glycogen.

allosteric regulation:

A process where the binding of a molecule at one site on an enzyme affects the activity of the enzyme at a different site, influencing its activity.