Glycogenolysis
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Glycogenolysis is the catabolism of glycogen (requiring removal of glucose unit from glycogen and addition of phosphate) thus producing glucose 1-phosphate, and subsequently reconfigured (C-1 -> C-6) to yield glucose 6-phosphate, a potent reaction intermediary leading to glucose available to the blood and brain, pyruvic acid (yet another potent intermediate) or reverting to glycogen if not immediately needed, as metabolically necessary. Both glucagon and epinephrine stimulate glycogenolysis.
Glycogenolysis requires three enzymes :
- Glycogen phosphorylase
- Debranching enzyme transferase & α-1,6-Glucosidase (bifunctional enzyme)
- Phosphoglucomutase
The liver also contains an additional enzyme, glucose 6-phosphatase, which cleaves the phosphate group to form free glucose.
Glycogenolysis transpires in the muscle and liver tissue, where glycogen is stored, as a hormonal response to epinephrine (e.g., adrenergic stimulation) and/or glucagon, a pancreatic peptide triggered by low blood glucose concentrations.
Of note, parenteral administration of glucagon is a common human medical intervention in diabetic emergencies when sugar cannot be given orally.