Metabolic pathways: glycogen

Question 1

What is glycogenesis?

Answer 1

Creation of glycogen from excess glucose

Question 2

What is glycogen?

Answer 2

Storage form of glucose

Question 3

Where is glycogen found?

Answer 3

Skeletal muscle and liver

Question 4

Why is glycogen stored in the liver?

Answer 4

For maintenance of blood glucose levels: glucose broken down from storage glycogen can be released into the bloodstream for use by other tissues

Question 5

Why is glycogen stored in skeletal muscle?

Answer 5

For use as fuel by that muscle: glucose here cannot be released into the bloodstream

Question 6

What is glycogenolysis?

Answer 6

Breakdown of glycogen into glucose

Question 7

What happens to glycogen during physical activity?

Answer 7

In skeletal muscle: glycogenolysis occurs to produce glucose to be used in TCA cycle, producing ATP

Question 8

When does glycogenolysis tend to occur and why?

Answer 8

Between mealtimes to keep blood sugar at a constant level

Question 9

What process keeps blood sugar constant when glycogen levels in the liver have become depleted?

Answer 9

Gluconeogenesis

Question 10

What are the links between glucose molecules in the glycogen polymer?

Answer 10

Alpha 1-4 glycosidic bonds

Question 11

Which glycosidic bonds introduce branches into the glycogen polymer?

Answer 11

Alpha 1-6 glycosidic bonds

Question 12

Where are free glucose molecules added during glycogen synthesis?

Answer 12

The ends of branches

Question 13

What protein is at the centre of glycogen?

Answer 13

Glycogenin

Question 14

What is the purpose of glycogenin?

Answer 14

It can add a small number of glucose molecules to itself - acts as a “primer” for glycogen

Question 15

Why is glycogenin so important in glycogen synthesis?

Answer 15

Glycogen synthase - the enzyme which allows glycogen synthesis - can only add glucose to existing glycogen chains
Needs a glycogen primer - this is glycogenin

Question 16

Glucose must be phosphorylated before it can be used in any metabolic pathways. What enzyme facilitates this reaction, and what is the end product?

Answer 16

Hexokinase
Glucose-6-phosphate
This also involves the conversion of ATP to ADP

Question 17

What two fates are there for glucose-6-phosphate depending on blood glucose levels?

Answer 17

If a cell requires energy, glucose-6-phosphate can be broken down in glycolysis
If there is plenty of energy, glucose can be converted to be stored as glycogen

Question 18

In glycogen synthesis, which enzyme facilitates the conversion of glucose-6-phosphate to glucose-1-phosphate?

Answer 18

Phosphoglucomutase

Question 19

In glycogen synthesis, what must glucose-1-phosphate be converted to?

Answer 19

Glucose-1-phosphate must be activated by being converted to UDP glucose

Question 20

What is required for the conversion of glucose-1-phosphate to UDP-glucose in glycogen synthesis?

Answer 20

1 UTP molecule

The enzyme UDP-glucose pyrophosphorylase

Question 21

What does glycogen synthase do?

Answer 21

Takes the glucose part of UDP-glucose and bonds it to the end of the glycogen strand

Question 22

What happens to the UDP after glycogen synthase has removed the glucose from UDP-glucose?

Answer 22

It is rephosphorylated to UTP, requiring 1 ATP molecule

Question 23

How many ATP are consumed for each molecule of glucose added to glycogen?

Answer 23

1 - this is used to rephosphorylate UDP to UTP

Question 24

What is the significance of convering glucose to UDP glucose in the glycogen synthesis pathway?

Answer 24

UDP-glucose is an activated form of glucose - conversion is required to make it reactive

Question 25

Can glycogen synthase start new molecules of glycogen?

Answer 25

No - glycogenin required

Question 26

Can glycogen synthase introduce new branches of glycogen?

Answer 26

No - requires branching enzyme, a transglycosylase which introduces an alpha 1-6 glycosdic bond every 10 glucose molecules or so

Question 27

What enzyme catalyses glycogenolysis?

Answer 27

Glycogen phosphorylase

Question 28

What reaction does glycogen phosphorylase catalyse?

Answer 28

[Glucose]n + phosphate –> glucose-1-phosphate + [glucose]n-1
i.e. shortens glycogen by one glucose molecule

Question 29

In glycogenolysis, which enzyme converts glucose-1-phosphate back to glucose-6-phosphate?

Answer 29

Phosphoglucomutase

Question 30

What happens to glucose-6-phosphate in the liver after glycogenolysis?

Answer 30

Glucose-6-phosphate can be dephosphorylated back to glucose for release into the bloodstream

Question 31

What happens to glucose-6-phosphate in skeletal muscle after glycogenolysis?

Answer 31

It cannot be dephosphorylated back to glucose, so it is used to provide energy via glycolysis and the TCA cycle

Question 32

Which two enzymes regulate the occurrence of glycogenesis and glycogenolysis?

Answer 32

Glycogen synthase and glycogen phosphorylase

Question 33

Which enzyme - glycogen synthase or glycogen phosphorylase - does insulin inhibit?

Answer 33

Glycogen phosphorylase - this is because insulin is the hormone of the fed state, and in the fed state there is no need for more glucose to be produced

Question 34

Which enzyme - glycogen synthase or glycogen phosphorylase - does glucagon inhibit?

Answer 34

Glycogen synthase - this is because glucagon is the hormone of the starving state, and in the starving state glucose is needed in the bloodstream, not in storage

Question 35

What other two hormones signal for glycogen breakdown by stimulating glycogen phosphorylase?

Answer 35

Adrenaline and cortisol - both hormones signal a need for energy, and rapid mobilisation of glucose