Glycogen phosphorylase (GP) targets the non-reducing ends of glycogen to cleave glycogen and produce one glucose-1P at a time. GP will do this until it is three glucose molecules from the glucose molecule with the branch point - at which time another enzyme takes over the degradation. Which glucose molecule(s) on glycogen are substrates for GP based on this information

Glucose molecules bound together by a-1,4 glycosidic linkages, and they must be >4 glucose molecules away from a branch point.

Explanation:

Glycogen phosphorylase can not degrade the glucose polymer close to the branch point because these sections of the glycogen molecule are to short for the glucose polymer to fit properly into the active site of the GP enzyme. The GP enzyme can therefore only degrade the 'straight' portions of glycogen. To degrade a branch point, a debranching enzyme is required. The debranching enzyme has transferase (cleaves off glucose molecules right before branch point and moves them to the end of another branch) and a-1,6 glycosidic activity which removes the branching glucose.

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