Beta oxidation

Inbiochemistryandmetabolism,beta oxidation(also β-oxidation) is thecatabolic processby whichfatty acidmolecules are broken down in thecytosolin prokaryotes and in themitochondriain eukaryotes to generateacetyl-CoA.Acetyl-CoA enters thecitric acid cycle,generatingNADHandFADH₂,which are electron carriers used in theelectron transport chain.It is named as such because thebeta carbonof the fatty acid chain undergoes oxidation and is converted to acarbonylgroup to start the cycle all over again. Beta-oxidation is primarily facilitated by themitochondrial trifunctional protein,an enzyme complex associated with theinner mitochondrial membrane,althoughvery long chain fatty acidsare oxidized inperoxisomes.

The overall reaction for one cycle of beta oxidation is:

C_n-acyl-CoA + FAD + NAD⁺+ H₂O + CoA → C_n-2-acyl-CoA + FADH₂+ NADH + H⁺+ acetyl-CoA

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 specifictransport proteins,such as theSLC27family fatty acid transport protein.Once in thecytosol,the following processes bring fatty acids into the mitochondrial matrix so that beta-oxidation can take place.

1. Long-chain-fatty-acid—CoA ligasecatalyzes the reaction between a fatty acid withATPto give a fatty acyl adenylate, plus inorganic pyrophosphate, which then reacts with freecoenzyme Ato give afatty acyl-CoA esterandAMP.
2. If the fatty acyl-CoA has a long chain, then thecarnitine shuttlemust be utilized (shown in the table below):
   • Acyl-CoA is transferred to the hydroxyl group of carnitine bycarnitine palmitoyltransferase I,located on the cytosolic faces of theouterandinner mitochondrial membranes.
   • Acyl-carnitine is shuttled inside by acarnitine-acylcarnitine translocase,as a carnitine is shuttled outside.
   • Acyl-carnitine is converted back to acyl-CoA bycarnitine palmitoyltransferase II,located on the interior face of theinner mitochondrial membrane.The liberated carnitine is shuttled back to the cytosol, as an acyl-carnitine is shuttled into the matrix.
3. If the fatty acyl-CoA contains a short chain, theseshort-chain fatty acidscan simply diffuse through the inner mitochondrial membrane.

Step 1	Step 2	Step 3	Step 4
A diagrammatic illustration of the process of lipolysis (in a fat cell) induced by highepinephrineand lowinsulinlevels in the blood. Epinephrine binds to abeta-adrenergicreceptor in the cell wall of the adipocyte, which causescAMPto be generated inside the cell. The cAMP activates aprotein kinase,which phosphorylates and activates ahormone-sensitive lipasein 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.	A diagrammatic illustration of the transport offree fatty acidsin the blood attached toplasma albumin,its diffusion across the cell membrane using a protein transporter, and its activation, usingATP,to formacyl-CoAin thecytosol.The illustration is of a 12 carbon fatty acid.	A diagrammatic illustration of the transfer of an acyl-CoA molecule across the inner membrane of themitochondrionbycarnitine-acyl-CoA transferase(CAT). The illustrated acyl chain is 12 carbon atoms long. CAT is inhibited by high concentrations ofmalonyl-CoA(the first committed step infatty acid synthesis) in the cytoplasm. This means that fatty acid synthesis and fatty acid catabolism cannot occur simultaneously in any given cell.	A diagrammatic illustration of the process of thebeta-oxidationof 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 5ATPmolecules are the other products of each beta-oxidative event, until the entire acyl-CoA molecule has been reduced to a set ofacetyl-CoAmolecules.

General mechanism of beta oxidation

General Mechanism of Beta Oxidation

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

1. A long-chain fatty acid isdehydrogenatedto create a transdouble bondbetween C2 and C3. This is catalyzed byacyl CoA dehydrogenaseto produce trans-delta 2-enoyl CoA. It uses FAD as an electron acceptor and it is reduced to FADH₂.
2. Trans-delta 2-enoyl CoA is hydrated at the double bond to produce L-3-hydroxyacyl CoA byenoyl-CoA hydratase.
3. 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.
4. Thiolysisoccurs between C2 and C3 ( Alpha and beta carbons) 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 (TCA 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 thered blood cellsof mammals (which do not contain mitochondria) and cells of thecentral nervous systemdo not use fatty acids for their energy requirements, but instead use carbohydrates (red blood cells and neurons) orketone bodies(neurons only).

