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Glycogenolysis is the chemical process by which glycogen is broken down into smaller molecules - normally glucose-6-phosphate. In the liver, the glucose-6-phosphate is further modified to glucose, and is released into the bloodstream. In muscle tissue, the glucose-6-phosphate is used as fuel by the muscle cell and is not released.

Glycogen is a complex polymer with a 'trunk' and 'branches'. Addition and removal of glucose subunits occurs only at the tips of the 'branches', not at the 'trunk'.

Glycogenolysis has two main steps; the first step, in which a glucose molecule is removed from the glycogen molecule as a glucose-1-phosphate molecule, and a second step in which the glucose-1-phosphate is converted to glucose-6-phosphate.

The enzyme that clips a glucose-1-phosphate off the tip of a branch, glycogen phosphorylase, stops working when it is less than 5 glucose subunits from a branching point. A second set of two molecules, the debranching enzymes, are then required to remove the branching point so that glycogen phosphorylase can continue working.

Glycogenolysis is stimulated by the endocrine hormones glucagon and/or epinepherine.