1. Lipid Catabolism

2. Lesson Learning Outcome
Upon completion of this lecture, students should be able to:
understand lipid catabolism

3. Lipid catabolism
Fatty acids are the major source of energy in catabolism of lipids
Sterols (eg steroids) are not catabolized but are excreted out
Thus lipid molecules that contain fatty acids are the ones that can be catabolized for energy production
The release of fatty acids from the glycerol backbone is done by enzyme lipases.
The release of fatty acids from triacyglycerides (TAG) in adipose tissue is hormone-controlled

4. Liberation of fatty acids from TAG in adipose tissue is hormone-dependant.

5. Fatty Acids and Energy
Fatty acids in triacylglycerols are the principal storage form of energy for most organisms
their carbon chains are in a highly reduced form
the energy yield per gram of fatty acid oxidized is greater than that per gram of carbohydrate oxidized

6. Fig. 21-1, p. 592

7. 1. Activation of fatty acid
A thioester bond is formed between the carboxyl group of the fatty acid and the thiol group of the CoA  acyl-CoA
Enzyme responsible is acyl-CoA synthase, reaction require ATP
Acyl-CoA can cross the outer mitochondrial membrane but not the inner membrane

8. 1. Activation of fatty acid

9. Transport of Acyl-CoA
In the inter membrane space, acyl group is transferred to carnithine by transesterification by carnitine acyltransferase (located in the inner membrane)
Acyl carnitine can then cross the inner membrane via specific transporter called carnitine translocase

11. Β-oxidation
In the matrix, repeated sequence of reactions cleaves two-carbon units from the fatty acids starting from the carboxyl end  process called β-oxidation
The whole cycle requires four reactions.
Each cycle produces single molecules of FADH2, NADH, and acetyl-CoA
and yields a fatty acid shortened by two carbons.
This series of reactions is then repeated on the shortened fatty acyl chain and continues until the entire fatty acid chain is degraded to acetyl-CoA

13. Reaction 1: oxidation of the , carbon-carbon single bond to a carbon-carbon double bond

14. Reaction 2: hydration of the carbon-carbon double bond

15. Reaction 3: oxidation of the -hydroxyl group to a carbonyl group

16. Reaction 4: cleavage of the carbon chain by a reverse Claisen reaction

17. the overall equation for oxidation of stearic acid can be obtained by adding the equations for b-oxidation, the citric acid cycle, and oxidative phosphorylation

18. Odd-Numbered Fatty Acids
The last -oxidation cycle of a fatty acid with an odd number of carbons gives propionyl-CoA

19. Ketone Bodies
Ketone bodies: acetone, -hydroxybutyrate, and acetoacetate
formed principally in liver mitochondria
can be used as a fuel in most tissues and organs
Formation occurs when the amount of acetyl-CoA produced is excessive compared to the amount of oxaloacetate available to react with it
intake high in lipids and low in carbohydrates
diabetes not suitably controlled
starvation

20. Ketone Bodies

21. End of lecture