Glycogen Metabolism

Question 1

Define:

Starch

Answer 1

A polysaccharide made up of straight-chain and branched glucose polymers.

It comes in two key forms, amylose (straight-chain) and amylopectin (branched-chain).

Question 2

Define:

Glycogen

Answer 2

A stored form of of fuel made up of many branching molecules of glucose.

It is a type of polysaccharide stored in the liver and skeletal muscle.

Question 3

Define:

Glycogen metabolism

Answer 3

The breakdown or synthesis of stored glucose molecules in their polysaccharide form.

Breakdown is triggered during exercise or periods of fasting by glucagon levels rising.

Question 4

What kind of linkages are found in starch?

Answer 4

α-linkages

Question 5

Which kind of starch polymer is straight-chained?

Answer 5

Amylose

Question 6

What kind of linkages make up amylose?

Answer 6

α-1,4-glycosidic linkages

This gives its straight-chain structure.

Question 7

Which kind of starch polymer is branch-chained?

Answer 7

Amylopectin

Question 8

What kind of linkages does amylopectin contain?

Answer 8

Both α-1,4-glycosidic and α-1,6-glycosydic linkages.

The α-1,6-glycosidic linkages occur every ~24-30 glucose units, and give rise to its branched structure.

Question 9

What is the key difference between α-linkages and β-linkages for humans?

Answer 9

α-linkages are readily digested, whilst β-linkages are not.

Question 10

What is the main stored form of glucose/carbohydrates in plants?

Answer 10

Starch

For humans, most of our intake of glucose is in the form of starch from our food, such as potatoes.

Question 11

True or False:

Glycogen is similar to amylopectin of starch, but less branched.

Answer 11

False

Glycogen IS similar to amylopectin, but is MORE branched.

Question 12

Where is glycogen most concentrated?

Answer 12

In the liver.

(5-6% by mass)

It is also found in the muscles at up to around 1-2% by mass.

Question 13

What role does α-amylase serve in digestion?

Answer 13

It randomly cleaves α-1,4-linkages (of polysaccharides).

However, it cannot cleave a linkage within four units of each branch point.

Question 14

What is one major restriction of α-amylase’s cleavage function?

Answer 14

It CANNOT cleave linkages within four units of a branch point and leaves behind highly-branched ‘limit dextrins’.

In plants, ** β-amylase** is able to cut off 2-glucose units.

Question 15

Where is α-amylase found in animals?

Answer 15

The saliva and pancreatic juice.

Question 16

How are the ‘limit dextrins’ left behind by α-amylase processed afterwards?

Answer 16

In a two-step reaction carried out by a debranching enzyme.

Question 17

Describe:

The TWO reactions carried out by the debranching enzyme to further break down ‘limit dextrins’ left behind by α-amylase.

Answer 17

1. Transfer of a trisaccharide (taking branch and adding to end of chain).
2. Cleavage of the remaining α-1,6-linkage (via α-1,6-glucosidase activity).

After this, α-amylase is able to continue cleaving α-1,4-linkages along the straight chain.

Question 18

Which enzyme is vital for metabolism of tissue glycogen?

Answer 18

Glycogen phosphorylase

It cleaves and phosphorylates at the non-reducing end (i.e. phosphorolysis).

Question 19

What is the reaction catalysed by glycogen phosphorylase, and what is the end product formed?

Answer 19

Phosphorolysis of the non-reducing end of a glycogen residue to form α-D-glucose-1-phosphate.

This reaction has a negative ΔG due to the high intracellular concentration of phosphate.

Question 20

What happens to the α-D-glucose-1-phosphate resulting from the glycogen phosphorylase catalysed reaction after it is formed?

Answer 20

Phosphoglucomutase converts it into glucose-6-phosphate.

Which can be fed into the remaining steps of glycolysis.

Question 21

How are the two monosaccharides, galactose and mannose related to glucose?

Answer 21

They are epimers.

Thus, after a few extra steps and conversions, they too can be fed into the glycolytic pathway, as seen in the above diagram.

Question 22

Which enzyme reaction needs to be bypassed for glycogen synthesis to occur?

Answer 22

The reaction carried out by glucose phosphorylase, since it is spontaneous/exergonic.

This is done by activating glucose.

Question 23

Which enzyme is responsible for activating glucose to bypass glucose phosphorylase in glycogen synthesis?

Answer 23

UDP-glucose pyrophosphorylase

Question 24

What is the product of the glycogen synthetic reaction catalysed by UDP-glucose pyrophosphorylase?

Answer 24

An activated sugar nucleotide called UDP-glucose.

TWO phosphates are cleaved off UTP during this reaction.

Question 25

Describe:

Glycogenin, and its association with UDP-glucose.

Answer 25

Glycogenin is a protein dimer whose tyrosine sidechain covalently binds with the first glucose molecule from UDP-glucose.

Question 26

After the first glucose is bound to glucogenin, how are remaining UDP-glucose molecules added to form glycogen?

Answer 26

Glycogen synthase transfers glucose from UDP-glucose to the C₄ hydroxyl at the non-reducing end of the forming glycogen chain.

This forms α-1,4-linkages.

Question 27

What is the role of the branching enzyme in glycogen synthesis?

Answer 27

It introduces α-1,6-linkages and thus branches to the growing glycogen polysaccharide.

Once it finds a chain of >11 units, it cleaves 6-7 off and transfers it to a C₆ hydroxyl of the same/a different chain.

Question 28

Carbohydrate metabolism is highly regulated by…

Answer 28

…hormones.

Question 29

When is insulin secreted?

Answer 29

When blood-glucose levels are high.

Question 30

What does insulin stimulate in regards to hormonal control of glycogen metabolism?

Answer 30

Glycogen synthesis

This is in order to lower blood-glucose levels.

Question 31

Where is insulin synthesised?

Answer 31

In β cells of the pancreas.

it is then secreted into portal veins.

Question 32

What transporter is responsible for glucose uptake into muscle, liver, and adipose cells?

Answer 32

GLUT4

Question 33

How is glycogen synthase activated?

Answer 33

• Inactivation of glycogen synthase kinase 3 (GSK3).
• Allosteric activation via glucose-6-phosphate.

Insulin triggers a cascade that leads to the inactivation of GSK3.

Question 34

Which TWO hormones have the opposite effects to insulin in regards to carbohydrate metabolism?

Answer 34

Glucagon & adrenaline.

(adrenaline is also known as epiniephrine).

Question 35

What do glucagon and adrenaline stimulate in regards to hormonal control of glycogen metabolism?

Answer 35

Glucose metabolism / glycogen breakdown.

(i.e. glycolysis).

They simultaneously ‘switch-off’ gluconeogenesis.

Question 36

Where does glucagon come from, and what kind of molecule is it?

Answer 36

It is a peptide hormone secreted from the pancreas.

Question 37

Where does adrenaline (epinephrine) come from, and what kind of molecule is it?

Answer 37

It is a catecholamine derived from tyrosine, and released from the adrenal glands.

Question 38

What cell types does glucagon act on?

Answer 38

Liver & adipose.

Question 39

What cell types does adrenaline (epinephrine) act on?

Answer 39

Muscle & liver.

Question 40

What is the second messenger that both glucagon and adrenaline (epinephrine) act via?

Answer 40

cAMP

(cyclic AMP).