L17 - β-Oxidation Pathway

Question 1

Efficiency of triglycerides for long-term fuel storage:

Answer 1

1. Compact storage - stored as fat droplets in fat cells of adipose tissues.
2. Large body stores - can store large amount as it is compact.
3. Efficiency on weight basis - Has lots of energy.
   1g fat has 38kJ
   1g protein has 21kJ (emergency supply)
   1g carbohydrate has 17kJ

Question 2

Breakdown of triglyceride in adipose tissue:

Answer 2

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
TRIACYLGLYCEROL (TAG) LIPASE

Diacylglycerol –> Monoacylglycerol + FA
DAG LIPASE

Monoacylglycerol –> Glycerol + FA
MAG LIPASE

Free fatty acids (FA) travel in plasma bound to albumin as FA hydrophobic so needs to be carried in a protein.

Glycerol diffuses in bloodstream to all tissues.

Question 3

β-Oxidation summary:

Answer 3

Occurs in mitochondrial matrix.

Intermediates present as CoA thioesters.

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

4 enzyme reactions - remove 2 carbon unit as Acetyl CoA.

Question 4

Activation of long chain FA:

Answer 4

β-Oxidation as it is split between Cα and Cβ.

Links FA to CoA.

Energetically favourable (no phosphorylation).

CH3-(CH2)n-CH2-CH2-C(=O)-O-H
To
CH3-(CH2)n-C(β)H2-C(α)H2-C(=O)-S-CoA
Fatty acyl -CoA

FATTY ACYL-CoA SYNTHETASE
ATP converted to AMP + PPi (pyrophosphate)
Occurs in cytosol.

Question 5

Transport of FA into mitochondria via the carnitine shuttle system:

Answer 5

1. Acyl CoA transferred to carnitine.
   Preserves high energy bond so conserves energy from thioester linkage.
2. Fatty acyl-CoA can cross OMM and enter IMS but can’t cross IMM.
3. Acyl transfer occurs from fatty acyl-CoA to carnitine:

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

4. FA carnitine complex transferred across IMM by a translocase.
5. Carnitine released in matrix and CoA transferred to fatty acid:

Fatty acyl-carnitine+CoA–>carnitine + FA-CoA
USES CARNITINE ACYLTRANSFERASE II

6. Carnitine returns to IMS by the translocase.

This process is called carnitine shuttle system as it allows transfer of FA into mitochondria.
High-energy nature of acyl bond is preserved during transport allowing to reform the fatty acyl-CoA inside mitochondria.

Question 6

β-Oxidation: THE PATHWAY

Answer 6

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

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

Reaction 3: Remove 2H atoms
Hydroxyacyl-CoA --> β-ketoacyl-CoA 
HYDROXYACYL-CoA DEHYDROGENASE
NAD+ --> NADH + H+
Forms Acetyl-CoA group which can be split off.

Reaction 4: Removal of 2C units
β-ketoacyl-CoA–>fatty acyl-CoA+ 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 7

Summary of β-oxidation pathway:

Answer 7

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.

Question 8

Regulation of fat metabolism:

Answer 8

1. Release of FA from adipose tissue.
   Adrenaline and glucagon activate lipase enzyme.
2. Rate of entry into mitochondria via carnitine shuttle.
   If a lot of carbohydrate, then you don’t want a lot of FA to be broken down. So enzyme used to inhibit entry of FA in mitochondria so more is stored as fat.
3. Rate of reoxidation of cofactors NADH and FADH2 by cytochrome chain.

Question 9

Metabolism of odd numbered fats:

Answer 9

β-oxidation will result in 1 odd no. Carbon on Acetyl CoA:

15C–> 13C–> 11C–> 9C–> 7C–> 5C–> 3C

1. CH3-CH2-C(=O)-S-CoA    
          Propionyl-CoA 
                   to
-OOC-CH(CH3)-C(=O)-S-CoA   
  Methyl malonyl-CoA 

Enzyme: PROPIONYL-CoA CARBOXYLASE
CO2 added which requires ATP to ADP and Pi.

2. -OOC-CH(CH3)-C(=O)-S-CoA   
            Methyl malonyl-CoA 
                       to 
-OOC-CH2-CH2-C(=O)-S-CoA   
             Succinyl-CoA 

Enzyme: METHYL MALONYL-CoA MUTASE
Succinyl-CoA added into TCA cycle.

Question 10

Ketone body formation (definition of ketogenesis and ketone bodies and what they do to excess Acetyl CoA):

Answer 10

Ketogenesis occurs when fat metabolism is the main source of energy in starvation and type 1 diabetes.

Excess acetyl CoA (that is needed by TCA) are converted into ketone bodies in liver.

Ketone bodies are produced during metabolism of fats e.g. Acetoacetate and β-hydroxybutyrate.
They can be utilised for energy by most tissues. They are released into the bloodstream. In most cell types, they can be converted back into TCA cycle intermediates (Acetyl CoA + Succinate).