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 (acetyl-CoA), occurs in a sequence of four reactions:

Description	Diagram	Enzyme	End product
DehydrogenationbyFAD:The first step is the oxidation of the fatty acid by Acyl-CoA-Dehydrogenase. The enzyme catalyzes the formation of a trans-double bondbetween 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 thehydrationof the bond between C-2 and C-3. The reaction isstereospecific,forming only the Lisomer.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
OxidationbyNAD+:The third step is theoxidationof L-β-hydroxyacyl CoA by NAD+.This converts thehydroxylgroup into aketogroup.		3-hydroxyacyl-CoA dehydrogenase	β-ketoacyl CoA
Thiolysis:The final step is the cleavage of β-ketoacyl CoA by thethiolgroup of another molecule ofCoenzyme A.The thiol is inserted between C-2 and C-3.		β-ketothiolase	Anacetyl-CoAmolecule, and anacyl-CoAmolecule 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

Propionyl-CoA modification after beta oxidation of odd-chain fatty acid

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 ofcarbonsare oxidized in the same manner as even-numbered chains, but the final products arepropionyl-CoAand acetyl-CoA.

Propionyl-CoA is first carboxylated using abicarbonateioninto a D-stereoisomer ofmethylmalonyl-CoA.This reaction involves abiotinco-factor,ATP and the enzymepropionyl-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 bymethylmalonyl-CoA epimerase.It then undergoes intramolecular rearrangement, which is catalyzed bymethylmalonyl-CoA mutase(requiring B₁₂as a coenzyme) to form succinyl-CoA. Thesuccinyl-CoAformed then enters thecitric acid cycle.

However, whereas acetyl-CoA enters the citric acid cycle by condensing with an existing molecule ofoxaloacetate,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 exceedscatapleroticdemand (such as foraspartateorglutamatesynthesis), some of them can be extracted to thegluconeogenesispathway, in the liver and kidneys, throughphosphoenolpyruvate 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-Δ²bond 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 isomeraseand2,4 Dienoyl CoA reductase.

Complete beta oxidation oflinoleic acid(an unsaturated fatty acid).

β-oxidation occurs normally until the acyl CoA (because of the presence of a double bond) is not an appropriate substrate foracyl CoA dehydrogenase,orenoyl CoA hydratase:

• If the acyl CoA contains acis-Δ³bond,thencis-Δ³-Enoyl CoA isomerasewill convert the bond to atrans-Δ²bond, which is a regular substrate.
• If the acyl CoA contains acis-Δ⁴double bond,then its dehydrogenation yields a 2,4-dienoyl intermediate, which is not a substrate for enoyl CoA hydratase. However, the enzyme2,4 Dienoyl CoA reductasereduces the intermediate, using NADPH, intotrans-Δ³-enoyl CoA. This compound is converted into a suitable intermediate by 3,2-Enoyl CoA isomeraseand β-Oxidation continues.

Peroxisomal beta-oxidation

Fatty acid oxidation also occurs inperoxisomeswhen the fatty acid chains are too long to be processed by the mitochondria. The same enzymes are used in peroxisomes as in the mitochondrial matrix and acetyl-CoA is generated. Very long chain (greater than C-22) fatty acids, branched fatty acids,someprostaglandinsandleukotrienesundergo initial oxidation in peroxisomes untiloctanoyl-CoAis formed, at which point it undergoes mitochondrial oxidation.

One significant difference is that oxidation in peroxisomes is not coupled toATPsynthesis. Instead, the high-potential electrons are transferred to O₂,which yieldshydrogen peroxide.The enzymecatalase,found primarily in peroxisomes and thecytosoloferythrocytes(and sometimes inmitochondria), converts thehydrogen peroxideintowaterandoxygen.

Peroxisomal β-oxidation also requires enzymes specific to the peroxisome and to very long fatty acids. There are four key differences between the enzymes used for mitochondrial and peroxisomal β-oxidation:

1. The NADH formed in the third oxidative step cannot be reoxidized in the peroxisome, so reducing equivalents are exported to the cytosol.
2. β-oxidation in the peroxisome requires the use of a peroxisomalcarnitine acyltransferase(instead of carnitine acyltransferase I and II used by the mitochondria) for transport of the activated acyl group into the mitochondria for further breakdown.
3. The first oxidation step in the peroxisome is catalyzed by the enzymeacyl-CoA oxidase.
4. Theβ-ketothiolaseused in peroxisomal β-oxidation has an altered substrate specificity, different from the mitochondrialβ-ketothiolase.

Peroxisomal oxidation is induced by a high-fat diet and administration of hypolipidemic drugs likeclofibrate.

