Here, glycogen phosphorylase cleaves the bond at the 1 position by substitution of a phosphoryl group. It breaks down glucose polymer at ?-1-4 linkages until 4 linked glucoses are left on the branch. (Furthermore, glycogen phosphorylase (EC 188.8.131.52) can be used as a marker enzyme to determine glycogen breakdown).
The 2nd step involves the debranching enzyme that moves three remaining glucose units to another 1,4 end of glycogen. The final action of the debranching enzyme is the hydrolysis of the glucose attached as a 1,6-branched, giving one free glucose. This is the only case in which glucose-1-phosphate is not a glycogen metabolite.
The 3rd and last stage converts G1P (glucose-1-phosphate) to G6P (glucose-6-phosphate) through the enzyme phosphoglucomutase
Regulation: The key regulatory enzyme of the process of glycogenolysis is glycogen phosphorylase: