1. Fatty acid breakdown The oxidation of fatty acids
proceeds in three stages

2. b-oxidation b-oxidation is catalyzed by four enzymes
Acyl-CoA dehydrogenase
Enoyl-CoA hydratase
b-hydroxyacyl-CoA dehydrogenase
Acyl-CoA acetyltransferase (thiolase)

3. First step Isozymes of first enzyme confers substrate specificity
FAD-dependent enzymes
Reaction analogous to succinate
dehydrogenase in citric acid
cycle

4. Electrons on FADH2 transferred to respiratory chain

5. Second step Adding water across a double bond
Analogous to fumarase reaction in
citric acid cycle

6. Third step Dehydrogenation (oxidation) using NAD
NADH is transferred to respiratory
chain for ATP generation
Analogous to malate dehydrogenase
reaction of citric acid cycle

7. Fourth step Splits off the carboxyl-end
Acetyl-CoA and replaces it with
Co-A – Thiolase

8. b-oxidation bottomline
The first three reactions generate a much less stable, more easily broken C-C bond subsequently producing
two carbon units
through thiolysis

9. The process gets repeated over and over until no more acetyl-CoA can be generated
16:0-CoA + CoA + FAD + NAD + H2O  14:0-CoA + acetyl-CoA + FADH2 + NADH + H+
Then..
14:0-CoA + CoA + FAD + NAD + H2O  12:0-CoA + acetyl-CoA + FADH2 + NADH + H+
Ultimately..
16:0-CoA + 7CoA + 7FAD + 7NAD + 8H2O  8acetyl-CoA + 7FADH2 + 7NADH + 7H+

10. Acetyl-CoA can be fed to the citric acid cycle resulting in reducing power

11. Breakdown of unsaturated fatty acids requires additional reactions
Bonds in unsaturated fatty acids are in the cis conformation, enoyl-CoA hydratase cannot work on as it requires a trans bond
The actions of an isomerase and a reductase convert the cis bond to trans, resulting in a substrate for b-oxidation

12. In some instances (monounsaturated), enoyl-CoA isomerase is sufficient

13. For others (polyunsaturated), both are needed

14. Complete oxidation of odd-number fatty acids requires three extra reactions
Although common fatty acids are even numbered, odd numbered fatty acids do occur (ie. propionate)
Oxidation of odd numbered fatty acids uses same pathway as even numbered
However, ultimate substrate in breakdown has five, not four carbons, which is cleaved to form acetyl-CoA and propionyl-CoA

15. Propionyl Co-A enters the citric acid cycle using three steps
Propionyl Co-A is carboxylated to form methyl-malonyl CoA (catalyzed by the biotin containing propionyl-CoA carboxylase)
Recall that methyl-malonyl CoA is also a intermediate in the catabolism of methionine, isoleucine, threonine and valine to succinyl-CoA

17. Methyl-malonyl-CoA undergoes two isomerization steps to form succinyl-CoA
Methyl-malonyl epimerase catalyzes the first reaction
Methyl-malonyl-CoA mutase (a vitamin B12 dependent enzyme) catalyzes the second to form succinyl-CoA

18. Vitamin B12 catalyzes intramolecular proton exchange

19. Vitamin B12 is a unique and important enzyme cofactor
Contains cobalt in a corrin ring system (analogous to heme in cytochrome)
has a 5’ deoxy adenosine (nucleoside component
Has a dimethylbenzimidazole ribonucleotide component

21. Attachment of upper ligand is second example of triphosphate liberation from ATP
Cobalamin 
Coenzyme B12
The other such reaction
where this is observed
is formation of Ado-Met

22. Proposed mechanism for methyl-malonyl CoA mutase
Same hydrogen
always accounted
for

23. Regulation of fatty acid oxidation
Fatty acids in the cytosol can either be used to form triacylglycerols or for b-oxidation
The rate of transfer of fatty-acyl CoA into the mitochondria (via carnitine) is the rate limiting step and the important point of regulation, once in the mitochondria fatty acids are committed to oxidation

24. Malonyl-CoA is a regulatory molecule
Malonyl-CoA (that we will talk about in more detail next week in lipid biosynthesis) inhibits carnitine acyltransferase I

26. Also…
When [NADH]/[NAD] ratio is high b-hydroxyacyl-CoA dehydrogenase is inhibited
Also, high concentrations of acetyl-CoA inhibit thiolase

27. Diversity in fatty acid oxidation
Can occur in
multiple cellular
compartments

28. b-oxidation in peroxisomes and glyoxysomes is to generate biosynthetic precursors, not energy

29. Distinctions among isozymes

30. Fatty acids can also undergo w oxidation in the ER
Omega oxidation occurs at the carbon most distal from the carboxyl group
This pathway involves an oxidase that uses molecular oxygen, and both an alcohol and aldehyde dehydrogenase to produce a molecule with a carboxyl group at each end
Net result is dicarboxylic acids

32. Omega oxidation is a minor pathway
Although omega oxidation is normally a minor pathway of fatty acid metabolism, failure of beta-oxidation to proceed normally can result in increased omega oxidation activity. A lack of carnitine prevents fatty acids from entering mitochondria can lead to an accumulation of fatty acids in the cell and increased omega oxidation activity

33. Alpha oxidation is another minor pathway

34. Ketone bodies are formed from acetyl CoA
Can result from fatty acid oxidation or amino acid oxidation (for a few that form acetyl-CoA)

35. Formation of ketone bodies

36. Ketone bodies can be exported for fuel

37. Then broken down to get energy (NADH)