Week 6: Glycogen

Question 1

What is glucose energy for?

Answer 1

1. Brain
2. Cells lacking mitochondria
3. Cells with few mitochondria
4. Exercising muscles
5. Sudden and strenuous activity

Question 2

How is blood glucose obtained?

Answer 2

1. Diet
2. Gluconeogenesis
3. Degradation of glycogen

Question 3

Where is glycogen stored?

Answer 3

Liver

Question 4

Where would glycogen degradation and synthesis occur?

Answer 4

Cytosol of the cell

Question 5

What is glycogen made from?

Answer 5

1. Excess blood glucose

2. Recycling glucogenic metabolites

Question 6

What occurs when glycogen is degraded?

Answer 6

Energy is produced

Question 7

Describe the functions of glycogen storage in the muscle?

Answer 7

1. Serves as a fuel reserve for ATP synthesis
2. Store are replenished after depletion from exercise
3. When glycogen is used up, proteins degrade into amino acids enter gluconeogenesis

Question 8

Describe the functions of glycogen storage in the liver?

Answer 8

1. Storage increases during the well-fed state and depletes during a fast
2. Used to maintain blood glucose concentration
3. Maintains glood glucose for 10-18hrs

Question 9

Describe the structure of glycogen

Answer 9

1. Granules within the cytoplasm
2. The more branch points the more terminal residues become available
3. It’s a branched chain polysaccharide (homopolymer)
4. Each a-D-glucose is joined by a-1,4 glycosidic bonds
5. Branch points occur every 8-10 glycosyl residue by a-1,0 glycosidic bond

Question 10

What is the purpose for glycogen to be highly branched?

Answer 10

Makes it possible for several glucose residues to be released at once to meet energy needs

Question 11

What are the 3 steps of glycogen metabolism?

Answer 11

1. Release of glucose-1-phosphate from glycogen
2. Remodeling of glycogen for continued degradation
3. Conversion of glucose 1-phosphate into glucose 6-phosphate

Question 12

What occurs during the 1st reaction of glycogen breakdown?

Answer 12

Glycogen is phosphorolyzed by glycogen phosphorylase cleaving a glucose 1-phosphate molecule and a shortened glycogen molecule

Question 13

Does the 1st reaction of glycogen breakdown require ATP?

Answer 13

No

Question 14

Explain the process of debranching glycogen?

Answer 14

1. Glycogen phosphorylase works on noreducing ends to reach 4 residues from branch point
2. Branch point is removed by 2 enzymatic activities
3. Debranching enzyme transfers a block of 3 residues to the nonbranching end of the chain
4. Debranching enzyme cleaves the single remaining glucose which is becomes a free glucose unit
5. Glycogen phosphorylase continues

Question 15

What occurs during the 2nd reaction of glycogen breakdown?

Answer 15

Glucose-1-phosphate is isomerized by phosophoglucomutase to form glucose-6-phosphate

Question 16

Describe glycogen breakdown in the liver

Answer 16

1. Liver glycogen phosphorylase breaks down glycogen to glucose-1-phosphate
2. Through glucose-6-phosphotase, glucose-1-phosphate is converted to glucose-6-phosphate
3. Glucose and Pi are shuttled back to cytoplasm by transporters
4. Glucose is released from cell by transporter, GLUT2

Question 17

Describe glycogen breakdown in the muscle

Answer 17

1. Muscle glycogen phosphorylase breaks down glycogen to glucose-1-phosphate
2. Glucose-1-phosphate is converted to glucose-6-phosphate without glucose-6-phosphotase
3. Glucose-6-phosphotate enters the glycolytic pathway directly
4. Saves 1 ATP

Question 18

What are the fates of glucose-6-phosphate?

Answer 18

1. Processing by the glycolytic pathway
2. Conversion into free glucose for the release into the blood (liver)
3. Processing by the pentose phosphate pathway

Question 19

What is glycogen synthesis?

Answer 19

Synthesis of glycogen requiring more enzymes and metabolic intermediates than glycogen degradation

Question 20

Blood glucose must meet what criteria to initiate glycogen synthesis?

