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?

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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: 1) Identify the chain-shortening enzyme: glycogen phosphorylase.2) Recognize the stalling at branch points (limit dextrin).3) Assign debranching enzyme to clear the branch via transferase + glucosidase.4) Conclude that both enzymes are required for uninterrupted degradation. 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

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