Fat as Fuel

Question 1

Fatty acids are long chain hydrocarbons, what does that mean?

Answer 1

It means they are long carbon chains with hydrogen atoms attached.

Question 2

What are fatty acids called when they are bound to a glycerol molecule?

Answer 2

Triglycerides

Question 3

What are the difference between saturated and unsaturated fats/fatty acids?

Answer 3

Unsaturated = have some double bonds between the carbons

Question 4

What is the difference between cis and trans unsaturated fats?

Answer 4

Cis unsaturated = the hydrogens connected to the double bond are both on the same side

Trans = the hydrogens connected to the double bond are on opposite sides

Question 5

Is a fully saturated fatty acid oxidised or reduced?

Answer 5

It is fully reduced?

Question 6

Are saturated or unsaturated fatty acids better at producing larger amounts of energy?

Answer 6

Saturated, as they are completely reduced therefore have greater capacity to be oxidised and reduce coenzymes in the process.

Question 7

What is special about membrane lipids such as phospholipids and cholesterol and our diets?

Answer 7

These membrane molecules are reliant on our diet alone. We do NOT have a genetic code to synthesise these biomolecules.

Question 8

Where are triglycerides that are long term fuels found?

Answer 8

They are stored in adipocytes as fat droplets and released into the blood by hormones to act as a fuel source between meals.

Question 9

What fraction of a 70kg man is triglyceride stores?

How much is glycogen stores?

Answer 9

Triglycerides = 11kg = roughly 1/7

Glycogen = 150 grams!

Question 10

Explain the bonding of a triglyceride mentioning which functional groups are involved from each molecule.

Answer 10

The alcohol group (-OH) of the glycerol molecule bonds with the carboxyl (COOH) to form an Ester bond.

Question 11

Which fatty acid has the structure C16:0?

Answer 11

Palmitic acid.

Question 12

Which fatty acid has the structure C18:0?

Answer 12

Stearic acid.

Question 13

Which fatty acid has the structure C18:1?

Answer 13

Oleic acid.

Question 14

Which fatty acid has the structure C18:2?

Answer 14

Linoleic acid.

Question 15

Which fatty acid has the structure C18:3?

Answer 15

Linolenic acid.

Question 16

Which enzyme class breaks down triglycerides?

How are these named depending on how many fatty acid chains are attached the glycerol molecule?

Where does this process occur?

Answer 16

Lipases.

They are named “tri/di/mono - acylglyceride lipase” depending on how many fatty acid chains are attached to the glycerol molecule.

This happens in the cytosol of adipose tissue, where the fatty acids are stored as fat droplets.

Question 17

What happens to the free fatty acids that have been broken down from the triacyglycerides?

Answer 17

They travel in the blood bound to albumin,

Question 18

What happens to the glycerol molecule once all the fatty acids have been broken away from it from where it was a triacyglyceride?

Answer 18

It travels in the blood to all tissues for further breakdown via glycolysis, as it is converted to dihydroxyacetone and then converted from this to glyceraldehyde-3-phosphate by triose phosphate isomerase.

In the liver in cases of starvation it enters the glycolysis pathway as glyceraldehyde-3-phosphate, but is then converted to glucose by gluconeogenesis as it can generate more energy this way.

Question 19

Why is the term ‘free fatty acid’ ambiguous?

Answer 19

They are ‘free’ in terms of not being bound to glycerol as a triglyceride, but they travel bound to albumin therefore are not completely ‘free’.

Question 20

Where does the beta-oxidation pathway occur?

Answer 20

In the mitochondrial matrix.

Question 21

How many enzymatic reactions are in the beta-oxidation pathway?

How many times does a fatty acid go through these reactions?

Answer 21

X4 reactions.

It depends on the number of carbon atoms in the fatty acid chain.

Question 22

How many ATP are synthesised in the beta-oxidation pathway?

Answer 22

None - all the energy produced is in the form of reduced coenzymes.

Question 23

What is the outcome of X1 cycle of the 4 enzyme reactions?

Answer 23

X1 FADH2
X1 NADH
X1 2-carbon unit removed as acetyl CoA

Question 24

What has to happen to fatty acids in the cell cytosol after they have become unbound to albumin and entered the cell which requires energy but before they enter the mitochondrial matrix to proceed with the beta-oxidation pathway? Where can this molecule now travel to?

How does this process occur?

What enzymes are required for this?

What other molecules are required?

Answer 24

They must be activated and turned into fatty acyl-CoA’s, which can now freely pass into the space between the outer and inner mitochondrial membrane.

This means a CoA molecule is also required as this is what joins with the fatty acid to form a fatty acyl-CoA.

The enzyme responsible for this = acyl CoA synthase.

This reaction also requires energy in the form of ATP —> AMP + PPi (pyrophosphate).