Energy yield

Even-numbered saturated fatty acids

Theoretically, the ATP yield for each oxidation cycle where two carbons are broken down at a time is 17, as each NADH produces 3 ATP, FADH₂produces 2 ATP and a full rotation of Acetyl-CoA in citric acid cycle produces 12 ATP.In practice, it is closer to 14 ATP for a full oxidation cycle as 2.5 ATP per NADH molecule is produced, 1.5 ATP per each FADH₂molecule is produced and Acetyl-CoA produces 10 ATP per rotation of the citric acid cycle(according to theP/O ratio). This breakdown is as follows:

Source	ATP	Total
1FADH2	x 1.5 ATP	= 1.5 ATP (Theoretically 2 ATP)
1NADH	x 2.5 ATP	= 2.5 ATP (Theoretically 3 ATP)
1Acetyl CoA	x 10 ATP	= 10 ATP (Theoretically 12 ATP)
1Succinyl CoA	x 4 ATP	= 4 ATP
Total		= 14 ATP

For an even-numbered saturated fat (C_n), 0.5 * n - 1 oxidations are necessary, and the final process yields an additional acetyl CoA. In addition, two equivalents ofATPare lost during the activation of the fatty acid. Therefore, the total ATP yield can be stated as:

$(0.5\times n-1)\times 14+10-2=ATP$

or

$7n-6$

For instance, the ATP yield ofpalmitate(C₁₆,n = 16) is:

$7\times 16-6=106ATP$

Represented in table form:

Source	ATP	Total
7FADH2	x 1.5 ATP	= 10.5 ATP
7NADH	x 2.5 ATP	= 17.5 ATP
8Acetyl CoA	x 10 ATP	= 80 ATP
Activation		= -2 ATP
Total		= 106 ATP

Odd-numbered saturated fatty acid

Steps in beta-oxidation of odd-numbered saturated fatty acids

For an odd-numbered saturated fat (C_n), 0.5 * n - 1.5 oxidations are necessary, and the final process yields 8 acetyl CoA and 1 propionyl CoA. It is then converted to a succinyl CoA by a carboxylation reaction and generates additional 5 ATP (1 ATP is consumed in carboxylation process generating a net of 4 ATP). In addition, two equivalents ofATPare lost during the activation of the fatty acid. Therefore, the total ATP yield can be stated as:

$(0.5\times n-1.5)\times 14+4-2=ATP$

or

$7n-19$

For instance, the ATP yield ofNonadecylic acid(C₁₉,n = 19) is:

$7\times 19-19=114ATP$

Represented in table form:

Source	ATP	Total
8FADH2	x 1.5 ATP	= 12 ATP
8NADH	x 2.5 ATP	= 20 ATP
8Acetyl CoA	x 10 ATP	= 80 ATP
1Succinyl CoA	x 4 ATP	= 4 ATP
Activation		= -2 ATP
Total		= 114 ATP

Clinical significance

There are at least 25 enzymes and specific transport proteins in the β-oxidation pathway.Of these, 18 have been associated with human disease asinborn errors of metabolism.

Furthermore, studies indicate that lipid disorders are involved in diverse aspects of tumorigenesis, and fatty acid metabolism makes malignant cells more resistant to a hypoxic environment. Accordingly, cancer cells can display irregular lipid metabolism with regard to both fatty acid synthesis and mitochondrialfatty acid oxidation(FAO) that are involved in diverse aspects of tumorigenesis and cell growth.Several specific β-oxidation disorders have been identified.

Medium-chain acyl-coenzyme A dehydrogenase (MCAD) deficiency

Medium-chain acyl-coenzyme A dehydrogenase (MCAD) deficiencyis the most common fatty acid β-oxidation disorder and a prevalent metabolic congenital error It is often identified through newborn screening. Although children are normal at birth, symptoms usually emerge between three months and two years of age, with some cases appearing in adulthood.

Medium-chain acyl-CoA dehydrogenase(MCAD) plays a crucial role in mitochondrial fatty acid β-oxidation, a process vital for generating energy during extended fasting or high-energy demand periods. This process, especially important when liver glycogen is depleted, supports hepatic ketogenesis. The specific step catalyzed by MCAD involves the dehydrogenation of acyl-CoA. This step converts medium-chain acyl-CoA to trans-2-enoyl-CoA, which is then further metabolized to produce energy in the form of ATP.

Symptoms

• Affected children, who seem healthy initially, may experience symptoms like low blood sugar without ketones (hypoketotic hypoglycemia) andvomiting
• Can escalate tolethargy,seizuresandcoma,typically triggered by illness
• Acute episodes may also involve enlarged liver (hepat Omega ly) and liver issues
• Sudden death

Treatments

• Administering simple carbohydrates
• Avoiding fasting
• Frequent feedings for infants
• For toddlers, a diet with less than 30% of total energy from fat
• Administering 2 g/kg of uncooked cornstarch at bedtime for sufficient overnight glucose
• Preventing hypoglycemia, especially due to excessive fasting.
• Avoiding infant formulas with medium-chain triglycerides as the main fat source

Schematic demonstratingmitochondrialfatty acidbeta-oxidationand effects oflong-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency, LCHAD deficiency

Long-chain hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency

Long-chain hydroxyacyl-CoA dehydrogenase (LCHAD) deficiencyis a mitochondrial effect of impaired enzyme function.

