Control of Glycogen Metabolism

Question 1

What are the two types of control of the pathways for the synthesis and breakdown of glycogen?

Answer 1

1) Allosteric

2) Hormonal control by covalent modification

Question 2

Which enzyme in glycogen metabolism is activated allosterically by AMP? Which pathway is it the RDS in?

Answer 2

Glycogen phosphorylase; it is part of the glycogenolysis pathway (breaks down glycogen)

Question 3

Which enzyme is activated by G6P? Which pathway is it part of?

Answer 3

Glycogen synthase; it is part of the glycogen synthesis pathway.

Question 4

Which molecules inhibit glycogen phosphorylase and, therefore, the breakdown of glycogen?

Answer 4

ATP and G6P

Question 5

How does a low concentration of ATP affect glycogen metabolism?

Answer 5

It stimulates glycogen phophorylase and, therefore, glycogen breakdown

Question 6

How do high concentrations for AMP affect glycogen metabolism?

Answer 6

Activates glycogen phosphorylase and, therefore, the breakdown of glycogen.

Question 7

How do low concentrations of G6P affect metabolism of glucose?

Answer 7

Inhibits glycogen synthase and, therefore, inhibits glycogen synthesis.

Question 8

How do high concentrations of G6P and ATP affect glycogen metabolism?

Answer 8

Activates glycogen synthase and inhibits glycogen phosphorylase, thereby stimulating glycogen synthesis.

Question 9

What molecules move glycogen phosphorylase from the T to the R form?

Answer 9

AMP

Question 10

What molecules move glycogen phosphorylase from the R to the T form?

Answer 10

ATP and G6P

Question 11

True or false: glucose moves glycogen phosphorylase from the T to the R form.

Answer 11

False: glucose moves phosphorylase from the R to the T form

Question 12

True or false: Phosphorylase a is only active when it has been covalently modified?

Answer 12

False; it is active even without covalent modification

Question 13

True of false: Glycogen synthase is essentially inactive unless it has been dephosphorylated (covalently modified) and G6P is present.

Answer 13

True

Question 14

What kind of control does the concentration of AMP exert on glycogen phosphorylase?

Answer 14

Allosteric control

Question 15

What kind of control does phosphorylase kinase and phosphoprotein phosphatase exert on glycogen phosphorylase?

Answer 15

Covalent modification

Question 16

How does covalent modification affect the control provided by allosteric affectors?

Answer 16

It provides sophisticated control that modulates the enzyme’s responsiveness to allosteric effectors.

Question 17

How is the interconversion of a and b forms of phosphorylase and synthase accomplished?

Answer 17

By hormone regulated enzyme catalyzed phosphorylation and dephosphorylation.

Question 18

What is the result of phosphorylation on glycogen phosphorylase?

Answer 18

b form (inactive) goes to a form (active)

Question 19

What is the result of phosphorylation on glycogen synthase?

Answer 19

a form goes to b form

Question 20

What is the result of dephosphorylation on glycogen synthase?

Answer 20

b form (inactive) goes to a form (active)

Question 21

What is the net result of dephosphorylation on glycogen phosphorylase?

Answer 21

a form goes to b form

Question 22

Which hormones control glycogen metabolism in the liver?

Answer 22

Insulin and glucagon (synthesized in the pancreas)

Question 23

Which hormones controls glycogen metabolism in muscles and other tissues?

Answer 23

Insulin and epinephrine/norepinephrine

Question 24

In which direction is the metabolic flux when a large fraction of glycogen enzymes are phosphorylated?

Answer 24

Toward glycogen breakdown (phosphorylase is active and synthase is inactive)

Question 25

What happens when the concentration of cAMP drops?

Answer 25

phosphorylation rate decreases, synthase is activated, and metabolic flux moves toward glycogen synthesis

Question 26

What does the binding of glucagon on a liver cell result in?

Answer 26

Generates intracellular cAMP, and results in glucose mobilization from glycogen.

Question 27

When is glucose release from the pancrease?

Answer 27

When [blood glucose] is less than 5 mM

Question 28

True or false: Glucagon causes muscle cells to convert glycogen into glucose.

Answer 28

False; muscle cells do not have glucagon receptors and do not, therefore, respond to glucagon.

Question 29

What is the second messenger of alpha-adrenergic receptors?

Answer 29

Ca2+

Question 30

What is the second messenger of beta-adrenergic receptors?

Answer 30

cAMP

Question 31

Which types of cells have both beta- and alpha-adrenergic receptors?

Answer 31

liver

Question 32

Which types of cells have only beta adrenergic receptors?

Answer 32

muscle

Question 33

True or false: Muscle cells respond to epinephrine.

Answer 33

True (via beta-adrenergic receptors)

Question 34

What does epinephrine and norepinephrine cause in muscle cells?

Answer 34

Break down glycogen for glycolysis, thereby generating ATP, and helps muscles cope with stress.

Question 35

What effect does epinephrine and norepinephrine have on the liver?

Answer 35

Promotes the release of glucagon from the pancreas and, therefore, the breakdown of glycogen to glucose in liver cells.

Question 36

True or false: epinephrine binds only beta-adrenergic receptors.

Answer 36

False; it binds both beta and alpha adrenergic receptors.

Question 37

What does the binding of epinephrine by beta-adrenergic receptors cause?

Answer 37

Release of cAMP and glycogen breakdown.

Question 38

What does the binding of epinephrine by alpha-adrenergic receptors cause?

Answer 38

Stimulates release of Ca2+, reinforcing the cell's response to cAMP and inactivating glycogen synthase.

Question 39

True or false: Insulin and epinephrine are antagonists.

Answer 39

True

Question 40

What triggers the release of insulin?

Answer 40

High blood glucose

Question 41

Which types of cells have both insulin receptors and insulin-sensitive GLUT 4? Which do not?

Answer 41

Muscle and fat; liver and brain

Question 42

What happens to cAMP when insulin concentration increases? What affect does this have on glycogen metabolism?

Answer 42

cAMP concentration decreases; causes glycogen metabolism to shift from breakdown to synthesis.

Question 43

What does insulin promote in muscle?

Answer 43

Glucose storage as glycogen by stimulation of synthesis and inhibition of breakdown.

Question 44

What does insulin promote in liver?

Answer 44

Glycogen synthesis

Question 45

True or false: Glucose activates phosphorylase in the liver by binding to the R state.

Answer 45

False; it inhibits phosphorylase by binding to the T state.

Question 46

Which organs do glycogen storage diseases usually affect?

Answer 46

The liver (cause hepatomegaly and hypoglycemia)

Question 47

What affect do glycogen storage diseases have on muscle?

Answer 47

Cramps and weakness