Glycogenolysis
Glycogenolysis, the breakdown of glycogen into monomeric glucose, occurs during times of increased energy demand in the liver and muscle, the major stores of glycogen.
Cleavage of glycogen by glycogen phosphorylase yields glucose 1-phosphate, which is converted into glucose-6-phosphate by phosphoglucomutase.
Glucose-6-phosphate cannot exit the cell, and so, in the liver, glucose 6-phosphate is transported into the endoplasmic reticulum, where it is converted into glucose by glucose 6-phosphatase. Glucose then exits the cell through the Glut-2 transporter.
Muscle Use of Glycogen
Muscle use of glycogen as fuel occurs immediately after onset of exercise and last for about an hour.
Glycogenolysis is induced by three factors in muscle:
- Nerve impulses that activate muscle contraction act to release calcium stores from the sarcoplasmic reticulum. Ca2+-calmodulin increases glycogen phosphorylase
- Epinephrine binding to its cell surface receptor activate the AC/cAMP/PKA pathway, inducing glycogen phosphorylase
- Muscle contraction uses ATP, and production of AMP from ADP is another activator of glycogen phosphorylase
In liver, glycogenolyis is positively regulated by glucagon and epinephrine and negatively regulated by insulin.
Glycogenolysis can also be controlled by allosteric regulators.