LCHAD performs the dehydrogenation of hydroxyacyl-CoA derivatives, facilitating the removal of hydrogen and the formation of aketo group.This reaction is essential for the subsequent steps in beta oxidation that lead to the production of acetyl-CoA, NADH, and FADH2, which are important for generating ATP, the energy currency of the cell.

Long-chain hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency is a condition that affects mitochondrial function due to enzyme impairments. LCHAD deficiency is specifically caused by a shortfall in the enzymelong-chain 3-hydroxyacyl-CoA dehydrogenase.This leads to the body's inability to transform specific fats into energy, especially during fasting periods.

Symptoms

• Severe Phenotype: symptoms appear soon after birth and includehypoglycemia,hepat Omega ly,brain dysfunction (encephalopathy) and oftencardiomyopathy
• Intermediate Phenotype: characterized byhypoketotic hypoglycemiaand is triggered by infection or fasting during infancy
• Mild (Late-Onset) Phenotype: presents as muscle weakness (myopathy) and nerve disease (neuropathy)
• Long-Term Complications: can includeperipheral neuropathyand eye damage (retinopathy)

Treatments

• Regular feeding to avoid fasting
• Use ofmedium-chain triglyceride(MCT) ortriheptanoinsupplements andcarnitinesupplements
• Low-fat diet
• Hospitalization with intravenous fluids containing at least 10% dextrose
• Bicarbonate therapy for severemetabolic acidosis
• Management of high ammonia levels and muscle breakdown
• Cardiomyopathymanagement
• Regular monitoring of nutrition, blood and liver tests with annual fatty acid profile
• Growth, development, heart and neurological assessments and eye evaluations

Very long-chain acyl-Coenzyme A dehydrogenase (VLCAD) deficiency

Very long-chain acyl-coenzyme A dehydrogenase deficiency(VLCADdeficiency) is a genetic disorder that affects the body's ability to break down certain fats. In the β-oxidation cycle, VLCAD's role involves the removal of two hydrogen atoms from the acyl-CoA molecule, forming a double bond and converting it into trans-2-enoyl-CoA. This crucial first step in the cycle is essential for the fatty acid to undergo further processing and energy production. When there is a deficiency in VLCAD, the body struggles to effectively break down long-chain fatty acids. This can lead to a buildup of these fats and a shortage of energy, particularly during periods of fasting or increased physical activity.

Symptoms

• Severe Early-Onset Cardiac and Multiorgan Failure Form: symptoms appear within days of birth and includehypertrophic/dilated cardiomyopathy,fluid around heart (pericardial effusion), heart rhythm problems (arrhythmias),hepat Omega lyand occasionalintermittent hypoglycemia

• Hepatic or Hypoketotic Hypoglycemic Form: typically appears in early childhood withhypoketotic hypoglycemia

• Later-Onset Episodic Myopathic Form: presents with muscle breakdown after exercise (intermittent rhabdomyolysis), muscle cramps and pain, exercise intolerance andlow blood sugar

Treatments

• Low-fat diet

• Use ofmedium-chain triglyceride(MCT) supplements

• Regular, frequent feeding, especially for infants and children
• Snacks high in complex carbohydrates before bedtime
• Guided and limited exercise for older individuals
• Administration of high-energy fluids intravenously

• AvoidingL-carnitineand IV fats
• Plenty of fluids and urine alkalization for muscle breakdown

See also

• Fatty acid metabolism
• Fatty-acid metabolism disorder
• Lipolysis
• Omega oxidation
• Alpha oxidation

Further reading

• Berg JM, Tymoczko JL, Stryer L (2002)."Certain Fatty Acids Require Additional Steps for Degradation".Biochemistry(5th ed.). New York: W H Freeman.ISBN978-0-7167-4684-3.

External links

• Silva P."The chemical logic behind fatty acid metabolism".Universidade Fernando Pessoa. Archived fromthe originalon 16 March 2010.
• "Fatty acid oxidation animation".Cengage Learning.Archived fromthe originalon 2012-05-08.Retrieved2007-05-02.
• Jain, P.; Singh, S.; Arya, A. (2021)."Integrated formulae for calculating fatty acid ATP yield".Biochemistry and Molecular Biology Education.49(3): 492–499.doi:10.1002/bmb.21486.PMID33427394.S2CID231577993.