Liver Glycogen Metabolism

Question 1

what’s the importance of storing glycogen?

Answer 1

maintaining blood sugar. the liver breaks down glycogen to export glucose to cells that need it (i.e. RBCs and brain cells)

Question 2

describe the structure of glycogen

Answer 2

polysaccharide
highly branched polymer of glucose
mostly alpha-1,4 glycosidic linkages with alpha-1,6 glycosidic linkages at the beginnings of a branch

Question 3

what tissues store significant amounts of glycogen?

Answer 3

muscle cells and liver cells

muscle cells use glycogen for fuel while liver cells do not. liver cells break down glycogen for glucose to maintain blood sugar. muscle cells can use glycogen as fuel even in anaerobic conditions

Question 4

define glycogenolysis

Answer 4

glycogen breeakdown

Question 5

define glycogenesis

Answer 5

glycogen synthesis

Question 6

explain why it is beneficial for glucagon signals to stimulate both glycogenolysis and gluconeogenesis

Answer 6

both add sugar to the blood

Question 7

name the enzyme that allows liver cells to convert glucose-6-phosphate —> glucose.
why is this enzyme needed for raising blood sugar?

Answer 7

glucose-6-phosphatase

necessary for liver glycogen to end up as blood sugar; once the phosphate is removed, the glucose is exportable

Question 8

location of glucose-6-phosphatase

Answer 8

endoplasmic reticulum

Question 9

name the enzyme:
glucose-1-phosphate <—–> glucose-6-phosphate

when would it go each direction?

Answer 9

phosphoglucomutase

in fasting state or during exercise (glycogenolysis), it goes the direction from G1P –> G6P

in glycogenesis, goes the direction where G6P —> G1P

Question 10

donor of glucose rings to variety of building products, including addition of glucose rings to ends of branches of growing glycogen molecule in glycogenesis

Answer 10

UDP-glucose

Question 11

name enzyme that phosphorylates glucose in muscle cells

Answer 11

hexokinase

Question 12

name enzyme that phosphorylates glucose in liver cells

Answer 12

glucokinase

Question 13

describe the rxn that forms UDP-glucose

Answer 13

glucose-1-phosphate reacts with UTP to produce pyrophosphate and UDP-glucose

Question 14

name the enzyme that makes a pre-existing glycogen molecule bigger by one glucose ring

who’s the donor?

what’s the product?

Answer 14

glycogen synthase

UDP-glucose

UDP is a product

Question 15

what type of linkages does glycogen synthase produce?

Answer 15

alpha-1,4 linkages

Question 16

what enzyme helps to create branching in glycogen molecules by adding alpha-1,6 glycosidic linkages?

Answer 16

branching enzyme or 4:6 transferase

• does NOT use UDP-glucose
• does NOT make glycogen molecule any bigger, only restructures it

Question 17

what enzyme breaks an alpha-1,4 linkage at the end of a branch to remove a glucose ring?

Answer 17

glycogen phosphorylase

Question 18

what type of rxn is the glycogen phosphorylase rxn?

Answer 18

phosphorylysis rxn

uses a Pi instead of water to break the alpha 1,4 linkage

Question 19

product in the glycogen phosphorylase rxn

Answer 19

glucose-1-phosphate

Question 20

what enzyme breaks down the alpha-1,6 linkages in glycogen?

what type of rxn is this

Answer 20

debrancher enzyme
hydrolysis rxn
- water goes in, breaking the alpha 1,6 linkage and releasing a small amount of glucose (bare naked glucose)

Question 21

what enzyme breaks an alpha 1,4 linkage and then will reattach the shortchain of glucoses, forming a 1,4 linkage elsewhere

Answer 21

4:4 transferase

Question 22

what enzyme catalyzes hydrolysis of 1,6 linkage in glycogen molecule, leavingone molecule of bare naked glucose?

Answer 22

alpha 1,6 glucosidase

Question 23

catalytic cofactor that resides in the active site of glycogen phosphorylase

Answer 23

pyridoxal phosphate (PLP), from vitamin B6 (pyrodoxin)

Question 24

predict effect of insulin signals on glycogenesis and glycogenolysis

Answer 24

speed up glycogenesis
slow down glycogenolysis

insulin stimulates glycogensis after a high carb meal

Question 25

predict the effect of glucagon signals on glycogenesis and glycogenolysis

Answer 25

slow down glycogenesis speed up glycogenolysis in the fasting state, we want the liver to break down glycogen until we eat again to maintain blood. sugar. glucagon is the hormone for stimulating glycogenolysis in the liver during the fasting state

Question 26

predict the effect of epinephrine on glycogenesis and glycogenolysis

Answer 26

slow down glycogenesis speed up glycogenolysis during exercise, we breakdown glycogen in for muscle cell use as energy. the hormone for this is epinephrine. this means glygogenolysis would speed up

Question 27

predict the effects of an increase in cytosolic level of cAMP

Answer 27

glucagon signals and epinephrine signals can increase levels of cAMP in cytosol, speeding up glycogenolysis

