Glycogen Metabolism

Question 1

where is glucose stored?

Answer 1

skeletal muscle and liver cells

Question 2

what is glucose stored as?

Answer 2

glycogen

Question 3

what is glycogen?

Answer 3

polymer of a bunch of glucose molecules connected by glycosidic bonds

it is compact!

glycogen has a main chain and branches sprouting off of it

Question 4

why is glycogen branching important?

Answer 4

it allows glycogen to be compact

it allows for rapid addition and removal of glucose

Question 5

UDP

Answer 5

uridine diphosphate

Question 6

glycogenin

Answer 6

glycogen primer

fools glycogen synthase

catalyzes addition of 4 glucose molecules to itself using alpha 1,4 glycosidic linkages

Question 7

hormonal response to decrease in blood glucose levels + exercise

Answer 7

pancreas secretes glucagon

adrenal glands secrete epinephrine to increase heart rate

Question 8

glucagon

Answer 8

tells liver cells to break down glycogen

Question 9

epinephrine

Answer 9

increases heart rate

tells skeletal muscle cells to break down glycogen

Question 10

phosphorylated glycogen synthase is…

Answer 10

inactive

Question 11

phosphorylated glycogen phosphorylase is…

Answer 11

active

Question 12

what type of bonds between glucose in glycogen?

Answer 12

alpha 1,4 glycosidic bonds

Question 13

what type of bonds at branching points in glycogen?

Answer 13

alpha 1, 6 bonds

Question 14

After eating a starchy meal,

Answer 14

1) increase in insulin acts on GLUTs (glucose transporters on cell membrane) which bring more glucose into our cells
2) Hexokinase adds phosphate to C6 of glucose (makes G6P)
3) G6P broken down to pyruvate during glycolysis (increases ATP)
4) ATP inhibits some glycolysis enzymes
- -> now we use G6P to make glycogen (liver + muscle)

Question 15

Glycogen synthesis step 1

Answer 15

ATTACH UDP TO GLUCOSE
1) phosphoglucomutase moves phosphate from C6 to C1 (creates G1P and UTP (energy))
2) UDP-glucose phosphorylase cuts 2 phosphates off UTP (gives energy)
1 phosphate on glucose + 1 on UMP = UDP-glucose

Question 16

Glycogen synthesis step 2

Answer 16

ATTACH UDP-GLUCOSE TO GLYCOGENIN

1) glycogen synthase adds glucose to chains of at least 4 glucose
* if there are no chains this long, glycogenin catalyzes addition of 4 glucose molecules to itself using alpha 1,4 glycosidic linkages

this forms a short, linear glycogen chain

Question 17

Glycogen synthesis step 3

Answer 17

ADD MORE GLUCOSE

glycogen synthase elongates the chain

Question 18

Glycogen synthesis step 4

Answer 18

ADD BRANCHES

1) branching enzyme cuts off end of chain and attaches it to main chain using alpha 1,6 glycosidic bond
2) after branching enzyme shortens linear chain, glycogen synthae goes back in and elongates it

Question 19

Glycogen breakdown step 1

Answer 19

START WITH BRANCHES

1) glycogen phosphorylase:
- cleaves alpha 1,4 bonds
- catalyzes transfer of phosphate group to free glucose
2) 1 G1P is released at a time until 4 glucose remain on branch

Question 20

Glycogen breakdown step 2

Answer 20

DEBRANCHING ENZYME

1) cuts off branches
- has 4-alpha-glucotransferase activity: transfers 3 glucose off branch and onto main chain
- has alpha-1,6 glucosidase activity: cleaves alpha 1,6 bond (releases free glucose)

Question 21

Glycogen breakdown step 3

Answer 21

1) after each glucose is removed via phosphorolysis, a G1P is freed.
2) phosphoglucomutase converts it to G6P

Question 22

why is glycogen breakdown different in liver and muscle?

Answer 22

liver has glycogen-6-phosphatase, muscle DOES NOT

Question 23

Glycogen breakdown in liver

Answer 23

HAS glucose-6-phosphatase

• removes phosphate from C6
• releases free glucose into bloodstream for other organs and tissues

Question 24

Glycogen breakdown in muscle

Answer 24

NO glucose-6-phosphatase

-uses G6P by sending it to glycolysis to make energy

Question 25

When is glycogen synthase active?

Answer 25

when it is not phosphorylated

Question 26

When is glycogen phosphorylase active?

Answer 26

when it is phosphorylated

Question 27

Regulation depends on…

Answer 27

insulin and glucagon!

Question 28

regulation by insulin step 1

Answer 28

1) insulin attaches to tyrosine kinase receptor on cell surface

Question 29

regulation by insulin step 2

Answer 29

2) this activates a phosphatase which:
- removes phosphates from glycogen synthase (activates it)
- removes phosphates from glycogen phosphorylase (inactivates it)

increase in glycogen synthesis, decrease in glycogen degradation

Question 30

regulation by glucagon step 1

Answer 30

1) glucagon binds to GCPR on liver cell surface

Question 31

regulation by glucagon step 2

Answer 31

2) GCPR activates adenylyl cyclase (converts ATP to cAMP)

Question 32

regulation by glucagon step 3

Answer 32

3) cAMP activates protein kinase A

- protein kinase A adds phosphate to glycogen phosphorylase kinase (activates it)

Question 33

regulation by glucagon step 4

Answer 33

4) glycogen phosphorylase kinase adds phosphate to:
- glycogen phosphorylase (activates it)
- glycogen synthase (inactivates it)

increase in glycogen breakdown, decrease in glycogen synthesis