MEH 4. Glycogen And Fat

Question 1

Which tissues have an absolute requirement for glucose as an energy source?

Answer 1

Red blood cells
Neutrophils
Innermost cells of kidney medulla
Lens of the eye

Question 2

What are the 2 locations of glycogen storage?

Answer 2

1. Muscle

2. Liver - hepatocytes

Question 3

Glycogen is stored as ___________.

Answer 3

Granules

Question 4

Glycogen is a polymer consisting of chains of which monomer?

Answer 4

Glucose

Question 5

Which two glycosidic bonds are present in glycogen?

Answer 5

Alpha 1-4 glycosidic bonds between glucose residues.

Alpha 1-6 glycosidic bonds forming branches every 8-10 residues.

Question 6

Which protein dimer is present at the core of glycogen?

Answer 6

Glycogenin

Question 7

Outline the first step of glycogen synthesis from glucose, naming the enzyme required.

Answer 7

Glucose + ATP -> Glucose-6-phosphate + ADP.

Hexokinase or glucokinase (in liver)

Question 8

The second step of glycogenesis converts glucose-6-phosphate to glucose 1-phosphate. Which enzyme is responsible for this?

Answer 8

Phosphoglucomutase ( this step is reversible)

Question 9

In the 3rd reaction of glycogenesis, what is glucose-1-phosphate converted to?

Answer 9

UDP-glucose

Question 10

What form of glucose is able to be incorporated to glycogen?

Answer 10

UDP-glucose

Question 11

Which 2 enzymes are responsible for the addition of UDP-glucose to glycogen?

Answer 11

Glycogen synthase (alpha 1,4 glycosidic bonds)
Branching enzyme (alpha-1,6-glycosidic bonds)

Question 12

Which enzymes are responsible for degradation of glycogen to produce glucose 1-phosophate?

Answer 12

Glycogen phosphorylase

De-branching enzyme

Question 13

Why is glycogenolysis not simply the reversal of glycogenesis?

Answer 13

Different enzymes are involved in order to allow inhibition of one pathway and stimulation of another.

Question 14

Why can only liver glycogen (and not muscle glycogen) act as a buffer for blood glucose levels?

Answer 14

In the liver, Glucose 6-phosphate is converted to glucose by glucose-6-phosphatase and exported to blood.

Muscle lacks the enzyme glucose-6-phosphatase, so glucose 6-phosphate enters glycolysis for energy production.

Question 15

What is the rate limiting enzyme in glycogen synthesis?

Answer 15

Glycogen synthase

Question 16

What is the rate limiting enzyme in glycogen degradation?

Answer 16

Glycogen phosphorylase

Question 17

Which 3 hormones regulate glycogen metabolism?

Answer 17

Glucagon, adrenaline and insulin

Question 18

What effect do glucagon and adrenaline have on glycogen synthase?

Answer 18

Decrease it’s activity

Question 19

What effect do glucagon and adrenaline have on glycogen phosphorylase?

Answer 19

Increase its activity

Question 20

What effect does insulin have on glycogen synthase?

Answer 20

Increase its activity

Question 21

What effect does insulin have on glycogen phosphorylase?

Answer 21

Decreases activity

Question 22

How does regulation of muscle stores differ to liver stores?

Answer 22

Glucagon has no effect on muscle glycogen stored (no receptors).
AMP is an allosteric activator of muscle glycogen phosphorylase, but NOT the liver form of the enzyme.

Question 23

How do the mechanisms of glucagon and adrenaline differ to insulin?

Answer 23

Glucagon + adrenaline - phosphorylate

Insulin - de-phosphorylates

Question 24

Give 2 examples of glycogen storage diseases.

Answer 24

1. Von Gierke’s disease

2. McArdle disease

Question 25

Which enzyme is deficient in von Gierke's disease?

Answer 25

Glucose 6-phosphatase

Question 26

Which enzyme is deficient in McArdle disease?

Answer 26

Muscle glycogen phosphorylase

Question 27

Where does gluconeogenesis occur?

