3. Energy Storage - 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

How is glycogen stored?

Answer 2

As granules either in muscle or the liver

Question 3

Why is glucose not stored?

Answer 3

Glucose would have great osmotic effect

Question 4

How are chains of glycogen organised?

Answer 4

Have branches originating from a dimer of the protein glycogenin
Glucose residues liked by alpha 1-4 glycosidic bonds with alpha 1-6 glycosidic bonds forming branch points every 8-10 residues

Question 5

Why is there branching in glycogen?

Answer 5

It allows efficient release of glucose residues as can be from many sections of glycogen

Question 6

What is the first reaction in glycogenesis?

Answer 6

Glucose + ATP -> glucose 6-phosphate + ADP
Uses hexokinase (glucokinase in liver)

Question 7

What is the second reaction in glycogenesis?

Answer 7

Glucose 6-phosphate glucose 1-phosphate

Using phosphoglucomutase

Question 8

What is the third reaction in glycogenesis?

Answer 8

Glucose 1-phosphate + UTP + H2O -> UDP-glucose + PPi

Using G1P uridylyltransferase

Question 9

What is the fourth reaction in glycogenesis?

Answer 9

Glycogen(n residues) + UDP-glucose -> glycogen(n+1 residues) +UDP
Using glycogen synthase (1-4) or branching enzyme (1-6)

Question 10

What is the first reaction in glycogenolysis?

Answer 10

Glycogen(n residues) + Pi -> glucose 1-phosphate + glycogen(n-1 residues)
Using glycogen phosphorylase or de-branching enzyme

Question 11

What is the second reaction in glycogenolysis?

Answer 11

Glucose 1-phosphate glucose 6-phosphate

Using phosphoglucomutase

Question 12

How are glycogen stores used in liver?

Answer 12

Glycogen -> glucose 1-phosphate -> glucose 6-phosphate -> glucose
G6P converted to glucose and exported to blood
Liver glycogen is a buffer of blood glucose levels

Question 13

How are glycogen stores used in muscle?

Answer 13

Glycogen -> glucose 1-phosphate -> glucose 6-phosphate -> glycolysis
Muscle lacks enzyme glucose-6-phosphatase
G6P enters glycolysis for energy production

Question 14

What is the rate limiting enzyme in glycogen synthesis?

Answer 14

Glycogen synthase

Question 15

What is the rate limiting enzyme for glycogen degradation?

Answer 15

Glycogen phosphorylase

Question 16

How are muscle glycogen stores different?

Answer 16

Glucagon has no effect as it has no glucagon receptors on membrane
AMP is an allosteric activator of muscle glycogen phosphorylase

Question 17

Where do glycogen storage diseases arise from?

Answer 17

Deficiency or dysfunction of enzymes of glycogen metabolism

Question 18

What happens in glycogen storage diseases?

Answer 18

Liver and/or muscle can be effected
Excess glycogen storage can lead to tissue damage
Diminished glycogen stores can lead to hypoglycaemia and poor exercise tolerance

Question 19

Give 2 examples of glycogen storage diseases

Answer 19

Von Gierke’s disease - glucose 6-phosphatase deficiency

McArdle disease - muscle glycogen phosphorylase deficiency

Question 20

What is gluconeogenesis?

Answer 20

Production of new glucose

Used after 8 hours of fasting as liver glycogen stores start to deplete

Question 21

Where does gluconeogenesis occur?

Answer 21

Liver and to lesser extent in kidney cortex

Question 22

What are the 3 major precursors of gluconeogenesis?

Answer 22

Lactate from anaerobic glycolysis (Cori cycle)
Glycerol released from adipose tissue 9breakdown on triglycerides)
Amino acids (mainly alanine)

Question 23

Why is there no net synthesis of glucose from acetyl CoA?

Answer 23

Acetyl CoA cannot be converted into pyruvate because the pyruvate dehydrogenase reaction is irreversible

Question 24

What are the 3 key enzymes in gluconeogenesis?

Answer 24

PEPCK
Fructose 1,6-bisphosphatase
Glucose-6-phosphatase

Question 25

What does PEPCK do?

Answer 25

Converts oxaloacetate to phosphoenolpyruvate

Question 26

What does fructose 1,6-bisphosphatase do?

Answer 26

Converts fructose 1,6-bisphosphate to fructose 6-phosphate

Question 27

What does glucose 6-phosphatase do?

Answer 27

Converts glucose 6-phosphate to glucose

Question 28

What are the 2 key enzymes released in response to starvation/fasting, prolonged exercise or stress?

Answer 28

Fructose 1,6-bisphosphatase

PEPCK

Question 29

What is excess energy intake converted to?

Answer 29

Triacylglycerol (TAG) for storage

Question 30

Why are TAGs stored in adipose tissue?

Answer 30

Hydrophobic so stored in anhydrous form

Highly efficient energy store

Question 31

When are TAGs utilised?

Answer 31

In prolonged exercise, stress, starvation, during pregnancy

Question 32

What control is TAG storage and mobilisation under?

Answer 32

Hormonal control

Question 33

Where does fatty acid synthesis take place?

Answer 33

Mainly in liver

Question 34

What happens in fatty acid synthesis?

Answer 34

Glucose -> pyruvate in cytoplasm
Pyruvate into mitochondria and forms acetyl CoA and OAA, which then condense to form citrate
Citrate -> cytoplasm and cleaved back to acetyl CoA and OAA
Acetyl CoA carboxylate produces malonyl-CoA from acetyl CoA
Fatty acid synthase complex builds fatty acids by sequential addition of 2 carbon units provided by malonyl CoA

Question 35

What is the key regulatory enzyme in liver lipogenesis?

Answer 35

Acetyl CoA carboxylase

Question 36

What mobilises fat (lipolysis) from adipose tissue?

Answer 36

Hormone sensitive lipase