Lecture 25: Glycogen Metabolism

Question 1

What is glycogen a polymer of?

Answer 1

Glucose residues

Question 2

How are glucose residues combined to form glycogen?

Answer 2

Linked by:
* a(1->4) glycosidic bonds [primary, linear]
* a(1->6) glycosidic bonds [branch points]

Question 3

What does glycogen maintain?

Answer 3

Blood glucose levels

Question 4

How does glycogen maintain blood sugar level?

Answer 4

• When blood sugar is high, the body converts glucose into glycogen to reduce BSL
• When blood sugar is low, glycogen breaks down to release glucose to raise the BSL

Question 5

Where is gluclose predominately stored as glycogen?

Answer 5

Liver and muscle cells

Question 6

Glycogen Degradation (catabolism)

What is the enam that removes glucose residues from glycogen?

Answer 6

Glycogen Phosphorylase

Question 7

Glycogen Degradation (catabolism)

What end can glycogen phosphorylase work at?

reducing or non-reducing

Answer 7

Non-reducing end

can only go linear

Question 8

Glycogen Degradation (catabolism)

What does glycogen phosphorylase catalyze and what does it release?

Answer 8

• Catalizes: phosphorolytic cleavage of a(1->4)
• Releases: glucose-1-phosphate

cannot cut a(1->6)

Question 9

Glycogen Degradation (catabolism)

What does glucose-1-phosphate have to undergo to be used for metabolism? What does it become?

Answer 9

Isomerization to become glucose-6-phosphate

this is the active form for glycolosis and pentose phosphate pathway

Question 10

Glycogen Degeneration (catabolism)

What enzyme is used to isomerize the glucose-1-phospahte?

Answer 10

phosphoglucomutase

is reverable

Question 11

Glycogen Degradation (catabolism)

What must happen to glucose-6-phosphate so it be released into blood?

Answer 11

it must be dephosphorylated

Question 12

Glycogen Degradation (catabolism)

What is the phosphate added to glucose and what is that addition called?

Answer 12

Phosphate: orthophosphate
This reaction is called: phosphorolysis

Question 13

Glycogen Degradation (catabolism)

What is phosphorolysis similar to and how?

Answer 13

Hydrolysis

The main difference is that hydrolysis adds water while phosphorolysis adds a phosphate

Question 14

Branching Points in Glycogen

Where does glycogen phosphorylase stop on glycogen when coming to branches?

Answer 14

4 residues away from the branch point

called the limit branch

Question 15

Branching Points in Glycogen

What enzyme is used to cleave the branching points?

Answer 15

Debranching enzyme

Question 16

Branching Points in Glycogen

What are the two active sites of the Debranching Enzyme?

Answer 16

• Transferase
• a(1->6) glucosidase

Question 17

Branching Points in Glycogen

What does the trasferase site do?

in debranching enzyme

Answer 17

transfers 3 glucose residues from the 4-residue limit to the end of another branch so the other branch is only a single residue

Question 18

Branching Points in Glycogen

What does the a(1->6) glucosidase site do?

in debranching enzyme

Answer 18

catalyzes hydrolysis of that linkage to free that glucose

Question 19

Regulation of Glycogen Degradation

What is glycogen phosphorylase regulated by?

Answer 19

1. allosteric effectors that signal the energy state of the cell (if the cell has enough energy storage, ex: ATP, glucose, regulates how much is released based on what is already there)
2. reversible phosphorylation

Question 20

Regulation of Glycogen Degradation

Glycogen phosphorylase exists in what 2 interconvertible forms?

Answer 20

1. Phosphorylase a (usually in Relaxed state, R, active)
2. Phosphorylase b (usually in Tense state, T, inactive)

Question 21

Regulation of Glycogen Degradation

Phosphorylase kinase does what for glycogen phosphorylase?

Answer 21

converts phosphorylase b to a by adding a phosphate (activates the glycogen phosphorylase)

Question 22

Regulation of Glycogen Degradation

What are critical hormones that signal for glycogen breakdown?

Answer 22

• Glucagon
• Epinephrine

Question 23

Regulation of Glycogen Degradation

What situlation would we need these hormones to be released?

Answer 23

When we need a sudden amount of energy released for use in something like running

Question 24

Regulation of Glycogen Degradation

What is the general pathway from the hormones to glycogen phosphorylase?

Answer 24

1. Activate the adenylase cyclase to produce cAMP
2. cAMP activates Protein kinase A
3. Protein Kinase A activeates phosphorylase kinase
4. phosphorylase kinase adds phosphate to phosphylase b to create phosphorylase a (in glycogen phosphorylase)

Question 25

Glycogen Synthesis

What is the basic definition of glycogen synthesis?

Answer 25

addition of glucose onto an existing glycogen

Question 26

Glycogen Synthesis

What is the active form of glucose that is the precursor for glycogen synthesis?

Answer 26

Uridine diphosphate glucose (UDP-glucose)

bonding of UTP to glucose-1-phosphate

Question 27

Glycogen Synthesis

What initiated glycogen synthesis?

Answer 27

Glycogenin (small protein/enam)

Question 28

Glycogen Synthesis

What does gycogenin also do?

Answer 28

Acts as the primer

builds a short linear glucose polymer behind it

Question 29

Glycogen Synthesis

What enam catalizes the elongation of the glycogen chains created by glycogenin?

Answer 29

Glycogen Synthase

Question 30

Glycogen Synthesis

Which enam controls the rate of glycogen synthesis?

glycogen synthase or glycogenin

Answer 30

Glycogen synthase

Question 31

Glycogen Synthesis

What enam controls the additions of branches onto glycogen chains?

Answer 31

Branching enam

a(1->6)

Question 32

Glycogen Synthesis

Why is branching in glycogen chains useful?

Answer 32

It increases both the rates of synthesis and the degredation by increasing potential sites of action

Question 33

Regulation of Glycogen Synthesis

What enam regulates glycogen synthesis?

Answer 33

Insulin

Question 34

Regulation of Glycogen Synthesis

What does insulin do?

Answer 34

Produced in response to High blood glucose, triggers the phosphorylation and inactivation of the glycogen synthase kinase. Prevents the phosphorylation of glycogen synthase

Phosphorylation of glycogen synthase stops it from creating glycogen

Question 35

Glycogen General

What does phosphorylation do to the enzymes for breakdown vs the enam for synthesis?

Answer 35

Breakdown: activates
Synthesis: Inactivates