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

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

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

Electrons on FADH2 transferred to respiratory chain

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

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

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

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

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+

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

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

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

For others (polyunsaturated), both are needed

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

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

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

Vitamin B12 catalyzes intramolecular proton exchange

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

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

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

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

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

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

Diversity in fatty acid oxidation Can occur in multiple cellular compartments

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

Distinctions among isozymes

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

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

Alpha oxidation is another minor pathway

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

Formation of ketone bodies

Ketone bodies can be exported for fuel

Then broken down to get energy (NADH)