Glycogen breakdown (glycogenolysis): which enzyme set is specifically required to cleave α-1,4 linkages along the chain and to remove branch points so that degradation can proceed?

Introduction / Context:Glycogenolysis mobilizes glucose from glycogen stores. Two enzymatic activities are essential for efficient degradation: one to remove successive glucose units from non-reducing ends and another to clear branch obstacles so phosphorylase can continue. Recognizing this pair is fundamental for understanding hepatic and muscle energy release.

Given Data / Assumptions:

• Glycogen is branched with α-1,6 linkages at branch points.
• Phosphorylase removes α-1,4-linked residues until it reaches a branch limit.
• A debranching system is needed to process α-1,6 linkages and transfer short oligos.

Concept / Approach:Glycogen phosphorylase cleaves α-1,4 glycosidic bonds to produce glucose-1-phosphate. When four residues remain before a branch (limit dextrin), the glycogen debranching enzyme acts via transferase (4-α-glucanotransferase) and α-1,6-glucosidase activities to move a trisaccharide and then hydrolyze the α-1,6 bond, allowing phosphorylase to resume.

Step-by-Step Solution:

Verification / Alternative check:Patients with debranching enzyme deficiency (Cori/Forbes disease, GSD III) accumulate limit dextrin, illustrating the necessity of the debranching step.

Why Other Options Are Wrong:

• Phosphoglucomutase participates downstream (G1P → G6P) but does not clear branches.
• Glycogen synthase is anabolism, not degradation.
• Other listed enzymes are glycolytic/gluconeogenic and irrelevant here.
• None of the above: incorrect because a correct pair exists.

Common Pitfalls:Assuming phosphorylase alone can bypass branches; forgetting the two distinct catalytic functions of the debranching enzyme complex.

Final Answer:Glycogen phosphorylase and glycogen debranching enzyme