Cytosolic glycogen breakdown occurs via the same chemical steps in all tissues but is separately regulated via tissue specific isozymes and signaling pathways that enable distinct physiological fates for liver glycogen and that in other tissues. Glycogen phosphorylase, which can be activated by phosphorylase kinase, catalyzes the removal of glucose residues as glucose 1-phosphate from the ends of glycogen branches. The final four residues of each branch are removed in two steps catalyzed by debranching enzyme, and further glycogen phosphorylase activity completes the process of glycogen breakdown. The figure shows the actions of phosphorylase and debranching enzyme. The first glucose residue in each branch is released as free glucose; all other residues are released as glucose 1-phosphate. The latter molecule can be converted to glucose 6-phosphate in a step shared with other pathways (Villar-Palasi and Larner 1970; Hers 1976).
Glycogen can also be taken up into lysosomes, where it is normally broken done by the action of a single enzyme, lysosomal alpha-glucosidase (GAA).
Enzymes in liver generate 1,5-anhydro-D-fructose from glycogen, which in turn can be reduced to 1,5-anhydro-D-glucitol, a sequence of events that may represent a novel minor pathway for glycogen breakdown (Kametani et al. 1996).