Answer 20

1. Phosphorylated
2. Attached to UDP
3. Added to glycogen

Question 21

What instigates Glucose-6-P to Glucose-1-P?

Answer 21

Phosphoglucomutase

Question 22

What occurs during stage 1 of glycogen synthesis?

Answer 22

1. Glucose-1-P is catalyzed by UDP-glucose pyrophosphorylase with UTP to produces UDP-glucose and PPi
2. PPi is hydrolyzed to Pi and Pi by pyrophosphatase

Question 23

What occurs during stage 2 of glycogen synthesis?

Answer 23

1. UDPG is added onto a growing glycogen chain by glycogen synthase
2. UTP is released once glycosidic bond is formed
3. UTP must be regenerated from UDP and ATP by nucleoside diphosphate kinase

Question 24

What is the general structure of a glycogen particle?

Answer 24

Glycogenin primer to initiate glycogen synthase

Question 25

How are glycogenin primers synthesized?

Answer 25

Glycogenin enzymes

Question 26

What is a branching enzyme?

Answer 26

Catalyzes the reactions needed to introduce a branch point during the synthesis of glycogen

Question 27

Describe the mode of action of the branching enzyme in glycogen synthesis

Answer 27

1. Transfers a segment (7 residues long) from the end of a growing chain to the branch point where it catalyzes the formation of the required
2. α-1,6 glycosidic linkage α-1,6 glycosidic linkage occurs at least 4 residues away from previous branch point
3. Glycogen synthase can then extend the branches

Question 28

What would be the major controlling factor in the synthesis and breakdown of glycogen?

Answer 28

Glycogen phosphorylase

Question 29

What are the regulatory steps to glycogen degradation?

Answer 29

1. Enzyme is subject to allosteric control and covalent modification
2. Phosphorylation leads to activation of glycogen phosphorylase
3. Dephosphorylation leads to inhibition of glycogen phosphorylase
4. Phosphorylation is controlled by the hormones epinephrine and glucagon (activates breakdown of glycogen)

Question 30

What are the regulatory steps to glycogen synthesis?

Answer 30

1. Inactive form is the phosphorylated form, and the active form is unphosphorylated
2. Phosphorylation is stimulated by hormonal signals (glucagon or epinephrine) via the enzyme cAMP-dependent protein kinase
3. Becomes inactive after phosphorylation at the same time the hormonal signal is activating phosphorylase

Question 31

Describe the allosteric control of glycogen metabolism in the liver

Answer 31

1. Glycogen synthase is allosterically activated by elevated glucose-6-phosphate
2. Glycogen phosphorylase is allosterically inhibited by glucose 6-phosphate and ATP.
3. Glucose inhibits glycogen phosphorylase in the liver only.

Question 32

Describe the allosteric control of glycogen metabolism in the muscle

Answer 32

1. Glycogen synthase is allosterically activated by elevated glucose-6-phosphate
2. Glycogen phosphorylase is allosterically inhibited by glucose 6-phosphate and ATP
3. Ca2+ release into the cytoplasm activates muscle glycogen phosphorylase to release glucose 1-phosphate to convert into glucose 6-phosphate to enter glycolysis and produce ATP.
4. In a low energy state, increased AMP levels activate glycogen phosphorylase.

Question 33

Describe the hormonal control using the insulin-signaling pathway

Answer 33

1. At high blood glucose levels, insulin would bind to the insulin receptor producing a net dephosphorylation
2. Protein phosphatase 1 inactivates glycogen phosphorylase
3. Protein phosphatase 1 activates glycogen synthase increasing glycogen synthesis
4. Increasing glucose import into the liver

Question 34

Describe the hormonal control using the glucagon-signaling pathway

Answer 34

1. At low blood glucose levels, glucagon would bind to the glucagon receptor producing net phosphorylation
2. Protein kinase A inactivates glycogen synthase
3. Protein kinase A activates phosphorylase kinase which activates glycogen phosphorylase
4. Glycogen degradation increase as well as glucose export from liver