Glycogen Metabolism - Glycogen Breakdown

Question 1

What are the two major storage tissues for glycogen?

Answer 1

Liver and muscle

Question 2

In muscles, what does the need for ATP result in?

Answer 2

The conversion of glycogen to glucose-6-phosphate (G6P) for entry into glycolysis

Question 3

In the liver, what is triggered when blood glucose concentration is low?

Answer 3

Glycogen breakdown to G6P, which gets hydrolyzed to glucose and released into the bloodstream to reverse this situation

Question 4

What enzymes are required for glycogen breakdown?

Answer 4

1) Glycogen phosphorylase
2) Glycogen debranching enzyme
3) Phosphoglucomutase

Question 5

What does glycogen phosphorylase catalyze and yied?

Answer 5

Catalyzes glycogen phosphorolysis (bond cleavage by the substitution of a phosphate group) to yield glucose-1-phospate (G1P)

Question 6

What does glycogen debranching enzyme remove, permit, and hydrolyze?

Answer 6

• Removes glycogen’s branches, thereby permitting the glycogen phosphorylase reaction to go to completion
• Hydrolyzes alpha(1–>6) linked glycosyl units to yield glucose
  *92% of glycogen’s glucose residues are converted to G1P and the remaining 8% at the branch points are converted to glucose

Question 7

What does phosphoglucomutase convert?

Answer 7

G1P to G6P which can either continue along the glycolytic pathway (as in muscle) or be hydrolyzed to glucose (as in liver)

Question 8

What is glycogen phosphorylase regulated by?

Answer 8

Both allosteric interactions and by covalent modification

Question 9

What is glycogen phosphorylase a dimer of?

Answer 9

Identical 842-residue subunits that catalyze the controlling step in glycogen breakdown

Question 10

Glycogen phosphorylase is involved in an enzyme-catalyzed modification/demodification process to yield what two forms of phosphorylase?

Answer 10

• Phosphorylase a which has a phosphoryl group esterified to Ser 14 in each of its subunits
• Phosphorylase b which lacks these phosphoryl groups

Question 11

What are phosphorylase’s allosteric inhibitor(s)?

Answer 11

• ATP
• G6P
• Glucose

Question 12

What is glycogen phosphorylase regulated by?

Answer 12

Both allosteric interactions and by covalent modifications

Question 13

What is glycogen phosphorylase regulated by?

Answer 13

Both allosteric interactions and by covalent modifications

Question 14

What are phosphorylase’s allosteric activator(s)?

Answer 14

AMP

Question 15

What do the structures of phosphorylase a and b both have?

Answer 15

1) N-terminal –> Interface domain: Covalent modification site (Ser 14), allosteric effector site, and glycogen-binding subdomain which contains the “glycogen storage site”
2) C-terminal domain: Catalytic site located at the center of the subunit where two subdomains come together

Question 16

How does the glycogen storage site presumably increase the catalytic efficiency of glycogen phosphorylase?

Answer 16

By permitting it to phosphorylate many glucose residues on the same glycogen particle without having to dissociate and reassociate completely between catalytic cycles

Question 17

What is an essential cofactor for glycogen phosphorylase

Answer 17

Pyridoxal-5’-phosphate (PLP) is a vitamin B6 derivative covalently linked to glycogen phosphorylase near the enzyme’s active site

Question 18

What does the glycogen phosphorylase reaction result in?

Answer 18

The cleavage of the C1-O1 bond from a nonreducing terminal glycosyl unit of glycogen, yielding G1P

Question 19

What is the reaction mechanism of glycogen phosphorylase?

Answer 19

1) Formation of an E x Pi x Glycogen ternary complex
2) Formation of a shielded oxonium ion intermediate from the alpha-linked terminal glycosyl residue involving acid catalysis by Pi as facilitated by proton transfer from PLP. The oxonium ion has a half-chair conformation.
3) Reaction of Pi with oxonium ion with overall retention of configuration about C1 to form alpha-D-glucose-1-phosphate. The glycogen, which has one less residue than before, cycles back to step 1.

Question 20

How does the glycogen storage site increase the catalytic efficiency of phosphorylase?

Answer 20

By permitting it to phosphorylize many glucose residues on the same glycogen particle without having to dissociate and reassociate completely between catalytic cycles

Question 21

The fact that the glycogen phosphorylase reaction proceeds with retention of configuration suggests what?

Answer 21

That the phosphorolysis occurs via a double displacement mechanism (two sequential nucleophilic substitutions, each occurring with inversion of configuration) involving a covalent glucosyl-enzyme intermediate *SN1

Question 22

What imitates the transient intermediate in the glycogen phosphorylase reaction?

Answer 22

1,5-Gluconolactone (therefore, it acts as an inhibitor)

Question 23

What does phosphoglucomutase convert?

Answer 23

Converts G1P, which was converted by phosphorylase from glycosyl units of glycogen, to G6P - either for entry into glycolysis in muscle or hydrolysis to glucose in liver

Question 24

How is the phosphoglucomutase reaction similar to that catalyzed by phosphoglycerate mutase?

Answer 24

A phosphoryl group is transferred from the active phosphoenzyme to G6P, forming glucose 1,6-bisphosphate (G1,6P), which then phosphorylates the enzyme to yield G1-

Question 25

What is an important difference between the phosphoenzyme, phosphoglucomutase, and phosphoglycerate mutase?

Answer 25

Phosphoryl group in phosphoglucomutase is covalently bound to a Ser hydroxyl group rather than to a His imidazole nitrogen

Question 26

What results in the inactivation of phosphoglucomutase?

Answer 26

When G1,6P occasionally dissociates from phosphoglucomutase

Question 27

What intermediate provides the necessary presence of small amounts of G1,6P to keep phosphoglucomutase fully active? How?

Answer 27

Phosphoglucokinase by catalyzing the phosphorylation of the C6-OH group of G1P by ATP

Question 28

How does the glycogen debranching enzyme form a new alpha(1–>4) linkage with three more units available for phosphorylase-catalyzed phosphorolysis?

Answer 28

Acts as an alpha(1–>4) transglycosylase/glycosyl transferase by transferring an alpha(1–>4) linked trisaccharide unit from a “limit branch” of glycogen to the nonreducing end of another branch

Question 29

How does the glycogen debranching enzyme yield free glucose and a debranched enzyme?

Answer 29

The alpha(1–>6) bond linking the remaining glycosyl residue in the branch to the main chain is hydrolyzed *not phosphorylized

Question 30

What does the glycogen debranching enzyme have different active sites for? Why?

Answer 30

The transferase reaction and the alpha(1–>6)-glucosidase reaction to improve the efficiency of the debranching process

Question 31

What % of the total glucose residues were generated from glycogen breakdown as free glucose? As G1P?

Answer 31

Approx. 10% as free glucose and 90% as G1P