Question 25

What type of bond is present in the formation of a fatty acyl-CoA?

Answer 25

A thioester bond.

Question 26

What system do fatty acyl-CoA molecules utilise to enter the mitochondrial matrix?

Answer 26

The mitochondrial-carnitine system or the cart It Is-shuttle system.

Question 27

How does the carnitine-shuttle system allow the fatty acylCoA to pass into the mitochondrial matrix?

Which enzymes are involved in this process?

Answer 27

The enzyme carnitine palmitoyltransferase 1 (which is located in the OUTER mitochondrial membrane) replaces the CoA part of the fatty acylCoA with carnitine, which acts as a transport molecule.

The newly formed long chain acylcarnitine can now enter the mitochondrial matrix via translocase proteins, where the carnitine palmitoyltransferase 2 (located on the INNER mitochondrial membrane) reverses the process and frees up the carnitine molecule for reuse. It replaces the carnitine with CoA forming a fatty acylCoA once again. This molecule can now enter the beta-oxidation pathway.

Question 28

What is the first reaction in the B-oxidation pathway?

What are the before and after molecules?

What enzyme catalyses this reaction?

What other molecules are involved in this process?

Answer 28

Fatty acylCoA —> enoyl CoA

Enzyme = acylCoA dehydrogenase

The fatty acylCoA is oxidised whilst FAD2+ is reduced to FADH2.

This forms a double bond, with the following X2 reactions forming a second double bond around which the molecule will be broken into an acetyl CoA and a remaining (2-carbon shorter) fatty acid.

Question 29

What is the second reaction in the B-oxidation pathway?

What are the before and after molecules?

What enzyme catalyses this reaction?

What other molecules are involved in this process?

Answer 29

Enoyl CoA —> 3-L-hydroxyacyl CoA

Enzyme = enoyl CoA hydratase (as water is being added across the double bond formed in the previous reaction)

Question 30

What is the third reaction in the B-oxidation pathway?

What are the before and after molecules?

What enzyme catalyses this reaction?

What other molecules are involved in this process?

Answer 30

3-L-hydroxyacyl CoA —> b-ketoacyl CoA

Enzyme = 3-L-hydroxyacyl CoA dehydrogenase.

3-L-hydroxyacyl CoA is being oxidised and NAD+ is reduced to NADH. This step forms the second carbonyl group causing an instability in the molecule, ready for separation in reaction 4.

Question 31

What is the fourth and final reaction in the B-oxidation pathway?

What are the before and after molecules?

What enzyme catalyses this reaction?

What other molecules are involved in this process?

Answer 31

B-ketoacyl CoA —> (fatty acylCoA -2 carbon unit) + (acetyl CoA)

Enzyme = b-ketoacyl thiolase (uses the ‘thiol’ (SH) group of a new CoA molecule to cleave the bond.

Therefore this step requires a new CoA molecule to be inputted.

The acetyl CoA can now enter the TCA cycle and the remaining fatty acid chain can repeat the X4 reactions of the b-oxidation pathway.

Question 33

What is the method for calculating the number of FADH2, NADH & acetyl CoA molecules a fatty acid with a certain number of carbons in its chain will produce?

Use a 16 carbon chain fatty acid for an example.

Answer 33

Number of carbons in the chain / 2 = number of acetyl CoA molecules produced.

Number of acetyl CoA molecules produced -1
= number of FADH2 molecules for that fatty acid chain
= number of NADH molecules for that fatty acid chain
(This is because the last molecule left is already an acetyl CoA molecule, and does not need to go through the pathway to be cleaved).

Question 34

What is the fate of odd chain fatty acids?

Answer 34

Odd chain fatty acids leave behind a propionyl CoA molecule, which undergoes X3 reactions to form succinyl CoA which enters the TCA cycle and can take part in reaction 5 onwards (therefore can form X1 GTP, X1 FADH2 and X1 NADH).

Question 35

Name X2 hormones which activate the breakdown of triglycerides from adipose tissue, in order to free fatty acids for use as fuel.

Answer 35

1) adrenaline

2) glucagon

Question 36

Which other processes can affect the regulation of the b-oxidation pathway?

Answer 36

1) the rate of entry of fatty acids into the cells via the carnitine shuttle.
2) the rate of reoxidation of the cofactors NADH & FADH2 by the ETC.

Question 38

NB: Just a note about the b-oxidation pathway.

The molecules of the 4 stages =

1) acyl CoA
2) ‘HYDROXY’ acylCoA
3) ‘KETO’ acylCoA
4) (fatty acylCoA -2 carbon atoms) + (acetyl CoA)

Answer 38

The enzymes of the 4 stages = the name of the starting molecule followed by:

1) dehydrogenase
2) hydratase
3) dehydrogenase
4) thiolase