Chapter 15: Glycogen Synthesis and Degradation

Question 1

Glycogen molecule

Answer 1

• Highly branched with non-reducing ends
• Glucose residues linked by alpha-1,4-glycosidic bonds
• Alpha-1,6-glycosidic bonds every 10 residues

Question 2

Glycogen

Answer 2

• The storage form of glucose

Question 3

Glycogen synthesis involves

Answer 3

• Phosphoglucomutase
• Formation of UDP-glucose
• Addition of UDP-glucose to glycogen polymer
• Formation of branches

Question 4

Glycogen degradation involves

Answer 4

• Phosphorolytic cleavage
• Debranching enzyme
• Phosphoglucomutase

Question 5

Alpha-1,4-glycosidic linkages

Answer 5

• Linear linkages of glycogen molecules

Question 6

Alpha-1,6-glycosidic linkages

Answer 6

• Branching linkages of glycogen molecules

Question 7

Liver glycogen

Answer 7

• Used for export to maintain blood sugar levels and for use by other tissues

Question 8

Glycogen synthesis (glycogenesis)

Answer 8

• Ongoing in all tissues
• Phosphoglucomutase
• Formation of active glucose
• Transfer of UDP-glucose to glycogen
• Formation of branches

Question 9

Phosphoglucomutase reaction

Answer 9

• Synthesis of glucose-1-phosphate from glucose-6-phosphate (hexokinase)
• Near equilibrium
• Intramolecular transfer of phosphate

Question 10

Formation of active glucose (UDPG)

Answer 10

• UDP-glucose phosphorylase

- Accomplished by combining a uridyl phosphate residue with the phosphate group on the #1 carbon of G1P

Question 11

Glycogen synthase catalyzes

Answer 11

• Breaking of ester bond between C1 of glucose and the pyrophosphate of UDP
• Formation of a new alpha-1,4-glycosidic bond (between C1 of glucose residue and C4 hydroxyl on a terminal glucose residue)

Question 12

The reducing end of the glucose molecule (C1)

Answer 12

• Is always added to the non-reducing end of the growing glycogen molecule that is longer than 4 residues

Question 13

Length of the polymer increases

Answer 13

• One by one

Question 14

Glycogen synthase cannot make

Answer 14

• Alpha-1,6-glycosidic linkages

Question 15

UDP-glucose is added to the glycogen chain via

Answer 15

• An alpha-1,4-glycosidic bond extending the glycogen polymer by 1 glucose unit

Question 16

1,4-alpha glucan

Answer 16

• Branching enzyme that forms branches on glycogen molecule

- Increases glycogen solubility

Question 17

Block of 7 glucose residues

Answer 17

• Moves from the end of a chain onto the C6 hydroxyl of a neighboring chain
• Forms an alpha-1,6-glycosidic bond
• Occurs at a point from at least 4 residues removed form nearest branch

Question 18

Andersen’s Disease

Answer 18

• Branching enzyme is defective
• Formation of long unbranched glycogen
• Glycogen precipitates in heart and liver
• Cirrhosis and death

Question 19

Anderson’s disease creates linear polymers

Answer 19

• Decreases glycogen solubility

Question 20

Primer

Answer 20

• Required for glycogen synthesis

Question 21

Glycogen synthase primer

Answer 21

• Tyrosine residue on the glycogenin enzyme

Question 22

Glycogenin

Answer 22

• Glycotransferase

- Has tyrosine residue that contains the glycogen synthase primer

Question 23

UDP-glucose

Answer 23

• Added to the glycosylated glycogen until it reaches optimal density

Question 24

Glycogen exerts a negative feedback effect on glycogen synthase

Answer 24

• Promotes the a –> b transition

Question 25

Glycogenin is self-glycosylating

Answer 25

- Capable of adding glucose/sugars to itself (scaffold and an enzyme)

Question 26

Glycogen phosphorylase

Answer 26

- Phosphorolytic cleavage

Question 27

Debranching enzymes

Answer 27

- 4-alpha-D-glucotransferase | - Amylo-alpha-1,6-glucosidase

Question 28

Glycogen degradation (glycogenolysis) involves

Answer 28

- Glycogen phosphorylase - Debranching enzymes - Phosphoglucomutase

Question 29

2 activities of the glycogen degradation bifunctional enzyme

Answer 29

- Move glucose units | - Hydrolytic activity where cleavage of 1,6 occurs

Question 30

Anaerboic glycolysis products

Answer 30

- Investment of 2 ATP - Extraction of 4 ATP - 2 net ATP overall

Question 31

Phosphorylated glucose from glycogen is glucose-1-phosphate

Answer 31

- Can enter glycolysis - Bypass the hexokinase step - Produces 3 ATP as opposed to 2 ATP from normal anaerobic glycolysis (where glucose enters unphosphorylated)

Question 32

Glycogen phosphorylase

Answer 32

- Phosphorolysis of the alpha-1,4-glycosidic bonds (yields G1P) - Requires pyridoxal phosphate prosthetic group - Cleaves the 1,4 bonds up to four residues from a branch point

Question 33

Phosphorolytic cleavage occurs at

Answer 33

- The non-reducing terminal of the glycogen molecule

Question 34

Phosphorolytic cleavage is reversible, but

Answer 34

- Favors formation of G1P - Produces 3 molecules of ATP - Anaerobic glycolysis (hexokinase not required)

Question 35

Complete aerobic oxidation produces

Answer 35

- 33 molecules of ATP

Question 36

Proteolytic cleavage does not require

Answer 36

- Utilization of ATP to produce a phosphorylated product

Question 37

G1P can enter the glycolysis

Answer 37

- Glucose comes in phosphorylated - Coverted to G6P - Glycolysis continues

Question 38

4-alpha-D-glucotransferase

Answer 38

- Transfers the glycogen molecules during debranching

Question 39

Amylo-alpha-1,6-glucosidase

Answer 39

- Final debranching enzyme | - Hydrolytic cleavage

Question 40

There is a limit to which the phosphorylase enzyme debranches

Answer 40

- It will move 3 glucose units to the core leaving one behind

Question 41

Transferase followed by hydrolytic cleavage

Answer 41

- Produces free glucose

Question 42

Phosphoglucomutase

Answer 42

- Catalyzes the conversion of G1P to G6P - Near equilibrium - Used in both synthesis and degradation of glycogen

Question 43

Phosphorylated compounds

Answer 43

- Do not readily diffuse out of cells

Question 44

G6P in the liver

Answer 44

- Can be converted to glucose and Pi-G6Pase

Question 45

G6P formed in the muscle

Answer 45

- From glycogen breakdown - Can only be used in the muscle - Muscle lacks G6Pase