NM - Fat as Fuel

Question 1

As a chemistry recap, what are the best types of molecules to be used as fuels?

Answer 1

Molecules that are highly reduced can be used as fuels, so you can extract a lot of energy through oxidation.

Question 2

For each of the fatty acids below, indicate how many carbons are on them.

a. butyric acid
b. stearic acid
c. formic acid
d. propionic acid
e. arachidic acid
f. acetic acid
g. palmatic acid

Answer 2

a. butyric acid - C$
b. stearic acid - C18
c. formic acid - C1
d. propionic acid - C3
e. arachidic acid - C20
f. acetic acid - C2
g. palmatic acid - C16

in order for memorisation:
C1 (formic acid)
C2 (acetic acid)
C3 (propionic acid) 
C4 (butyric acid)
C16 (palmitic acid)
C18 (stearic acid)
C20 (arachidic acid)

Question 3

What are the two types of fatty acids, and what is the difference between the two?

Answer 3

A fatty acid can be saturated (with no double bonds in the carbon chain) or unsaturated (with one or more double bonds on the carbon chain).

Question 4

What are the two configurations of an unsaturated fatty acid?

Answer 4

There is an unsaturated cis configuration and an unsaturated trans configuration.

The cis FAs are those where the double bond is going in the same direction as the bonds adjacent, giving it a kink or a bend.

The trans FAs are those where there is a double bond without a bend.

Question 5

Give an example of a saturated FA, an unsaturated cis FA and an unsaturated trans FA.

Answer 5

saturated FA: stearic acid (C18)

unsaturated cis FA: oleic acid (C18)

unsaturated trans FA: elaidic acid (C18)

Question 6

What is the different in nomenclature for fatty acids with one double bond as opposed to those with more than one double bond?

Answer 6

Monounsaturated - only one double bond

Polyunsaturated – many double bonds

Question 7

What does saturation do the movement of a lipid?

Answer 7

If they are unsaturated, the fats are less rigid. Adding a kink to a fatty acid chain increases the mobility of the side chains.

Question 8

List some biological functions of lipids.

Answer 8

1. They are components of cell membranes (phospholipidsand cholesterol).
2. They are precursors to hormones.
   (cholesterol → steroid hormones)
   (arachidonic acid → prostaglandins)
3. They are long terms fuels (triglycerides).

Question 9

What are the short term, medium term, and long term fuels of the body?

(not the starving state)

Answer 9

Short term fuel – glucose in blood

Medium term fuel – glycogen in liver, etc.

Long term – fatty stores as triglycerides

Question 10

Why are triglycerides a very good fuel?

Answer 10

Fats yield almost double the energy as a carbohydrate (which makes sense as they are more reduced, thus they can be oxidised for more energy).

1g fat - 38 KJ
1g protein - 21 KJ
1g carbohydrate - 17 KJ

Question 11

Describe the basic breakdown of a stored triglyceride fat in adipose tissue.

Answer 11

The enzyme llipase is activated by adrenaline and glucagon.

It acts on the triacylglycerol, making it a diacylglycerol, then a monoacylglycerol, then finally a glycerol.

Each of these reactions produces a fatty acid, until we have three free. The free fatty acids travel in the plasma, bound to albumin. They act as fuels for the muscles, heart and liver.

The glycerol diffuses in the blood stream to all the tissues.

Question 12

Describe the metabolism of glycerol after it has been sourced from broken down TAGs.

Answer 12

Glycerol is water-soluble, and is taken up by all tissues.

In most tissues, glycerol enters the glycolysis pathway to be converted to pyruvate, and then on to the TCA cycle for oxidation to CO2.

In the liver, during starvation, glycerol enters the glycolysis pathway and is converted to glucose by gluconeogenesis,

Question 13

Where do the β-oxidation pathway take place?

Answer 13

All the reactions occur in the mitochondrial matrix.

Question 14

What do the intermediates of the β-oxidation pathway present as?

Answer 14

The intermediates are present as CoA thioesters.

Question 15

How is the energy of the FA conserved during β-oxidation?

Answer 15

The biological energy of the FA molecules is conserved as the transfer of 2 H atoms to the cofactors NAD+ and FAD to form NADH and FADH2 (there is no direct ATP syntehsis with the β-oxidation pathway)

Question 16

What is the repetitive aspect of the β-oxidation pathway?

