L4 - Fat As Fuel

Question 1

What is the function of lipids:

Answer 1

1) Important structural component of membranes
2) Needed for steroid hormone synthesis
3) It can also be used as a long-term store

Question 2

Why is TAG an efficient long-term fuel storage molecule:

Answer 2

1) Has compact storage - can be stored as large fat droplets in adipose tissue

2) Can have large body stores of TAG:
In a 70 kg man, 11kg is due to fat stores, 10g is due to glucose stores and 150g is due to glycogen.

3) When metabolised, it produces a lot of energy (due to it being more reduced).
Fats - 38kJ/mol
Proteins - 21kJ/mol
Sugars - 17kJ/mol

Question 3

Structure of TAG:

Type of FAs:

The essential FAs:

Answer 3

• Has glycerol backbone
• Forms three ester bonds with FA chains

Fatty acids can either be saturated or unsaturated (cis or trans).
Omega-3 and omega-6 are the essential FAs.

Question 4

Breakdown of triglyceride in adipose tissue:

Answer 4

Lipase is hormone sensitive and activated by adrenaline and glucagon.
Adrenaline for fight-or-flight response.
Glucagon for when low blood glucose.

Triacylglycerol –> Diacylglycerol + FA

Diacylglycerol –> Monoacylglycerol + FA

Monoacylglycerol –> Glycerol + FA

Free fatty acids (FA) travel in plasma bound to albumin and is used as fuel by muscles and liver.

Glycerol diffuses in bloodstream to all tissues.

Question 5

Metabolism of Glycerol:

Answer 5

Glycerol is water soluble and can be used by all tissues. In most tissues, it enters glycolytic pathway and is converted to pyruvate. This can then enter TCA cycle where it is completely oxidised to CO2.

In liver and in starvation, it enters glycolytic pathway and is converted back to glucose via gluconeogenesis. This can be supplied to muscle and brain which require this.

Question 6

Overview of β-Oxidation:

Answer 6

Occurs in mitochondrial matrix.

Intermediates present as CoA thioesters.

Energy of FA conserved as 2H given to NAD+ and FAD forming NADH and FADH2.

Series of 4 enzyme catalysed reactions - remove 2 carbon unit as Acetyl CoA. This enters TCA cycle to produce energy.

Question 7

Activation of long chain FA:

Answer 7

Activation of FA is needed for β-oxidation and allows FAs to enter mitochondria.

Fatty Acid + ATP + CoA –> Fatty Acyl CoA + AMP + PPi
FATTY ACYL-CoA SYNTHETASE (in cytosol)

ATP converted to AMP + PPi (pyrophosphate)
CoA forms a thioester linkage with the carboxyl group of FAs.

Question 8

Transport of FA into mitochondria via the carnitine shuttle system:

Answer 8

1. In cytosol, Fatty acyl-CoA removes CoA and combines with carnitine to form a fatty acyl carnitine complex. Uses CARNITINE PALMITOYLTRANSFERASE I.

Carnitine + fatty acyl-CoA –> CoA + fatty acyl-carnitine
USES CARNITINE PALMITOYLTRANSFERASE I

2. FA carnitine complex transferred across OMM and IMM by a translocase.
3. In mitochondrial matrix, fatty acyl carnitine complex dissociates. Carnitine released in matrix and CoA transferred to fatty acid:

Fatty acyl-carnitine + CoA –> carnitine + Fatty acyl-CoA
USES CARNITINE PALMITOYLTRANSFERASE II

4. Carnitine returns to cytosol by the translocase.

This process is called carnitine shuttle system as it allows transfer of FA into mitochondria.

Question 9

β-Oxidation: THE PATHWAY

Answer 9

Reaction 1: Removal of 2H atoms (OXIDATION)
Fatty acyl-CoA –> enoyl-CoA
ACYL-CoA DEHYDROGENASE
FAD converted to FADH2.

Reaction 2: Addition of water (HYDRATION)
Enoyl-CoA –> hydroxyacyl-CoA
ENOYL-CoA HYDRATASE
Add water

Reaction 3: Remove 2H atoms (OXIDATION)
Hydroxyacyl-CoA –> β-ketoacyl-CoA
HYDROXYACYL-CoA DEHYDROGENASE
NAD+ –> NADH + H+

Reaction 4: Removal of 2C units (SPLITTING)
β-ketoacyl-CoA –> fatty acyl-CoA(2C shorter) + Acetyl-CoA
β-KETOACYL-CoA THIOLASE

Acetyl-CoA goes into TCA cycle and becomes CO2.
Fatty acyl-CoA is 2C atoms shorter so reenters reaction 1-4 until all of it has been converted to Acetyl-CoA.

Question 10

Summary of β-oxidation pathway:

Answer 10

Fatty acid with 16C atoms will pass through 7 repeats of β-oxidation pathway producing 7 NADH and 7 FADH2.
Produces 8 acetyl CoA molecules which then enter TCA cycle.
NADH and FADH2 reoxidised by oxidative phosphorylation - produce 2.5 and 1.5 molecules of ATP, respectively.

Question 11

Energy Yield from β-oxidation:

Answer 11

1) Fatty acid with 16C atoms produces 8 acetyl CoA:
Total amount of ATP molecules produced through TCA and OXPHOS is:
(2.5 x 3) + (1.5 x 1) + 1 GTP = 10 ATP molecules from TCA cycle of 1 acetyl CoA
8 Acetyl CoA = 8 x 10 = 80 ATP

2) Also produces 7 NADH and 7 FADH2, so total amount of ATP produced from OXPHOS is:
(7 x 2.5) + (7 x 1.5) = 28 ATP

Total Yield: 80 + 28 = 108 - 2 (Due to activation of FAs required two high energy phosphate bonds - PPi) = 106 molecules of ATP.

Question 12

Metabolism of odd numbered fats:

Answer 12

If have an odd number of FAs (e.g. 15C), β-oxidation leads to formation of a 3C FA.

15C–> 13C–> 11C–> 9C–> 7C–> 5C–> 3C

          Propionyl-CoA 
                   to
    Methyl malonyl-CoA 
                   to 
          Succinyl-CoA 

3C FA can combine with HCO3 (bicarbonate) to form a 4C FA. This requires ATP hydrolysis (ATP –> ADP + Pi). This is then converted to succinyl-CoA which can then enter TCA cycle.

Formation of succinyl CoA requires an enzyme which uses cobalamin (Vit B12).

Question 13

Regulation of fat metabolism:

Answer 13

1. Release of FA from adipose tissue.
   Adrenaline and glucagon activate lipase enzyme.
2. Rate of entry into mitochondria via carnitine shuttle.
3. Rate of reoxidation of cofactors NADH and FADH2 by cytochrome chain. If there is not enough NAD+ and FAD, then β-oxidation won’t occur.

Question 14

Metabolic Profiles of Organs:

Answer 14

1. Glucose is the major fuel for brain (can use ketone bodies in starvation)
2. Glucose and FAs is the main fuel for muscles
3. Adipose tissue has TAG stores - reservoir for metabolic fuel. Adipose do glycolysis to convert glucose to glyceraldehyde-3-phosphate where this can be converted to glycerol and used for TAG synthesis.
4. Liver provides a fuel for brain, muscles and other tissues.