Fatty acid breakdown

• The oxidation of fatty acids

proceeds in three stages

-oxidation

-oxidation is catalyzed by four enzymes– Acyl-CoA dehydrogenase– Enoyl-CoA hydratase -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 stepDehydrogenation (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

-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 -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 intramoelcular proton exchange

• Contains cobalt in a corrin ring system (analogous to heme in cytochrome)

• has a 5’ deoxy adenosine (nucleoside component

• Has a dimethylbenzimidazole ribonucleotide component

Vitamin B12 is a unique and important enzyme cofactor

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 -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 -hydroxyacyl-CoA dehydrogenase is inhibited

• Also, high concentrations of acetyl-CoA inhibit thiolase

Diversity in fatty acid oxidation

• Can occur in

multiple cellular

compartments

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

Distinctions among isozymes

Fatty acids can also undergo 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)