Answer 27

Liver and to a lesser extent in kidney cortex.

Question 28

What are the 3 major precursors of gluconeogenesis?

Answer 28

1. Lactate (from anaerobic glycolysis) 2. Glycerol ( from adipose tissue breakdown of triglycerides) 3. Amino acids (alanine)

Question 29

Why is Acetyl CoA unable to be converted to glucose?

Answer 29

Acetyl-CoA cannot be converted to pyruvate. Pyruvate deyhydrogenase enzyme regulated reaction is irreversible.

Question 30

After how many hours of fasting is gluconeogenesis required?

Answer 30

8 hours | Liver glycogen stores start to deplete at this point and alternative source is required.

Question 31

What are the 3 key regulatory enzymes in gluconeogenesis?

Answer 31

1. PEPCK 2. Fructose 1,6-bisphosphatase 3. Glucose-6-phosphatase

Question 32

Which hormones will function to stimulate gluconeogenesis? Through which enzymes does it exert this effect?

Answer 32

Glucagon Cortisol PEPCK Fructose 1,6 bisphosphatase

Question 33

Which hormone will function to decrease gluconeogenesis?

Answer 33

Insulin Through PEPCK Fructose 1,6 bisphosphatase

Question 34

What molecule are excess lipids stored as?

Answer 34

Triglycerol

Question 35

Under what conditions might a TAG store become utilised? How is this controlled?

Answer 35

Prolonged exercise Stress Starvation Pregnancy Hormonal control

Question 36

Outline the structure of an adipocyte.

Answer 36

Large lipid droplet (mainly TAG and cholesterol ester) | Cytoplasm and organelles pushed to the side of the cell

Question 37

How might adipocytes in an overweight individual differ to those in a healthy individual?

Answer 37

Upon weight gain, adipocytes increase in size before dividing to increase the total number of fat cells.

Question 38

Which is the role of hormone sensitive lipase?

Answer 38

Mobilises TAG stores in adipose tissue, converting them to FA and glycerol. FA is then transported by albumin to the tissues to be oxidised to produce energy.

Question 39

Where does lipogenesis occur?

Answer 39

Liver

Question 40

What is the major source of carbon used for lipogenesis?

Answer 40

Dietary glucose

Question 41

What is the function of the malate shuttle in lipogenesis?

Answer 41

Production of NADPH, which is required by fatty acid synthase

Question 42

What is the key regulatory enzyme in lipogenesis?

Answer 42

Acetyl-CoA carboxylase

Question 43

What effect does insulin have on acetyl-CoA carboxylase?

Answer 43

Increases activity to increase lipogenesis following a meal.

Question 44

In addition to insulin, what else will increase the activity of Acetyl-CoA carboxylase?

Answer 44

Citrate

Question 45

What hormones and molecules will decrease the activity of Acetyl-CoA carboxylase?

Answer 45

Glucagon Adrenaline AMP

Question 46

What is the substrate and product of the reaction catalysed by Acetyl-CoA carboxylase?

Answer 46

Acetyl Co A converted to Malonyl CoA

Question 47

In fatty acid synthesis, what molecule are C2 atoms added as?

Answer 47

Malonyl CoA

Question 48

Where does fatty acid synthesis occur?

Answer 48

Cytoplasm

Question 49

How do glucagon and insulin mobilise TAG stored in adipose tissue?

Answer 49

Phosphorylate and activate hormone sensitive lipase, which converts TAG to glycerol and fatty acids which can travel by albumin to tissues to undergo beta oxidation.

Question 50

How does insulin inhibit mobilisation of TAG stores in adipose tissue?

Answer 50

Insulin leads to de-phosphorylation and inhibition of hormone sensitive lipase.

Question 51

What is the fate of glycerol produced when TAG is mobilised from adipose stores?

Answer 51

Travels to the liver and utilised as carbon source for gluconeogenesis.

Question 52

What is the main difference in fuel stores in an obese man compared to a healthy man?

Answer 52

Significantly raised triacylglycerol levels