Beta oxidation

In biochemistry and metabolism, beta oxidation is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA. The acetyl-CoA product of this step of fatty acid metabolism enters the citric acid cycle, generating NADH and FADH₂, which are electron carriers used in the electron transport chain. It is named as such because the beta carbon of the fatty acid chain undergoes oxidation and is converted to a carbonyl group to start the cycle all over again. Beta-oxidation is primarily facilitated by the mitochondrial trifunctional protein, an enzyme complex associated with the inner mitochondrial membrane, although very long chain fatty acids are oxidized in peroxisomes. Beta oxidation occurs mainly in the heart, liver and kidney.
The overall reaction for one cycle of beta oxidation is:

Activation and membrane transport

Free fatty acids cannot penetrate any biological membrane due to their negative charge. Free fatty acids must cross the cell membrane through specific transport proteins, such as the SLC27 family fatty acid transport protein. Once in the cytosol, the following processes bring fatty acids into the mitochondrial matrix so that beta-oxidation can take place.

Long-chain-fatty-acid—CoA ligase catalyzes the reaction between a fatty acid 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 and AMP.
If the fatty acyl-CoA has a long chain, then the carnitine shuttle must be utilized :
* Acyl-CoA is transferred to the hydroxyl group of carnitine by carnitine palmitoyltransferase I, located on the cytosolic faces of the outer and inner mitochondrial membranes.
* Acyl-carnitine is shuttled inside by a carnitine-acylcarnitine translocase, as a carnitine is shuttled outside.
* Acyl-carnitine is converted back to acyl-CoA by carnitine palmitoyltransferase II, located on the interior face of the inner mitochondrial membrane. The liberated carnitine is shuttled back to the cytosol, as an acyl-carnitine is shuttled into the matrix.
If the fatty acyl-CoA contains a short chain, these short-chain fatty acids can simply diffuse through the inner mitochondrial membrane.

Step 1	Step 2	Step 3	Step 4
File:Metabolism1.jpg\|center\|thumb\|201x201px\|A diagrammatic illustration of the process of lipolysis induced by high epinephrine and low insulin levels in the blood. Epinephrine binds to a beta-adrenergic receptor in the cell wall of the adipocyte, which causes cAMP to be generated inside the cell. The cAMP activates a protein kinase, which phosphorylates and activates a hormone-sensitive lipase in the fat cell. This lipase cleaves free fatty acids from their attachment to glycerol in the adipocyte. The free fatty acids and glycerol are then released into the blood.	File:Metabolism2.jpg\|center\|thumb\|200x200px\|A diagrammatic illustration of the transport of free fatty acids in the blood attached to plasma albumin, its diffusion across the cell membrane using a protein transporter, and its activation, using ATP, to form acyl-CoA in the cytosol. The illustration is of a 12 carbon fatty acid.	File:Metabolism3.jpg\|center\|thumb\|200x200px\|A diagrammatic illustration of the transfer of an acyl-CoA molecule across the inner membrane of the mitochondrion by carnitine-acyl-CoA transferase. The illustrated acyl chain is 12 carbon atoms long. CAT is inhibited by high concentrations of malonyl-CoA in the cytoplasm. This means that fatty acid synthesis and fatty acid catabolism cannot occur simultaneously in any given cell.	File:Metabolism4.jpg\|center\|thumb\|200x200px\|A diagrammatic illustration of the process of the beta-oxidation of an acyl-CoA molecule in the mitochondrial matrix. During this process an acyl-CoA molecule which is 2 carbons shorter than it was at the beginning of the process is formed. Acetyl-CoA, water and 5 ATP molecules are the other products of each beta-oxidative event, until the entire acyl-CoA molecule has been reduced to a set of acetyl-CoA molecules.

General mechanism of beta oxidation

Once the fatty acid is inside the mitochondrial matrix, beta-oxidation occurs by cleaving two carbons every cycle to form acetyl-CoA. The process consists of 4 steps.

