Question

During glycogen degradation, the bifunctional debranching enzyme breaks _______ linkages. The debranching process releases a single _______ monomer.

(A) a-1,6; glucose 1-phosphate

(B) a-1,6; glucose 6-phosphate

(C) a-1,4; glucose 1-phosphate

(D) a-1,4 and a-1,6; glucose 1-phosphate

(E) a-1,4 and a-1,6; glucose

A young patient presents at the hospital with a deficiency in debranching enzyme transferase activity. You take a muscle biopsy and examine the structure of the glycogen in this patient’s tissue under a microscope. What would you expect to see?

(A) accumulation of glucose 1-phosphate

(B) accumulation of normal, intact glycogen

(C) no glycogen formed at all

(D) accumulation of very long glycogen branches with single glucose branch points

(E) accumulation of short glycogen branches

Which of the following statements correctly characterizes the regulation of glycogen synthase?

(A) The enzyme exists in an inactive a form and an active b form.

(B) Glycogen synthase is allosterically activated by AMP.

(C) Glycogen synthase b will transition to glycogen synthase a in the presence of protein phosphatase 1 (PP1).

(D) Glycogen synthase activity is increased by epinephrine.

(E) Glycogen synthase is activated after phosphorylation by GSK or protein kinase A.

Answer 1

1. (E)

Glycogen is the storage form of glucose in animal cells. It is branced, polymer consisting of glucose molecules joined by (1->4) linkages and branching occurs by (1->6) linkages.

Glycogen phosphorylase breaks down glycogen to release Glc-1-phosphate molecules from the non reducing ends. It however stops upon reaching a point 4 glucose residues away from the branch site.

A debranching enzyme also known as the oligo (1->6) to (1->4) glucan transferase catalyzes the debranching reaction. First it transfers the branch and then cleaves the (1->4) link to release the glucose molecules. Glycogen phosphorylase can now continue.

2. (E)

3. (C)