Biochemistry- Glycogen synthesis and breakdown

Question 1

What is glycogenesis?

Answer 1

The synthesis of glycogen from freely available glucose

Question 2

What is glycogenolysis?

Answer 2

The breakdown of glycogen to form glucose

Question 3

Why is glycogen important?

Answer 3

It is the major storage molecule for carbohydrates in liver and muscle cells

Question 4

How is glycogenolysis accomplished?

Answer 4

By phosphorolysis

Question 5

What is the function of liver glycogen?

Answer 5

It is broken down between meals to maintain blood glucose levels for red blood cells and the brain.

Question 6

What is the function of muscle glycogen?

Answer 6

It is not available for the maintenance of blood glucose levels.
The energy can only be consumed by muscle cells.
It provides energy via glycolysis and TCA during bursts of physical activity

Question 7

What is the primary source of glucose overnight?

Why?

Answer 7

Gluconeogenesis

Because at this time hepatic glycogen is depleted

Question 8

What is gluconeogenesis?

Answer 8

The generation of new glucose from non-carbohydrate precursors.

Question 9

What contributes most blood glucose straight after a meal?

Answer 9

Dietary carbohydrate

Question 10

What keeps the blood glucose level constant between meal?

Answer 10

Glycogenolysis of hepatic glycogen

Question 11

Describe the structure of glycogen

Answer 11

It is a polymer consisting of glucose molecules joined by alpha-1,4-glycosidic links
Branches are introduced by alpha-1,6-glycosidic links

Question 12

How is glycogen built up?

Answer 12

Glucose residues can only be added to an existing glycogen chain.
A glycogen primer containing at least 4 glucose residues is required.
The primer is covalently attached to a protein called glycogenin, which has enzymatic activity and can add these residues to itself.

Question 13

What enzyme synthesizes glycogen?

Answer 13

Glycogen synthase

Question 14

Why is glycogenin required?

Answer 14

Because glycogen synthase cannot start a glycogen from start- it can only add glucose onto existing glycogen.

Question 15

What must happen to glucose before it can be used to synthesise glycogen?

Answer 15

It must be phosphorylated by hexokinase to glucose-6-phosphate. This traps the glucose in the cell.
If there is an energy requirement, this can then undergo glycolysis. If there is an energy surplus, it can be used to synthesise glycogen.

Question 16

What happens to glucose-6-phosphate in the synthesis of glycogen?

Answer 16

Glucose-6-phosphate is converted to glucose-1-phosphate by phosphoglucomutase. This enzyme simply moves the phosphate from one carbon to another, and it can catalyse forward and backward reactions.
UDP-glucose phosphorylase then converts the glucose to UDP-glucose, which can be thought of as an active form of glucose.
UDP-glucose acts as a substrate for glycogen synthase. The enzyme takes up the glucose part of UDP-glucose and adds it onto an existing glycogen. Left over UDP can be phosphorylated again to form UTP, requiring one ATP equivalent. Therefore one ATP is consumed for each glucose added to glycogen.

Question 17

Why is UDP glucose highly active?

Answer 17

Because of the bond between the glucose and the phosphate of the UDP. The phosphate ester bond contains high energy levels which can be exploited to transfer the glucose onto another acceptor molecule.

Question 18

If ATP is considered as a phosphate transfer molecule, how can UDP be considered?

Answer 18

As a glucose transfer molecule

Question 19

What does glycogen synthase do?

What can it not do?

Answer 19

It synthesises glycogen from UDP-glucose, by adding one glucose to glycogen at a time. It can only extend the chains of glycogen- it cannot start a new chain or introduce new branches.

Question 20

Which enzyme can introduce new branches onto glucose?

Which bonds are formed in the formation of new branches?

Answer 20

Transglycosylase. This forms alpha-1,6-glycosidic bonds to introduce branches at approximately every 10 glucose residues.

Question 21

Which enzyme is the rate limiting enzyme in the synthesis of glycogen?

Answer 21

Glycogen synthase

Question 22

Describe glycogenolysis.

Answer 22

It is catalysed by glycogen phosphorylase. This frees glucose by phosphorlysis. It cleaves a glucose off the end of glycogen and adds a phosphate to the glucose to create glucose-1-phosphate. Phosphoglucomutase then transfers the phosphate from carbon 1 to carbon 6 to form glucose-6-phosphate.

Question 23

What is the rate limiting step in glycogenolysis?

Answer 23

The step carried out by glycogen phosphorylase- which cleaves off a glucose and adds a phosphate to it.

Question 24

What happens to the glucose-6-phosphate produced by glycogenonlysis in the liver?

Answer 24

It is de-phosphorylated and the resulting glucose is released into the bloodstream.

Question 25

What happens to the glucose-6-phosphate produced by glycogenolysis in skeletal muscle?

Answer 25

It cannot be dephosphorylated but is used to provide energy via glycolysis and the TCA cycle.

Question 26

What can glycogen phosphorylase not do?

Answer 26

It cannot remove the branches- this requires additional enzymes.

Question 27

Which transporter moves the glucose from the liver into the bloodstream?

Answer 27

GLUT-2

Question 28

What does insulin do?

Answer 28

It stimulates glycogen synthase and inhibits glycogen phosphorylase

Question 29

What does glucagon do?

Answer 29

It stimulates glycogen phosphorylase and inhibits glycogen synthase

Question 30

Name two other hormones which stimulates glycogen phosphorylase?

Answer 30

Adrenaline

Cortisol

Question 31

What happens in glycogen storage diseases?

Answer 31

In all but one of the diseases, there are increased glycogen deposits in the liver and muscle.