Beta Oxidation of Fatty Acids

An Overview

Beta oxidation of fatty acids takes place in the mitochondrial matrix for the most part.  However, fatty acids have to be activated for degradation by coenzyme A by forming a fatty acyl-CoA thioester.  For short and medium length fatty acids, they undergo this reaction in the mitochondria.  The long chain fatty acids can't go through the membrane though, so this reaction occurs at the outer mitochondrial membrane and the product has to be carried by carnitine across the inner mitochondrial membrane.  They are made into acylcarnitine derivatives by carnitine transferase I on the outer side of the inner membrane.  These are then transported across the membrane by a translocaseand then they are passed to carnitine acyltransferase II on the matrix side which puts the fatty acyl group back on CoA leaving the original fatty acyl-CoA.
Along with this "activation" step, Beta oxidation of saturated fatty acids consists of a recurring cycle of a series of four steps.

repetition of the cycle
odd numbers of carbons
unsaturated fatty acids
regulation of beta oxidation

What are the inputs of this pathway?

The molecules that start this cycle (the inputs) are the saturated fatty acid and coenzyme A products (fatty acyl-CoA). The fatty acids involved can be even numbered carbon chains with no double bonds.  (The ones with double bonds are unsaturated and will be discussed later.)  Some other inputs that are added after the cycle has started are FAD, water, ATP, and NAD+.

What are the outputs of this pathway?

The products of this pathway (the outputs) include FADH2, NADH, acetyl-CoA, and of course, the final products.  The final fatty acid products are acetyl-CoA for the even numbered fatty acids (without double bonds), and for those with an odd number of carbons, it is 3-carbon propionyl-CoA instead.

Reactions of this pathway.

"Activation Step"  (Coenzyme A Activates Fatty Acids for Degradation)

Transport into Mitochondria (important control point between synthesis and degradation)

The First Reaction of Beta Oxidation (Acyl-CoA Dehydrogenase)
Detailed mechanism -- Click here

This first reaction is the oxidation of the Ca-Cb bond.  It is catalyzed by acyl-CoA dehydrogenases.  This catalyst is a family of three soluble matrix enzymes.  These enzymes carry noncovalently bound FAD that is reduced during the oxidation of the fatty acid.  This is an oxidation reaction and it should be similar to that of the succinate dehydrogenase reaction of the TCA cycle because the first three steps of this pathway are directly analogous to the steps needed to get succinate to oxaloacetate.  The *G should therefore be approximately +0.4 kJ/mole.

The Second Reaction of Beta Oxidation (Enoyl-CoA Hydratase)

The second reaction in this pathway is one in which water is added across the new double bond to make hydroacyl-CoA.  The catalyst in this reaction is Enoyl-CoA hydratase.  This is also called a crotonase and it converts trans-enoyl-CoA to L-B-Hydroxyacyl-CoA.  This reactio would be classified as a hydration reaction because you are adding water.  The *G of this reaction should be similar to that of the Fumarase reaction in the TCA cycle, since the first three reactions are directly analogous to the steps to get succinate to oxaloacetate.  Therefore, it should be around -3.8 kJ/mole.

The Third Reaction in Beta Oxidation
(L-Hydroxyacyl-CoA Dehydrogenase)
Detailed mechanism-click here

The third reaction of this pathway is the oxidation of the hydroxyl group at the beta position which forms a beta-ketoacyl-CoA derivative.  This is the second oxidation step in this pathway and it is catalyzed by L-Hydroxyacyl-CoA Dehydrogenase.  This enzyme needs to have NAD+ as a coenzyme and the NADH produced represents metabolic energy because for every NADH produced, it drives the synthesis of 2.5 molecules of ATP in the electron transport pathway.  So, this reaction is classified as an oxidation reaction and its *G should be similar to that of the Malate Dehydrogenase reaction in the TCA cycle for the same reasons as the ones above.  Therefore, it should be approximately +29.7 kJ/mole.

The Fourth Reaction in Beta Oxidation (Thiolase)
Click for Mechanism of Thiolase

The fourth and final reaction of this pathway is the thiolase catalyzed reaction.  This reaction cleaves the beta-ketoacyl-CoA.  The products of this reaction are an acetyl-CoA and a fatty acid that has been shortened by two carbons.  So, this reaction is classified as a cleavage reaction and it is actually a reverse Claisen condensation which means that it should have about the same *G as the Isocitrate Dehydrogenase reaction in the TCA cycle.  It should be somewhere around -8.4 kJ/mole.

Repetition of the Beta Oxidation Cycle

The shortened fatty acyl-CoA that was the product of the last reaction now goes through another beta oxidation cycle.  This keeps happening until eventually you wind up with two molecules of acetyl-CoA in the final step.  This acetyl-CoA is then available to be further metabolized in the TCA cycle, or it can be used as a substrate in amino acid biosynthesis.  It cannot be used as a substrate for gluconeogenesis!

Beta Oxidation of Odd Carbon Fatty Acids

Fatty acids with an odd number of carbons are common in plants and marine organisms.  Therefore, humans and animals that include these things in their diets must metabolize them in the beta oxidation pathway.  Therefore, the end product, instead of acetyl-CoA, is propionyl-CoA which has three carbons.  This must then be changed to succinyl-CoA to enter the TCA cycle.

Beta Oxidation of Unsaturated Fatty Acids

Unsaturated fatty acids are catabolized by the beta oxidation pathway, but they require two additional enzymes to handle the cis-double bonds.  These fatty acids (with one cis-double bond) go through the beta oxidation cycle as many times as they can before coming to the double bond.  The Enoyl-CoA Isomerase makes the cis-double bond into a trans-double bond and moves it over one carbon.  This product can then continue through the beta oxidation pathway.
For polyunsaturated fatty acids (with more than one cis-double bond) it goes through the same thing, but it only goes through one more round of beta oxidation  because then you get to a fatty acid with a trans and a cis double bond.  For this we use 2,4-dienoyl-CoA reductase to produce a trans-3-enoyl product which is converted by an enoyl-CoA isomerase to a trans-2-enoyl-CoA which then goes normally through the pathway.  An example of this is on pg. 795 in the text book.

Regulation of Beta Oxidation

Malonyl-CoA can act to prevent fatty acyl-CoA derivatives from entering the mitochondria by inhibiting the carnitine acyltranferase that is responsible for this transport.  Thus, inhibiting the beta oxidation pathway.  When fatty acyl-CoA levels rise, beta oxidation is stimulated.  Increased citrate levels; however, inhibit beta oxidation.  Because this reflects an abundance of acetyl-CoA, it too inhibits beta oxidation.