28. Carbohydrate Metabolism

Question 1

Describe blood glucose levels in the body? (7pts)

Answer 1

1. The body aims to maintain blood glucose
2. The concentration of glucose in the blood needs to be maintained in order for the brain to function normally.
3. Critical blood glucose= 2.5mM
4. Blood concentration drops= Hypoglycaemia
5. Blood glucose concentration high= Hyperglycaemia
6. Resting level of glucose in the blood= 3mM
7. Glucose concentration in the blood rises rapidly after consumption of glucose but also falls back down rapidly.

Question 2

Describe the effects of Hypoglycaemia? (5pts)

Answer 2

1. Muscle weakness
2. Loss of coordination
3. Mental confusion
4. Sweating
5. Hypoglycemic coma and death

Question 3

What happens when Glucose enters the liver cell? (2pts)

Answer 3

1. Glucose enters the liver cell and is phosphorylated to give glucose-6-phosphate.
2. Glucose-6-phosphate leads to:
3. The production of glycogen
4. The production of pryuvate
5. The production of Ribose 5-phosphate via the pentose phosphate pathway.

Question 4

Describe Glycogenesis? (6pts)

Answer 4

1. Glycogenesis is glycogen synthesis.
2. Takes place in the liver- 100g of glycogen stored in the liver.
3. Takes place in skeletal muscle- 300-400g of glycogen stored in skeletal muscle.
4. Glycogenesis takes place when blood glucose levels are high
5. Glycogen is made as it is a safe way to store glucose.
6. Glycogenesis is regulated by insulin.

Question 4

Describe Glycogenesis? (6pts)

Answer 4

1. Glycogenesis is glycogen synthesis.
2. Takes place in the liver- 100g of glycogen stored in the liver.
3. Takes place in skeletal muscle- 300-400g of glycogen stored in skeletal muscle.
4. Glycogenesis takes place when blood glucose levels are high
5. Glycogen is made as it is a safe way to store glucose.
6. Glycogenesis is regulated by insulin.

Question 5

Describe Glycogen? (5pts)

Answer 5

1. Branched polymer of glucose
2. Glycogen is formed from linking glucose molecules via an alpha 1-4 link and alpha 1-6 link.
3. Large molecule
4. Glycogen is formed on the protein primer Glycogenin. In the absence of glycogenin glycogen does not form.
5. Synthesis of glycogen is regulated via the enzyme glycogen synthase.

Question 6

Describe Glycogen synthesis? (6pts)

Answer 6

1. Glucose 6 phosphate is converted to Glucose 1 phosphate via the enzyme phosphoglucomutase.
2. The Glucose 1 phosphate reacts with UTP to form UDP-glucose.
3. The UDP-Glucose then reacts with glcogenin to form Glycogenin with one glucose residue attached.
4. Glycogen synthase occurs which causes more glucose residues to be added to the glycogenin to increase the length of the chain of glucose molecules. Alpha 1-4 glycosidic bonds are formed.
5. Under the action of the branchig enzyme the 5 residues at the end of the glucose polymer a 1-6 alpha bond is formed.
6. The glucose is then added to the end of the two molecules.

Question 7

Why is Glycogen used? (3pts)

Answer 7

1. Glucose cannot be stored as it is osmotically active
2. 40 mM of glucose is stored as 0.01 uM glyocgen
3. The branched structure of glycogen means it can be rapidly synthesised.

