Glycogen Metabolism

Question 1

What are the names for glycogen synthesis and breakdown?

Answer 1

Synthesis - glycogenesis.

Breakdown - glycogenolysis.

Question 2

What is the difference in the function of liver and muscle glycogen?

Answer 2

Liver: broken down between meals and released to maintain blood glucose levels for RBCs and brain.
Muscle: provides energy via respiration during bursts of physical activity.

Question 3

What is the primary source of glucose overnight when hepatic glycogen is depleted?

Answer 3

Gluconeogenesis.

Question 4

What are the links joining glycogen and what links introduce branches?

Answer 4

α 1-4 glycosidic links, α 1-6 glycosidic links.

Question 5

What is the protein at the centre of glycogen?

Answer 5

Glycogenin.

Question 6

What is the function of glycogenin and why is this important?

Answer 6

It has catalytic properties to add glucose molecules to itself. Glycogen synthase can only add glucose molecules to existing glycogen so glycogen primer of at least 4 glucose required to be attached to glycogenin.

Question 7

Before glucose can be used in either glycolysis or glycogen synthesis, what needs to happen to it?

Answer 7

Needs to be phosphorylated by hexokinase to form glucose-6-phosphate.

Question 8

What determines whether glucose-6-phosphate will be used for glycolysis or glycogen synthesis?

Answer 8

If glucose conc is low, used for glycolysis and vice versa.

Question 9

What does glucose-6-phosphate need to be converted to before glycogenesis and what enzyme does this?

Answer 9

Glucose-1-phosphate. Phosphoglucomutase.

Question 10

What is a special feature of phosphoglucomutase?

Answer 10

It can do forwards and backwards reactions.

Question 11

What is glucoase-1-phosphate then converted to and what is required?

Answer 11

UDP-glucose. Uses a molecule of UTP and UDP-glucose phosphorylase.

Question 12

What is released when glycogen synthase adds UDP-glucose to the end of the strand?

Answer 12

UDP molecule.

Question 13

What happens to the released UDP molecule?

Answer 13

It is phosphorylated to form UTP. For each glucose added to glycogen, 1 ATP is consumed.

Question 14

What about UDP-glucose makes it highly active?

Answer 14

The bond between glucose and the second phosphate group of uridine diphosphate.

Question 15

What is the bond between UDP and glucose?

Answer 15

Phosphate ester linkage.

Question 16

What is the rate limiting enzyme of glycogenesis?

Answer 16

Glycogen synthase.

Question 17

What is the enzyme which introduces branching to glycogen and after how many residues does it do this?

Answer 17

Transglycosylase. Every 10.

Question 18

What enzyme is glycogenolysis catalysed by?

Answer 18

Glycogen phosphorylase.

Question 19

What does glycogen phosphorylase do?

Answer 19

Cleaves a glucose molecule from the glycogen and adds a phosphate to form glucose-1-phosphate.

Question 20

What is the rate limiting step of glycogenolysis?

Answer 20

The reaction catalysed by glycogen phosphorylase.

Question 21

What happens to the glucose-1-phosphate once it has been cleaved off glycogen?

Answer 21

It is the converted to glucose-6-phosphate by glucophosphomutase.

Question 22

Once glucose has been converted into glucose-6-phosphate, what then happens in the liver?

Answer 22

Can be de-phosphorylated by glucose-6-phosphatase and the resulting glucose released into the blood stream via GLUT-2 transporters.

Question 23

Once glucose has been converted into glucose-6-phosphate, what then happens in skeletal muscle?

Answer 23

Glucose-6-phosphate cannot be de-phosphorylated, is used to provide energy via glycolysis and the TCA cycle.

Question 24

What does debranching require?

Answer 24

Additional enzymes.

Question 25

What enzymes allow regulation in glycogenesis and glycogenolysis?

Answer 25

Glycogen synthase (glycogenesis), glycogen phosphorylase (glycogenolysis).

Question 26

What hormone allows regulation of glycogenesis and how?

Answer 26

Insulin, indicates high blood sugar so stimulates glycogen synthase and inhibits glycogen phosphorylase.

Question 27

What hormone allows regulation of glycogenolysis and how?

Answer 27

Glucagon, indicates low blood sugar so stimulates glycogen phosphorylase and inhibits glycogen synthase.

Question 28

What other hormones can stimulate glycogen phosphorylase?

Answer 28

Adrenaline and cortisol.

Question 29

What are glycogen storage diseases and how many types are there?

Answer 29

Group of diseases with increase glycogen deposits in liver or muscle or both. 10.