Lecture 5: Lipid metabolism - Breakdown

Question 1

Give three roles of lipids.

Answer 1

• main form of stored energy
• Major component of cell membranes
• hormones

Question 2

What is another name for a triglyceride?

Answer 2

Triacylgycerol

Question 3

What is a triglyceride?

Answer 3

The main storage form of fat made up of 3 fatty acids in ester linkage to one glycerol molecule.

Question 4

What is a fatty acid?

Answer 4

A long chain carboxylic acid

Question 5

With respect to fatty acids, what do the terms saturated, mono-unsaturated and polyunsaturated mean?

Answer 5

Saturated - no C=C double bonds
Mono-unsaturated - one C=C double bond
Polyunsaturated - more than one C=C double bond

Question 6

Explain the notation C18:0.

Answer 6

This means that the fatty acid being referenced has 18 carbon atoms and 0 C=C double bonds.

Question 7

Give the lengths of fatty acid chain which are classified as a) short, b) medium, c) long and d) very long. Indicate which are the commonest and rarest lengths.

Answer 7

a) Fewer than 8 carbons
b) 8 - 14 carbons
c) 15 - 22 carbons (commonest)
d) More than 22 carbons (rarest)

Question 8

Are fatty acids positively charged, negatively charged or neutral at physiological pH?

Answer 8

Negatively charged. (O=C-O-)

Question 9

Give two examples of saturated fatty acids and one example of a mono-unsaturated fatty acid.

Answer 9

Saturated: palmitate (protonated form of palmitic acid)(C16:0) and stearate (C18:0)
Mono-unsaturated: Oleate (C18:1)

Question 10

Where are triglycerides stored?

Answer 10

In adipose tissue (fat). Triglycerides form droplets and globules in cytoplasm of adipocytes (fat cells).

Question 11

Why do we store fats?

Answer 11

• Fats are a much better store of energy than carbohydrates (fats are much more reduced than carbohydrates and are very non-polar so can be stored in a very dense anhydrous form)
• Stored glycogen will only last 12-24 hours, whereas stored fat can last about 12 weeks (without eating anything)
• Fats are the main energy source of all tissues except the brain, red blood cells and rapidly contracting muscles.

Question 12

Give a general overview of the breakdown of lipids.

Answer 12

1) Mobilisation of free fatty acids from triglycerides in adipose tissue into blood
2) Free fatty acids transported in the blood to tissues by serum albumin (using its hydrophobic binding sites)
3) Free fatty acids activated by reaction with Coenzyme A
4) Transported into mitochondria (for long chain fatty acids this requires conjugation with a molecule called carnitine)
5) Beta-oxidation in matrix of mitochondria
6) Final oxidation of acetyl CoA in TCA cycle

Question 13

What stimulates the mobilisation of fatty acids from triglycerides?

Answer 13

Glucagon or adrenaline

Question 14

What happens when glucagon or adrenaline bind to their transmembrane receptor?

Answer 14

When glucagon/adrenaline binds to the transmembrane receptor, the attached G-protein exchanges its GDP for GTP, which moves along the membrane to activate adenylate cyclase. Adenylate cyclase converts ATP to cAMP, which activates cAMP-dependent protein kinase A. Protein kinase A then phosphorylates Triacylglycerol lipase, which hydrolyses the first fatty acid off the triacylglycerol, leaving a diacylglycerol. The diacylglycerol has its remaining two fatty acids removed by other lipases (through hydrolysis), releasing glycerol and free fatty acids.

Question 15

Give 2 molecules which Coenzyme A is derived from.

Answer 15

ATP and pantothenic acid

Question 16

What is pantothenic acid also known as?

Answer 16

Vitamin B5

Question 17

Describe the activation process by Coenzyme A.

Answer 17

Step 1) The carboxylate ion of the fatty acid attack ATP and displaces the outer two phosphates, producing AMP with the fatty acid attached.
Step 2) The lone pair of electrons on the SH group of CoASH nucleophilically attacks the AMP-fatty acid, releasing AMP and producing a fatty acyl-CoA (a fatty acid attached to Coenzyme A)
This activation process uses the equivalent of 2 ATP molecules and requires the enzyme Acyl-CoA synthetase.

Question 18

How are activated long chain fatty acids transported into the mitochondria?

Answer 18

The molecule carnitine (which is a zwitterion at physiological pH) displaces CoA in the fatty acid-CoA, releasing CoASH, via carnitine acyltransferase I.

Question 19

What inhibits the displacement of CoA by carnitine?

Answer 19

Malonyl-CoA

Question 20

Which membranes are carnitine acyltransferase I and carnitine acyltransferase II in?

Answer 20

Carnitine acyltransferase I is in the outer mitochondrial membrane and carnitine acyltransferase II is in the inner mitochondrial membrane.

Question 21

What are the functions of carnitine acyltransferase I and carnitine acyltransferase II?

Answer 21

Carnitine acyltransferase I replaces CoA with carnitine, so that the fatty acid can get into the matrix from the inter-membrane space (Acyl CoA crosses the outer mitochondrial membrane through a pore).
Acyl-carnitine is then transported across the inner mitochondrial membrane into the matrix by a carrier protein.
In the matrix carnitine acyltransferase II removes carnitine from the fatty acid and replaces it with CoA.