Question 8

Describe Glycogenolysis? (11pts)

Answer 8

1. The enzyme Phosphorylase breaks down the a-1-4 links resulting in the production of Glucose-1 Phosphate.
2. The glucose-1-phosphate is then converted to glucose 6-phosphate via the enzyme phosphoglcutomatase.
3. The de-branching enzyme (transferase) is used to break the alpha 1-6 bonds and removes 3 residues.
4. It then puts it on the non reducing end on the chain in transferase activity.
5. The phosphorylase will continue to break down the alpha 1-4 links and the de-branching enzyme transferase will continue to break down the alpha 1-6 links.
6. This produces Glucose
7. Glucose is phosphorylated in muscle to Glucose-6-phosphate and this will be used in glycolysis.
8. Phosphorylase will continue to break down chains removing one residue at a time producing glucose-1-phosphate.
9. The glucose 1-phosphate will be converted to Glucose-6-phosphate.
10. In the liver an enzyme called glucose 6 phosphatase will convert the glucose-6-phosphotase to glucose.
11. The glucose is then transported to general circulation to be used by tissues such as the brain.

Question 9

Describe the enzymes involved in Glycogenesis? (2pts)

Answer 9

1. Phosphorylase breaks the a 1-4 links

2. Transferase breaks the a 1-6 links and moves 3 residues to an existing chain.

Question 10

What are the 2 products of Glycogeolyiss? (2pts)

Answer 10

1. Glucose 1-phospate

2. Glucose

Question 11

Describe Glycogen phosphorylase? (4pts)

Answer 11

1. Phosphorylase is a large, multi-subunit enzyme that plays a key role in glycogenesis.
2. It is regulated by allosteric interactions that signal the energy state of the cell.
3. Regulated by reversible phosphorylation regulated by hormones such as insulin, glycagon, adrenaline and noroadrenaline.
4. Regulation of glycogen phosphorylase differs in the muscle and liver.

Question 12

Describe hormonal regulation of glycogenolysis? (4pts)

Answer 12

1. Insulin inhibits glycogenolysis
2. Glucagon stimulates glycogenolysis in the liver
3. Adrenaline stimulates glycogenolysis in the muscle
4. Cortisol is a weak stimulus

Question 13

Describe the control of glycogen phosphorylase? (4pts)

Answer 13

1. In muscle glyocgen phsophorylase b can be activated by 5 AMP without being phosphorylated.
   - 5AMP forms when ATP is depleted.
2. ATP binds to the same site and block activation
3. Glucose-6-phosphate also blocks 5-AMP activation
4. In the liver the activated phosphorylase a is inhibited by glucose.

Question 14

Describe the activation of Phosphorylase b kinase by Ca2+? (3pts)

Answer 14

1. Ca2+ ions activate phosphorylase b kinase.
2. In muscle, mediating glycogenloysis during muscle contraction occurs.
3. Only get max activity in the muscle when both calicium and phosphorylation are present. In the liver a-adrenergic activation stimulates CA2+ release.

Question 15

Summarise Glycogen synthase? (4pts)

Answer 15

1. Activated in times of excess glucose
   2, Stimulated by insulin
2. Inactivated by phosphorylation by protein kinase A.
3. Activated by dephosphrylation of protein phosphate-1.

Question 16

Summarise Glycogen Phosphorylase? (5pts)

Answer 16

1. Activated when blood glucose is low
2. Stimulated by glucagon, adrenaline and noradrenaline
3. Inactivated by ATP and G6P
4. Activated by phosphorylation by phosphorylase b kinase.
5. Inactivated by dephosrylation by protein phosphate-1.

Question 17

Describe the Pentose Phosphate pathway?

Answer 17

Glucose-6-Phopshate is converted to Ribose 5 Phosphate which leads to nucleotides and nucleic acid synthesis. NADPH is formed which is involved in the protection of the cell.

Question 18

Describes what happens in the liver? (5pts)

Answer 18

1. Pryuvae carboxylase is a mitochondrial enzyme which converts Pryuvate to Oxalocetate.
2. Pryuvtae synthesised in the cytosol has to be transported to the mitochondria.
3. The oxloacetate is then converted to Malate in order for it to be transported back out of the mitochodnria and into cytosol.
4. The malate is then converted to Oxaloacetate.
5. The enzyme phosphoenal pryuvate carboxykinase converts the oxaloacetate to Phosphoeneol Pryuvate.