Answer 16

A series of four enzyme reactions results in the removal of two carbon units as Acetyl CoA, and continues until there are no more 2Cs to be removed from the chain.

Question 17

What needs to happen to the FA before β-oxidation can take place, and why?

Answer 17

The FA needs to be activated. This is so it can participate in the system that allows it to cross the mitochondrial double membrane (it is a lipid, and can not cross easily).

Question 18

Describe long fatty acid chain activation.

Answer 18

We have to activate the enzyme, and spend some energy doing so.

The long chain fatty acid and the CoA are combined, with the help of the activating enzyme (in the cytosol), to result in a fatty acyl-CoA.

You need two units, so two phosphates are taken, converting ATP to AMP.

The Coenzyme A forms thioester bonds with carboxylic acids.

Question 19

What is the important of the mitochondrial carnitine system?

Answer 19

This is the shuttle system which helps bring FAs into the matrix of the mitochondria.

You start with the acyl CoA and you also end up with it at the end.

L-Carnitine and Acetyl-L-Carnitine are critically important for the transport of FAs.

Question 20

What are the steps to β-oxidation?

Answer 20

1) Dehydrogenation: removal of 2 H atoms.
2) Hydration: addition of water.
3) Dehydration: removal of 2 H atoms.
4) Acylation: removal of 2 C units.

The ‘cycle’ is then repeated.

Question 21

Describe the first dehydration step of β-oxidation.

Answer 21

The Fatty acyl-CoA is coverted to an Enoyl-CoA, catalysed by acyl-CoA dehydrogenase, and FAD is converted to FADH2.

The double bond is formed at the beta carbon, which is why it is called beta-oxidation.

Question 22

Describe the hydration step of β-oxidation.

Answer 22

The Enoyl-CoA is converted to 3-L-Hydroxyacyl-CoA, catalysed by enoyl-CoA hydratase. A water molecule is added to it.

This is not reduction or oxidation, as both the OH- and H+ in H2O are equivalent.

Question 23

Describe the second dehydration step of β-oxidation.

Answer 23

The 3-L-Hydroxyacyl-CoA is converted to β-Ketoacyl-CoA, catalysed by 3-L-hydroxyacyl-CoA dehydrogenase. NAD+ becomes NADH and H+ in the reaction.

NADH is normally involved when a keta and acyl group are being converted to one another.

Question 24

Describe the acylation step of β-oxidation.

Answer 24

The β-Ketoacyl-CoA is converted to A fatty acyl-CoA (2C shorter) and an Acetyl CoA molecule.

The Acetyl CoA goes on to the TCA cycle, and the fatty acyl CoA goes back into β-oxidation until it is fully oxidised.

Question 25

LIst the NADH and FADH2 (and Acetyl CoA yields) of a 16C atom FA going through complete β-oxidation.

Answer 25

A FA with 16 C atoms will pass through 7 repeats of the β-oxidation pathway, producing 7 NADH and 7 FADH2.

It will give rise to 8 Acetyl CoA, which will enter the TCA cycle.

Question 26

What is the overall energy yield from the complete β-oxidation of a 16 C FA?

Answer 26

From the 16 C FA, we get 8 Acetyl CoAs, so:
8 x 10 ATP = 80 ATP

We also get 7 NADH and 7 FADH2, so:
(7x2.5) + (7x1.5) = 28

80 + 28 = 108

108 -2 = 106

(we took away 2 for the 2 phosphates we used for activating the FA in the beginning)

Question 27

What happens to odd-chain fatty acids in β-oxidation?

Answer 27

If it is an odd-numbered chain, the last cycle will produce propionyl CoA (3C) instead of Acetyl CoA (2C).

The propionyl CoA is activated by hydrogen carbonate. This is an enzyme that required cobalamin (Vitamin B12).

The propionyl CoA is elongated slightly, adding onto it an extra carbon, and converted (through 2 reactions) to Succinyl CoA, and then shunted onto the TCA cycle.

Question 28

How is fat metabolism regulated?

