debranching enzyme

Meaning: Glycogen phosphorylase cleaves α-1,4 bonds from nonreducing ends using Pi, releasing glucose-1-phosphate, but it stops about four residues from an α-1,6 branch point. Debranching enzyme has two activities. A transferase moves a block of three glucose residues from the branch to the end of another chain, forming a new α-1,4 bond. An α-1,6-glucosidase then hydrolyzes the last glucose at the branch with water, releasing free glucose. Phosphorylase can then continue.

On the MCAT: Expect questions that match an enzyme to its linkage or product. Most glucose leaves glycogen as glucose-1-phosphate, which phosphoglucomutase converts to glucose-6-phosphate; only branch-point residues leave as free glucose. If a passage describes a missing or inhibited debranching enzyme, predict that breakdown stalls near the branches and less glucose is released.

Common confusion: Debranching enzyme is not the glycogen branching enzyme, which makes α-1,6 bonds during glycogen synthesis. Also, phosphorylase does not remove branches: it acts on α-1,4 bonds, not α-1,6 bonds, and the branch-point glucose is released by hydrolysis as free glucose, not as glucose-1-phosphate.