Difficulty: Medium
Correct Answer: Glycogen phosphorylase and glycogen debranching enzyme
Explanation:
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:
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:
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
Discussion & Comments