Glycogen debranching enzyme has two catalytic activities essential for complete glycogenolysis. In GSD type III (Cori disease), which two enzymatic functions are absent, and what accumulates in the tissue?

• A Glycogen phosphorylase and phosphoglucomutase; glucose-1-phosphate
• B Branching enzyme (α-1,4→1,6 transferase) and phosphorylase kinase; long-chain glycogen
• C Lysosomal α-glucosidase and cytoplasmic phosphorylase; lysosomal glycogen
• D Oligo-α-1,4→1,4-glucan transferase and α-1,6-glucosidase; limit dextrin (glycogen with short outer branches) ✓

Explanation

The glycogen debranching enzyme possesses two activities on the same polypeptide: 4-alpha-glucanotransferase (moves a trisaccharide from a branch onto the main chain) and alpha-1,6-glucosidase (cleaves the remaining single glucose at the branch point). Absence of both in Cori disease leaves limit dextrin — glycogen with very short outer chains — accumulating in liver, skeletal muscle, and heart. This distinguishes it from Pompe disease (lysosomal acid maltase deficiency) and GSD Ia.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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