During periods of fasting the pancreas secretes the hormone glucagon which tells the liver cells to breakdown glucose into its individual glucose molecules. Epinephrine, another hormone tells the muscle cells to do the same thing.
Steps in Glycogenolysis
- Glycogen phosphorylase cleaves α (1, 4) bonds at the non-reducing ends of glucose chains and catalyze transfer of phosphate groups unto the liberated glucose molecules to form molecules of glucose-1-phosphate.
- Glycogen phosphorylase continues to cleave off glucose until it gets to fours glucose residues away from the α (1, 6) branch
- Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase. In liver cells glucose-6-phosphate is converted to glucose by glucose-6-phosphatase. The glucose can then be transported to the blood to restore blood glucose levels. Since glucose-6-phosphatase is not present in muscle cells, glycose-6-phosphate cannot be converted to glucose in muscle cells. Glucose-6-phosphate therefore goes to the glycolysis pathway to produce ATP.
- A debranching enzyme with 1, 4 glucosidase and 1, 6 glucosidase activity removes the final four glucose residues from the side branches in two steps. First, the enzyme cuts off three glucose residues and transfers them to the elongated chain. Secondly, the enzyme cuts off the last glucose residue, releasing it. The glucose can then be transported to the blood to restore glucose levels.