MCAT Biochemistry Question 127: Answer and Explanation

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Question: 127

7. A biopsy is done on a child with an enlarged liver and shows accumulation of glycogen granules with single glucose residues remaining at the branch points near the periphery of the granule. The most likely genetic defect is in the gene encoding:

  • A. α-1,4 phosphorylase (glycogen phosphorylase).
  • B. α-1,4:α-1,6 transferase (branching enzyme).
  • C. α-1,4:α-1,4 transferase (part of debranching enzyme complex).
  • D. α-1,6 glucosidase (part of debranching enzyme complex).

Correct Answer: D

Explanation:

The pattern described for this child’s glycogen demonstrates appropriate production: there are long chains of glucose monomers, implying that glycogen synthase works. There are also branch points, implying that branching enzyme, choice (B) works. During glycogenolysis, it seems that the child is able to remove individual glucose monomers and process glycogen down to the branch point itself, which requires glycogen phosphorylase, choice (A), and α-1,4:α-1,4 transferase, choice (C). The metabolic problem here is removing the final glucose at the branch point, which is an α-1,6 (not α-1,4) link. This requires choice (D), α-1,6 glucosidase.

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