T2 L7: Glycogen metabolism in muscle and liver

Question 1

Where is most glucose stored in the body and why?

Answer 1

In the muscles because there is more muscle on the body that liver

Question 2

How much glycogen does the liver store?

Answer 2

Less than 24 hours worth

Question 3

Describe the structure of stored glucose in the body

Answer 3

Highly branched polysaccharide consisting of (α-1,4) linked glucose molecules with a (α-1,6) branch every 8-14 glucose residues

Question 4

What bonds are found between branches of a glycogen molecule?

Answer 4

α-1,6 bonds

Question 5

How is glycogen broken down is muscles?

Answer 5

Gluc-6-phosphate is broken down into pyruvate and then into lactate and CO2

Question 6

How is glycogen broken down in the liver?

Answer 6

Gluc-6-phosphate is broken down into glucose

Question 7

Why can’t glucose be broken down straight into glucose in muscles?

Answer 7

Because muscle cells don’t have the enzyme glucose-6-phosphotase

Question 8

Which enzyme breaks the α-1,4 of glycogen and what is this process called?

Answer 8

The enzyme Glycogen Phosphorylase breaks then by phosphorolysis . It can only break α-1,4 bonds

Question 9

Describe the process by which α-1,4 bonds are broken

Answer 9

Glycogen Phosphorylase removes single units from non-reducing ends of glycogen to form Glucose-1-Phosphate. ATP is not involved

Question 10

What is meant by phosphorolysis being analogous to hydrolysis?

Answer 10

The phosphate acts like water would in a hydrolysis reaction

Question 11

Which enzyme breaks the α-1,6 bonds in a glycogen molecule?

Answer 11

α-1.6-Glucosidase activity of the debranching enzyme. by hydrolysis After the debranching enzyme has moved all the glucoses attached and only a single one remains

Question 12

What is the importance of the debranching enzyme in glycogen degradation?

Answer 12

It’s a transferase enzyme that removes 3 residues from the branch of glycogen and transfers them to the end of a chain in a α-1,4 linkage so the phosphorylase enzyme can break them up.

Question 13

Why can’t glycogen broken down in the muscles be exported in blood?

Answer 13

Because muscles lack the enzyme glucose-6-phosphotase so it can’t leave

Question 14

What is uridine triphosphate (UTP)?

Answer 14

A molecule energetically equivalent to ATP

Question 15

What is UDP-Glucose?

Answer 15

A high energy form of glucose

Question 16

Describe the process that converts glucose into glycogen

Answer 16

Glucose is converted into glucose-6-phosphate by Hexokinase/Glucokinase (liver) using ATP. It’s then converted into glucose-1-phosphate and then UTP is added to produce UDP-Glucose. Glycogen synthase then converts it into Glycogen

Question 17

What does Glycogen synthase do?

Answer 17

It adds glucose units by α-1,4 linkage onto glycogen but only when there are more than 4 glycosyl residues

Question 18

Which protein carries out the priming function for glycogen synthesis?

Answer 18

Glycogenin. It remains in the centre of a glycogen molecule

Question 19

Describe the priming function for glycogen synthesis

Answer 19

UDP-glucose donates the first glycosyl residue and attaches it onto the amino acid tyrosine in glycogenin

Question 20

Which enzyme transfers a block of 7 residues from a growing glycogen chain to create a branch with a α-1,6 bond?

Answer 20

Branching enzyme

Question 21

A new glycogen branch can form within how many residues of a pre-existing branch? Why?

Answer 21

4 because the enzyme can’t access the residues otherwise

Question 22

Why is the sensitivity of phosphorylase and glycogen synthase beneficial?

Answer 22

They are very sensitive to hormones, stress, and muscle contraction so they can quickly respond to the metabolic needs of a cell

Question 23

What is 1 negative of storing glycogen?

Answer 23

It’s hydrophilic so it associates with water therefore a lot of water is also stored

Question 24

What is meant by allosteric regulation?

Answer 24

Changes in enzyme activity by molecules present in the cell

Question 25

What 3 molecules allosterically regulate activity of glycogen phosphorylase?

Answer 25

AMP, ATP, and glucose-6-phosphate

Question 26

What effect does a high [AMP] have on the activity of Glycogen phosphorylase?

Answer 26

Down regulation because it’s a sign that energy has been used up

Question 27

What effect does high [ATP] and high [glucose-6-phosphate] have on phosphorylase activity?

Answer 27

Inhibition because they’re a sign of high energy levels

Question 28

What allosteric effect does high [Glucose-6-phosphate] have on glycogen synthase activity?

Answer 28

Activation

Question 29

What do protein kinases do?

Answer 29

Add a phosphate group (phosphorylate)

Question 30

What do protein phosphatases do?

Answer 30

They remove phosphate groups (dephosphorylate)

Question 31

What does the cAMP cascade do to a glycogen phosphorylase molecule?

Answer 31

Causes phosphorylation of a hydroxyl group in a serine residue of glycogen phosphorylase, which promotes transition into the active state

Question 32

Why does the fact that a phosphorylated glycogen phosphorylase is less sensitive to allosteric inhibitors matter?

Answer 32

Because even if the ATP and glucose-6-phosphate levels are high, phosphorylase will remain activated

Question 33

What does phosphorylation of glycogen synthase do to the enzyme?

Answer 33

Converts it to the ‘b’ less active form

Question 34

What happens to glycogen synthesis when protein kinases are activated?

Answer 34

It’s inhibited because protein kinases phosphorylate

Question 35

What is the ‘a’ form of an enzyme?

Answer 35

The more active form

Question 36

What is the ‘b’ form of an enzyme?

Answer 36

The less active form