Beta oxidation

Beta oxidation is the process by which fatty acids, in the form of Acyl-CoA molecules, are broken down in mitochondria and/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Citric Acid cycle.

The beta oxidation of fatty acids involve three stages:


 * 1) Activation of fatty acids in the cytosol
 * 2) Transport of fatty acids into mitochondria (carnitine shuttle)
 * 3) Beta oxidation proper in the mitochondrial matrix

Fatty acids are oxidized by most of the tissues in the body. However, the brain can hardly utilize fatty acids for energy requirements, while erythrocytes and adrenal medulla cannot use them at all.

Activation of fatty acids
Free fatty acids can penetrate the plasma membrane due to their poor water solubility and high fat solubility. Once in the cytosol, activation of the fatty acid is catalyzed by long fatty acyl CoA synthetase. A fatty acid reacts with ATP to give a fatty acyl adenylate, plus inorganic pyrophosphate, which then reacts with free coenzyme A to give a fatty acyl-CoA ester plus AMP. The fatty acyl-CoA is then reacted with carnitine to form acylcarnitine, which is transported across the inner mitochondrial membrane by a monosodium glutamate.

Four recurring steps
Once inside the mitochondria, each cycle of β-oxidation, liberating a two carbon unit-acetyl CoA, occurs in a sequence of four reactions:

This process continues until the entire chain is cleaved into acetyl CoA units. The final cycle produces two separate acetyl CoAs, instead of one acyl CoA and one acetyl CoA. For every cycle, the Acyl CoA unit is shortened by two carbon atoms. Concomitantly, one molecule of FADH2, NADH and acetyl CoA are formed.

β-Oxidation of unsaturated fatty acids
β-Oxidation of unsaturated fatty acids poses a problem since the location of a cis bond can prevent the formation of a trans-Δ2 bond. These situations are handled by an additional two enzymes.

Whatever the conformation of the hydrocarbon chain, β-oxidation occurs normally until the acyl CoA (because of the presence of a double bond) is not an appropriate substrate for acyl CoA dehydrogenase, or enoyl CoA hydratase:


 * If the acyl CoA contains a cis-Δ3 bond, then cis-Δ3-Enoyl CoA isomerase will convert the bond to a trans-Δ2 bond, which is a regular substrate.


 * If the acyl CoA contains a cis-Δ4 double bond, then its dehydrogenation yields a 2,4-dienoyl intermediate, which is not a substrate for enoyl CoA hydratase. However, the enzyme 2,4 Dienoyl CoA reductase reduces the intermediate, using NADPH, into trans-Δ3-enoyl CoA. As in the above case, this compound is converted into a suitable intermediate by 3,2-Enoyl CoA isomerase.

To summarize:
 * Odd-numbered double bonds are handled by the isomerase.
 * Even-numbered double bonds by the reductase (which creates an odd-numbered double bond)

β-Oxidation of odd-numbered chains
In general, fatty acids with an odd number of carbon are found in the lipids of plants and some marine organisms. Many ruminant animals form large amount of 3-carbon propionate during fermentation of carbohydrate in rumen.

Chains with an odd-number of carbons are oxidized in the same manner as even-numbered chains, but the final products are propionyl-CoA and acetyl-CoA.

Propionyl-CoA is first carboxylated using a bicarbonate ion into D-stereoisomer of methylmalonyl-CoA, in a reaction that involves a biotin co-factor, ATP, and the enzyme propionyl-CoA carboxylase. The bicarbonate ion's carbon is added to the middle carbon of propionyl-CoA, forming a D-methylmalonyl-CoA. However, the D conformation is enzymatically converted into the L conformation by methylmalonyl-CoA epimerase, then it undergoes intramolecular rearrangement, which is catalyzed by methylmalonyl-CoA mutase (requiring B12 as a coenzyme) to form succinyl-CoA. The succinyl-CoA formed can then enter the citric acid cycle.

Because it cannot be completely metabolized in the citric acid cycle, the products of its partial reaction must be removed in a process called cataplerosis. This allows regeneration of the citric acid cycle intermediates, possibly an important process in certain metabolic diseases.

Oxidation in peroxisomes
Fatty acid oxidation also occurs in peroxisomes, when the fatty acid chains are too long to be handled by the mitochondria. However, the oxidation ceases at octanyl CoA. It is believed that very long chain (greater than C-22) fatty acids undergo initial oxidation in peroxisomes which is followed by mitochondrial oxidation.

One significant difference is that oxidation in peroxisomes is not coupled to ATP synthesis. Instead, the high-potential electrons are transferred to O2, which yields H2O2. The enzyme catalase, found exclusively in peroxisomes, converts the hydrogen peroxide into water and oxygen.

Peroxisomal β-oxidation also requires enzymes specific to the peroxisome and to very long fatty acids. There are three key differences between the enzymes used for mitochondrial and peroxisomal β-oxidation:
 * 1) β-oxidation in the peroxisome requires the use of a peroxisomal carnitine acyltransferase (instead of carnitine acyltransferase I and II used by the mitochondria) for transport of the activated acyl group into the peroxisome.
 * 2) The first oxidation step in the peroxisome is catalyzed by the enzyme acyl CoA oxidase.
 * 3) The β-ketothiolase used in peroxisomal β-oxidation has an altered substrate specificity, different from the mitochondrial β-ketothiolase.

Peroxisomal oxidation is induced by high-fat diet and administration of hypolipidemic drugs like clofibrate.

Energy yield
The ATP yield for every oxidation cycle is 14 ATP (according to the P/O ratio), broken down as follows:

For an even-numbered saturated fat (C2n), n - 1 oxidations are necessary, and the final process yields an additional acetyl CoA. In addition, two equivalents of ATP are lost during the activation of the fatty acid. Therefore, the total ATP yield can be stated as:


 * (n - 1) * 14 + 10 - 2 = total ATP

For instance, the ATP yield of palmitate (C16, n = 8) is:


 * (8 - 1) * 14 + 10 - 2 = 106 ATP

Represented in table form:

For sources that use the larger ATP production numbers described above, the total would be 129 ATP ={(8-1)*17+12-2} equivalents per palmitate.

Beta-oxidation of unsaturated fatty acids changes the ATP yield due to the requirement of two possible additional enzymes.