Fatty acid breakdown

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)