Note: available data cannot determine whether carnitine is added inside or outside the mitochondria.

Question 22

Describe the process of beta-oxidation.

Answer 22

The acyl CoA, which has now entered the matrix of the mitochondria, undergoes 4 reactions to release a molecule of acetyl CoA (2C). This cycle then repeats until the fatty acid is completely degraded.

The 4 reaction cycle is as follows:

1) Dehydrogenation: The acyl CoA is converted to an enoyl-CoA by Acyl-CoA dehydrogenase, with FAD being simultaneously being converted to FADH2.
2) Hydration: Enoyl-CoA is hydrolysed by 3-Hydroxyacyl-CoA by Enoyl-CoA hydrase (addition of water).
3) Dehydrogenation: 3-Hydroxyacyl-CoA is then converted to beta-Ketoacyl-CoA by 3-L-Hydroxyacyl-CoA dehydrogenase, with the simultaneous conversion of NAD+ to NADH + H+.
4) Thiolytic cleavage: Beta-ketoacyl-CoA is then converted to Acyl-CoA and acetyl CoA by thiolase, with the addition of CoA.

This acyl CoA then re-enters the start of the cycle, being converted to enoyl-CoA.

Question 23

What is significant about the enzyme Acyl-CoA dehydrogenase?

Answer 23

It has 3 isozymes (enzymes which catalyse the same reactions with slightly different substrate specificity (here it is chain length)).

Question 24

Which functional group is created in each step of the 4 reaction cycle of beta oxidation?

Answer 24

1) C=C double bond
2) Hydroxyl
3) Ketone
4) Thioester

Question 25

What is significant about odd-numbered fatty acids in animals and plants?

Answer 25

Animals and plants cannot synthesise odd-numbered fatty acids, but bacteria can, so they must be incorporated into the diet.
Odd-numbered fatty acids cannot be completely degraded to acetyl CoA (2C) by beta-oxidation, so the fragment remaining after the last acetyl CoA is released is Propionyl-CoA (3C), which is then degraded and enters the TCA cycle by another intermediate.

Question 26

How is propionyl-CoA converted to a TCA cycle intermediate after beta-oxidation of an odd-numbered fatty acid?

Answer 26

Propionyl-CoA is converted D-Methylmalonyl-CoA by the enzyme propionyl-CoA carboxylase, which is biotin dependent. This reaction uses up HCO3- (bicarbonate) and ATP and releases ADP + Pi.
Then D-Methylmalonyl-CoA is converted to L-Methylmalonyl-CoA by methylmalonyl-CoA epimerase in an isomerisation reaction.
Then L-Methylmalonyl-CoA is converted to Succinyl CoA (3C), a TCA intermediate, in another isomerisation reaction, by methyl-malonyl-CoA mutase and the coenzyme B12.

Question 27

Give the overall reactions for Beta-oxidation of Palmitate and compare the ATP yield to that of carbohydrate breakdown.

Answer 27

Parmitoyl CoA + 7 FAD + 7 NAD + 7 H2O + 7 CoA
—>
8 Acetyl CoA + 7 FADH2 + 7 NADH + 7 H+

Then, in the TCA cycle,
8 Acetyl CoA
—>
24 NADH + 8 FADH2 + 8 GTP

So OVERALL:
31 NADH = 2.5 x 31 ATP
15 FADH2 = 1.5 x 15 ATP
8 GTP = 8 ATP

In total = 108 - 2 (for activation of free palmitate with CoA) = 106 ATP

Glucose oxidation gives 30-32 ATP

So fat is much better at releasing energy than carbohydrate.

Question 28

How is fatty acid degradation controlled by hormones?

Answer 28

Free fatty acids are only released if glucose is low (glucagon) or if there is a potential need for a lot of energy (adrenaline). Inhibited by insulin.

When glucagon/adrenaline binds to the transmembrane receptor, the attached G-protein exchanges its GDP for GTP, which moves along the membrane to activate adenylate cyclase. Adenylate cyclase converts ATP to cAMP, which activates cAMP-dependent protein kinase A. Protein kinase A then phosphorylates Triacylglycerol lipase, which hydrolyses the first fatty acid off the triacylglycerol, leaving a diacylglycerol. The diacylglycerol has its remaining two fatty acids removed by other lipases (through hydrolysis), releasing glycerol and free fatty acids.

Question 29

How is fatty acid degradation controlled by allosteric inhibition?

Answer 29

Carnitine acyltransferase 1 is inhibited by malonyl CoA (the first intermediate in fatty acid synthesis), which indicates that acetyl CoA concentration is high, so there is no need to produce more.

Question 30

Where are longer fatty acids shortened so they can enter the mitochondria?

Answer 30

Peroxisomes

Question 31

Comment on the type of fatty acid which can be broken down by beta-oxidation?

Answer 31

Saturated fatty acids (no C=C double bonds). Unsaturated fatty acids (C=C double bonds) need extra enzymes to be oxidised.