Question

A accidental genetic mutation in the lab mice has occurred and affects the glycogen degradation pathway. Dr. Wilson is studying this unexpected mutation and has taken samples of their livers. What he found was excessive glycogen storage but the glycogen structure was unusual. There was a lot of glycogen but size of each glycogen molecule was smaller and each chain end had 4 glucose units (alpha 1,4 bonds) and then a SINGLE glucose attached as a branch point (alpha 1,6 bond) to the 5th carbon from the end. Which enzyme in the glycogen degradation pathway is not working?

phosphoglucomutase

glycogen phosphorylase

glycogen transferase

alpha-1,6-glucosidase

glucose 6-phosphatase

Answer 1

Answer: glycogen phosphorylase

Glycogen phosphorylase breaks up glycogen into glucose subunits. Glycogen is now with lesser number of free glucose molecule is in the form of glucose-1-phosphate. In order to be used for metabolism, it must be converted to glucose-6-phosphate by the enzyme phosphoglucomutase. Glycogen phosphorylase can act only on linear chains of glycogen (α1-4 glycosidic linkage), Its work will immediately come to a halt four residues away from α1-6 branch (which are exceedingly common in glycogen). In these situations, a debranching enzyme is necessary, which will straighten out the chain in that area. α1-6 glucosidase enzyme is required to break the remaining (single glucose) α1-6 residue that remains in the new linear chain. After all this is done, glycogen phosphorylase can continue.

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