CHEM 153C Midterm: Glycogen Metabolism study questions

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Glycogen Metabolism
Structure and Function
1. What is the major function of glycogen in mammalian liver/mammalian muscle?
Liver: maintain blood glucose levels; Muscle: provide glucose for ATP production.
2. Briefly describe the advantages of glycogen versus fat as a metabolic store of carbon-containing material to be used during stress or
More rapid mobilization for ATP production. Metabolism (and ATP production) can be carried out anaerobically). Animals cannot convert fats to
glucose (or precursors) needed for NADPH production or synthesis of essential biosynthetic precursors.
Glycogen Metabolism
1. Phosphorylase catalyzes the removal of glucose residues one residue at a time from the non-reducing end of a glycogen branch only if the
branch is at least 5 residues in length. Explain why the enzyme is unable to efficiently remove glucose residues from shorter branches.
The binding or recognition site of phosphorylase is complementary to 5 glucose residues which positions the non-reducing end at the active site for
phosphorolysis. Shorter lengths would not bind as strongly and would not locate the glycolytic bond appropriately for cleavage.
2. What 2 activities are associated with the Glycogen Debranching Enzyme?
1) alpha(1
4) transglycosylase; 2) alpha(1
6) glucosidase.
3. Briefly indicate how the enzyme pyrophosphatase contributes to the thermodynamic favorability of glycogen synthesis.
Pyrophosphatase cleaves the pyrophosphate product of UDP-glucose pyrophosphorylase making the synthesis of UDP-glucose thermodynamically
favorable for the subsequent synthesis of glycogen.
4. Glycogen synthase has been proposed to proceed through the formation of an oxonium ion intermediate. Suggest a possible test of this
Test a chemical analog (a stable molecule similar in structure to the proposed intermediate e.g. 1,5-gluconolactone) for its ability to competitively
inhibit the enzyme.
5. Briefly describe the function and mechanism of Glycogen Branching Enzyme.
Glycogen Branching Enzyme [Amylo-(1,4
1,6)-Transglycosylase] transfers ~7 glucose residues >= 4 residues from a branch to another branch
creating a new alpha-(1
6) branch.
6. Glycogen phosphorylase is thought to catalyze the initial reaction in the degradation of glycogen via a half-chair oxonium ion intermediate.
What observation which gave rise to this postulated mechanism?
The enzyme is inhibited by the 1,5-gluconolactone, a structural analog of the half-chair oxonium ion.
Regulation of Glycogen Metabolism
1. (a) Complete the following diagram including all relevant reactants, products, and reactions affecting the thermodynamics of glycogen
(b) Using the above diagram, explain how glycogen synthesis and breakdown are both thermodynamically favorable.
Glycogen synthesis is made thermodynamically favorable by the hydrolysis of both UTP and PPi to drive glycogen synthesis: synthesis of UDP-
glucose (formation of one new phosphoanhydride bond) is made thermodynamically favorable by cleavage of two phosphoanhydride bonds of UDP
and PPi. Glycogen synthesis is made thermodynamically favorable by breakage of phosphoester bond and formation of a glycosidic bond.
Glycogen breakdown is thermodynamically favorable due to large Pi/G1P ratio under physiological conditions.
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