Question 28

predict the effects of an increase in cytosolic level of calcium ion

Answer 28

in muscle cells, cytosolic calcium increases because of AP traveling down t-tubules causing calcium to stream out of the sarcoplasmic reticulum into the cytosol some of that calcium helps with muscle contraction by binding troponin C other calcium helps break down glycogen for fuel this speeds up glycogenolysis calcium also activates phosphorylase kinase

Question 29

effect of phosphorylating glycogen synthase

Answer 29

inactivates it

Question 30

effect of phosphorylating glycogen phosphorylase

Answer 30

activates it

Question 31

when would we want both glycogen synthase and glycogen phosphorylase to be phosphorylated? what causes both enzymes to be phosphorylated?

Answer 31

when glycogenolysis is desirable because this will activate glycogen phosphorylase and inactivate glycogen synthase glucagon signals in the fasting state and/or epinephrine during exercise causes this

Question 32

what enzymes can catalyze phosphorylation of glycogen synthase

Answer 32

a variety of enzymes (including phosphorylase kinase)

Question 33

what catalyzes phosphorylation of glycogen phosphorylase

Answer 33

phosphorylase kinase ONLY

Question 34

what enzyme catalyzes desphosphorylation of glycogen synthase and glycogen phosphorylase what is the effect? what hormone makes this happen?

Answer 34

protein phosphatase - hydrolysis of phosphoester bond glycogen synthase is active glycogen phosphorylase is inactive speeds up glycogenesis insulin in the fasting state

Question 35

explain how glucagon receptors lead to increase glucose in the blood

Answer 35

glucagon receptors are heptahelical, heterotrimeric G-protein coupled. 1. glucagon binds to heterotrimeric receptor 2. receptor changes shape 3. heterotrimeric g protein changes shape 4. alpha subunit drops GDP and picks up GTP 5. shape change activates adenylate cyclase 6. cAMP concentration increases, becomes second messenger 7. protein kinase A get activated which catalyzes phosphorylation of both phosphorylase kinase (activating it) & glycogen synthase (inactivating) 8. catalyzes the breakdown glycogen (glycogenolysis) 9. formation of glucose-1-phosphate that can become glucose-6-phosphate by phosphoglucomutase 10. glucose-6-phosphatase rxn turns glucose-6-phosphate to glucose that can be exported to the blood

Question 36

insulin signals lead to activation of a protein phosphatase called ______________ what happens next?

Answer 36

protein phosphatase 1 (PP1) causes glycogen synthase to be dephosphorylated, making it active UDP-glucose can then be used to make glycogen

Question 37

enzymes that can be desphosphorylated by PP1

Answer 37

``` glycogen synthase glycogen phosphorylase (inhibits) phosphorylase kinase (inhibits) ```

Question 38

what enzyme catalyzes the hydrolysis of cAMP ---> AMP what happens after this rxn occurs? what hormone causes this?

Answer 38

cAMP phosphodiesterase the effects: protein kinase A won't be activated anymore insulin signals

Question 39

effects of glucagon signals on cAMP | effect of insulin signals on cAMP

Answer 39

glucagon raises cAMP levels | insulin lowers cAMP levels

Question 40

receptors for epinephrine are ___________ receptors

Answer 40

adrenergic

Question 41

signal transduction mechanism for when epinephrine binds to a beta-adrenergic receptor of a liver cell explain.

Answer 41

adenylate cyclase system 1. epinephrine binds to heptahelical receptor of liver cell 2. receptor changes shape. 3. heterotrimeric G protein senses shape change 4. alpha subunit drop GDP and picks up GTP 5. adenylate cyclase activated 6. cAMP concentration increases, becomes second messenger 7. protein kinase A activated, catalyzing phosphorylation of glycogen synthase (turning off) and phosphorylase kinase which turns on glycogen phosphorylase for the breakdown of glycogen increase in glycogenolysis

Question 42

signal transduction when epinephrine binds to an alpha-1 adrenergic receptor of a liver cell explain

Answer 42

phospholipase c. 1. epinephrine binds to alpha-1 heptahelical receptor of a liver cell 2. receptor changes shape. 3. heterotrimeric G protein senses shape change 4. alpha-subunit drops GDP and picks up a GTP 5. activates phospholipase C (membrane-associated enzyme) 6. breaks the membrane phospholipid molecule into two pieces that serve as second messengers (DAG & inositol triphosphate) 7. IP3 receptors on the endoplasmic reticulum opens doors for calcium ion 8. Ca2+ passively flows out of ER into cytosol DAG and Ca2+ released from the ER activate several protein kinases (including phosphorylase kinase) 9. Ca2+ binds to calmodulin (direct allosteric activation of phosphorylase kinase) 10. phosphorylase kinase catalyzes phosphorylation of glycogen phosphorylase & glycogen synthase 11. glycogenolysis is sped up by epinephrine