Answer 28

• the release of FAs from adipose tissue is regulated by hormones (adrenaline and glucagon activate the lipase enzyme)
• the rate of entry into the mitochondria is regulated via the carnitine shuttle
• the rate of reoxidation of cofactors NADH and FADH2 by the cytochrome/ respiratory chain is also regulated

Question 29

What is the metabolic profile of organs (brain, muscle, and role of liver)?

Answer 29

Glucose is almost exclusively the fuel for the human brain (along with some ketone bodies).

Glucose and fatty acids (ketone bodies) are the major fuel for muscle. Like the brain, the muscle lacks glucose 6-phosphatase, so it cannot breakdown glycogen.

Triacylglycerols are stored in adipose tissue, and are a reservoir for metabolic fuel.

The liver provides fuel for brain, muscle and other organs.

Question 30

(from tutorial)

Describe, in detail, the biochemical role of carnitine in the β-oxidation pathway.

Answer 30

Carnitine is necessary for the transport of long chain fatty acids from the cytosol into the matrix of
mitochondria, as follows:

carnitine + long chain acyl CoA → carnitine-acyl complex

catalysed by carnitine- palmitoyl transferase I
(on the cytosolic side of the mitochondrial membrane)

The acyl-carnitine complex passes through the membrane via a specific transport protein.

On the matrix side of the membrane, another form of the enzyme carnitine- palmitoyl transferase II catalyses the transfer of the acyl group from carnitine to a molecule of CoA from the mitochondrial pool of cofactor.

Question 31

(from tutorial)

What ate the natural sources of the carnitine that is present in tissues such as muscle and liver?

Answer 31

Carnitine (4-trimethylamino-3-hydroxybutyrate) can be synthesised in the liver by enzyme-catalysed
methylation of the amino acid lysine, or it can be absorbed from the diet (from both animal and plant sources).

Question 32

(from tutorial)

What would be the biochemical and physiological consequences of reduced carnitine concentrations in tissues such as the liver, heart and skeletal muscle?

Answer 32

Reduced carnitine concentrationss would result in a slow rate of entry of long chain fatty acids into the mitochondrial matrix of these tissues, and subsequently a very slow rate of fat metabolism and reduced ATP production.

Cellular processes which are energy dependent will therefore be impaired.

Question 33

(from tutorial)

A patient has reduced tissue concentrations of carnitine.

Explain, in biochemical or physiological terms, why the concentrations of free fatty acids increased in the patient’s blood if there is a prolonged interval between their meals.

Answer 33

Between meals, glucagon is the predominant hormone involved in controlling the degradation of stored fuel molecules (triglyceride fat in adipose tissue and glycogen in the liver).

Glucagon activates the enzyme adipose
tissue lipase (hormone sensitive lipase), resulting in an increased rate of release of free fatty acids (and glycerol)
into the blood stream.

However, the metabolism of the fatty acids is reduced in muscle and liver in this patient,
and this will probably slow down the rate of diffusion of fatty acids into these cells.

High concentrations of free
fatty acids will therefore accumulate in the plasma.

The low blood glucose concentrations between meals, observed
in this patient, are presumably a result of disturbed glycogen metabolism or reduced gluconeogenesis
because of impaired liver function.

Question 34

(from tutorial)

Explain why the liver and muscle biopsy samples from a patient with reduced tissue carnitine showed more than the usual amount of triacylglycerols stored as fat droplets, when examined by histological techniques.

Answer 34

Some of the long chain fatty acids will enter liver and muscle cells from the plasma, moving down their
concentration gradient.

However, they can only be catabolised slowly in this patient since the carnitine
transport step is rate limiting.

Some of the free fatty acids may be esterified within these tissues as
triacylglycerols and stored in the cells as fat droplets.

Question 35

(from tutorial)

What is the explanation for the raised concentrations of the two intracellular enzymes (ALT and CK MM) in the patient (with reduced carnitine)’s blood sample?

Answer 35

Impaired metabolism in these tissues may result in low ATP production and low activity of Na+/K+ ATP
dependent pumps or other cell maintenance processes.

The cell membranes may become slightly leaky, and
intracellular enzymes leak into the plasma.

Question 36

(from tutorial)

The patient with reduced tissue carnitine was later shown to have a defect in the liver enzyme system for the synthesis of carnitine.

Which long-term treatment do you think would be effective in keeping them in good health?

Answer 36

They will require carnitine supplementation of their diet for the rest of their life.