Lipid Metabolism

Question 1

How does Polysaccharide storage form ?

Answer 1

Forms of glucose in the body

Question 2

Where is this predominately stored in ?

Answer 2

Liver and muscle as an energy reserve

Question 3

What is the difference between liver glycogen and muscle glycogen ?

Answer 3

Liver glycogen is utilised to maintain plasma glucose levels between meals, whereas muscle glycogen is required to sustain muscle contraction

Question 4

Glucose-1-Phosphate and Glucose-6-Phosphate are interconvertible by ?

Answer 4

Phosphoglucomutase

Question 5

What is the pathway of Glycogen synthesis ?

Answer 5

Glucose-1-Phosphate + UTP = Uridine Diphosphate Glucose (UDP-Glucose)

Question 6

What enzyme allows this formation of UDP-Glucose?

Answer 6

The enzyme is UDP-glucose pyrophosphorylase

Question 7

Explain the formation of Glycogen from UDP-Glucose ?

Answer 7

Glycogen synthase adds glucose units in α-1:4- linkage onto the non-reducing end of the glycogen chain

Question 8

What does synthesis involve ?

Answer 8

Addition of new glucose units (sometimes called residues) onto the end of an existing chain -sometimes called a primer

Question 9

Glycogen synthase can add glucose to only ?

Answer 9

A pre-existing chain of more than four glucosyl residues

Question 10

What is the priming function carried out by ?

Answer 10

A protein, glycogenin

Question 11

Where is Glycogenin found ?

Answer 11

Free in the cell but also remains at the core of the glycogen granule it has initiated

Question 12

What is Glycogenin ?

Answer 12

It is a protein dimer composed of two identical subunits of Mr 37,000. An oligosaccharide of four glucosyl units is built up on each molecule. Glycogen synthase can then elongate the chains. A separate branching enzyme introduces the branches

Question 13

What does UDP donate ?

Answer 13

The first glucosyl residue to glycogenin. Carbon 1 off the glucose forms a covalent link with the hydroxyl group of a specific tyrosine residue in the glycogenin protein sequence

Question 14

How is Glycogen synthesis controlled by ?

Answer 14

Insulin

Question 15

What can Glycogen synthase only catalyse ?

Answer 15

The formation of α-1,4 glycosidic linkages. Another enzyme is required to generate the α-1,6 linkages needed to form a branch

Question 16

The branching enzyme transfers ?

Answer 16

A block of 7 residues (including the non-reducing end) to a more interior site, creating a new α-1,6 glycosidic linkage

Question 17

What must the branch not be ?

Answer 17

Within 4 residues of a pre-existing branch

Question 18

Why is Glycogen a good energy store ?

Answer 18

Because it can be mobilised very rapidly:

• The enzymes phosphorylase and glycogen synthase are very sensitive to regulation by hormones, stress and muscle contraction;
• The branched structure helps it to be mobilised rapidly.

Question 19

But why is Glycogen also a bad energy store because ?

Answer 19

Because glucose is hydrophilic. Therefore water associates with glycogen granules, increasing the overall weight and bulk

Question 20

Glycogen Phosphorylase in muscle is subject to ?

Answer 20

Allosteric regulation by AMP, ATP, and glucose-6-phosphate

Question 21

What does AMP activate?

Answer 21

AMP (present significantly when ATP is depleted due to contraction) activates phosphorylase

Question 22

What does ATP & glucose-6-phosphate inhibit ? and how ?

Answer 22

ATP & glucose-6-phosphate, which both have binding sites that overlap that of AMP, inhibit phosphorylase. They are signals that energy status if OK

Question 23

So glycogen breakdown is inhibited when ?

Answer 23

ATP and glucose-6-phosphate are plentiful

Question 24

Glycogen synthase is allosterically activated by?

Answer 24

Glucose-6-phosphate (opposite to the effect on phosphorylase)

Question 25

So when is glycogen synthesis activated ?

Answer 25

When glucose-6-phosphate is plentiful

Question 26

Glycogen mobilisation is greatly accelerated:

Answer 26

• In liver during starvation, when Glucose is required for Glycolysis by the brain and blood cells
• In muscle to fuel glycolysis during vigorous exercise
• Glycogen synthesis is activated to replenish liver glycogen stores after feeding or muscle stores after feeding or muscle stores when exercise ceases. It is promoted by insulin
• The pathway for glycogen synthesis is NOT a simple reversal of breakdown and requires energy input

Question 27

Explain the structure of Fatty acids and its role ?

Answer 27

• Only the carboxyl group is polar. The hydrocarbon chains are hydrophobic
• This is important for their role as an energy store, as it means that their mass is not increased by the association of water
• They are also very reduced, giving high potential for energy yield when they are oxidised
• So, for this reason fats are a more efficient energy store for the body than glycogen