The decomposition of glycogen is not a reverse reaction of glycogen synthesis. Except for the glucose glucose mutase, other enzymes are different. The reactions include:
This converts one glycosyl group in the glycogen into one molecule of glucose, but the phosphorylase acts only on the α(1→4) glycosidic bond on the glycogen and catalyzes to four glucoses from the α(1→6) glycosidic bond. When the residue is no longer active, it is necessary to have the involvement of the debranching enzyme to completely decompose the glycogen. Debranching enzyme is a bifunctional enzyme that catalyzes two reactions of glycogen detachment. The first function is 4-α-glucanyltransferase activity, which is the glycosidic tetraglucan branching chain. The triglucan group is transferred to the enzyme protein and then to the same glycogen molecule or a glucose residue having a free 4 hydroxyl group at the end of the adjacent glycogen molecule to form an α(1→4) glycosidic bond, resulting in a linear elongation. 3 glucose, while leaving only 1 glucose residue in the α(1→6) branch, this glucosyl group is hydrolyzed by another function of the debranching enzyme, namely 1,6-glucosidase activity. Next, for free glucose, under the synergistic and repeated action of phosphorylase and de-branching enzyme, glycogen can be completely phosphorylated and hydrolyzed.
Decomposition of glycogen
