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.