A long-chain fatty acid is dehydrogenated to create a trans double bond between C2 and C3. This is catalyzed by acyl CoA dehydrogenase to produce trans-delta 2-enoyl CoA. It uses FAD as an electron acceptor and it is reduced to FADH₂.
Trans-delta 2-enoyl CoA is hydrated at the double bond to produce L-3-hydroxyacyl CoA by enoyl-CoA hydratase.
L-3-hydroxyacyl CoA is dehydrogenated again to create 3-ketoacyl CoA by 3-hydroxyacyl CoA dehydrogenase. This enzyme uses NAD as an electron acceptor.
Thiolysis occurs between C2 and C3 of 3-ketoacyl CoA. Thiolase enzyme catalyzes the reaction when a new molecule of coenzyme A breaks the bond by nucleophilic attack on C3. This releases the first two carbon units, as acetyl CoA, and a fatty acyl CoA minus two carbons. The process continues until all of the carbons in the fatty acid are turned into acetyl CoA.

This acetyl-CoA then enters the mitochondrial tricarboxylic acid cycle. Both the fatty acid beta-oxidation and the TCA cycle produce NADH and FADH₂, which are used by the electron transport chain to generate ATP.
Fatty acids are oxidized by most of the tissues in the body. However, some tissues such as the red blood cells of mammals and cells of the central nervous system do not use fatty acids for their energy requirements, but instead use carbohydrates or ketone bodies.
Because many fatty acids are not fully saturated or do not have an even number of carbons, several different mechanisms have evolved, described below.

Even-numbered saturated fatty acids

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

Description	Diagram	Enzyme	End product
Dehydrogenation by FAD: The first step is the oxidation of the fatty acid by Acyl-CoA-Dehydrogenase. The enzyme catalyzes the formation of a trans-double bond between the C-2 and C-3 by selectively remove hydrogen atoms from the β-carbon. The regioselectivity of this step is essential for the subsequent hydration and oxidation reactions.		acyl CoA dehydrogenase	trans-Δ²-enoyl-CoA
Hydration: The next step is the hydration of the bond between C-2 and C-3. The reaction is stereospecific, forming only the L isomer. Hydroxyl group is positioned suitable for the subsequent oxidation reaction by 3-hydroxyacyl-CoA dehydrogenase to create a β-keto group.		enoyl CoA hydratase	L-β-hydroxyacyl CoA
Oxidation by NAD⁺: The third step is the oxidation of L-β-hydroxyacyl CoA by NAD⁺. This converts the hydroxyl group into a keto group.		3-hydroxyacyl-CoA dehydrogenase	β-ketoacyl CoA
Thiolysis: The final step is the cleavage of β-ketoacyl CoA by the thiol group of another molecule of Coenzyme A. The thiol is inserted between C-2 and C-3.		β-ketothiolase	An acetyl-CoA molecule, and an acyl-CoA molecule that is two carbons shorter

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.

Odd-numbered saturated fatty acids

Fatty acids with an odd number of carbons are found in the lipids of plants and some marine organisms. Many ruminant animals form a large amount of 3-carbon propionate during the fermentation of carbohydrates in the rumen. Long-chain fatty acids with an odd number of carbon atoms are found particularly in ruminant fat and milk.
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 a D-stereoisomer of methylmalonyl-CoA. This reaction 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. It then undergoes intramolecular rearrangement, which is catalyzed by methylmalonyl-CoA mutase to form succinyl-CoA. The succinyl-CoA formed then enters the citric acid cycle.
However, whereas acetyl-CoA enters the citric acid cycle by condensing with an existing molecule of oxaloacetate, succinyl-CoA enters the cycle as a principal in its own right. Thus, the succinate just adds to the population of circulating molecules in the cycle and undergoes no net metabolization while in it. When this infusion of citric acid cycle intermediates exceeds cataplerotic demand, some of them can be extracted to the gluconeogenesis pathway, in the liver and kidneys, through phosphoenolpyruvate carboxykinase, and converted to free glucose.

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 which is essential for continuation of β-Oxidation as this conformation is ideal for enzyme catalysis. This is handled by additional two enzymes, Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase.
β-oxidation occurs normally until the acyl CoA 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. This compound is converted into a suitable intermediate by 3,2-Enoyl CoA isomerase and β-Oxidation continues.