Introduction to Fatty Acid Oxidation:
The fatty acids released in the adipocytes enter the circulation and are transported in a bound form to albumin. The free fatty acids enter various tissues and are utilized for the energy. About 95% of the energy obtained from fat comes from the oxidation of fatty acids.
Triacylglycerol (TG) /fatty acid cycle:
During starvation, TG stored in adipose tissue is hydrolysed to free fatty acids to provide energy to skeletal and cardiac muscle. However, about 65% of these free fatty acids are converted to TG, and sent back to adipose tissue for deposition. This process of lipolysis of TG and resterification of free fatty acids to TG is termed as triacylglycerol/fatty acid cycle.
Overview of Fatty acid oxidation:
β-oxidation is the primary process that oxidizes the body’s fatty acids. The oxidation of fatty acids on the β carbon atom, which results in the subsequent removal of the two-carbon fragment acetyl CoA, is one definition of oxidation.
The β-oxidation of fatty acids occurs in three phases.
Activation of fatty acids
Thiokinases or acyl CoA synthetases convert fatty acids into acyl CoA. ATP, coenzyme A, and Mg2+ are needed for the two-step process. ATP and fatty acid mix to generate acyladenylate, which subsequently joins coenzyme A to generate acyl CoA. Since ATP is changed into pyrophosphate (PPi), two high energy phosphates are used in the activation. PPi is hydrolyzed to phosphate (Pi) by the enzyme inorganic pyrophosphatase; this reaction is completely irreversible due to the instantaneous removal of PPi. Three different thiokinases, to activate long chain (10-20 carbon), medium chain (4-12carbon) and short chain (< 4 carbon) fatty acids have been identified.
Transport of acyl CoA into mitochondria
A specialized carnitine carrier system, also known as a carnitine shuttle, transports activated fatty acids from the cytosol to the mitochondria in four phases. The inner mitochondrial membrane is impermeable to fatty acids.
Carnitine acyltransferase-I (CAT-I), which is found on the outer side of the inner mitochondrial membrane, catalyzes the transfer of the acyl group of acyl CoA to carnitine. A particular carrier protein carries the acyl-carnitine across the membrane to the mitochondrial matrix. Acyl-carnitine is changed into acyl CoA by carnitine acyl transferase-II (CAT-II), which is located on the inner surface of the inner mitochondrial membrane. The liberated carnitine goes back to the cytosol to be used again.
Steps of β-Oxidation
Each cycle of β-oxidation, liberating a two-carbon unit-acetyl CoA, occurs in a sequence of four reactions
First Oxidation (Acyl-CoA Dehydrogenase)
The process kicks off with the enzyme acyl-CoA dehydrogenase, a flavoprotein reliant on FAD. This enzyme removes hydrogen atoms from the fatty acyl-CoA molecule, introducing a double bond between the α (C-2) and β (C-3) carbons — a crucial structural change that sets the stage for the next steps.
Hydration (Enoyl-CoA Hydratase)
The enzyme enoyl-CoA hydratase then adds a water molecule across the newly formed double bond. This hydration step transforms the molecule into β-hydroxyacyl-CoA, placing a hydroxyl group on the β-carbon.
Second Oxidation (β-Hydroxyacyl-CoA Dehydrogenase)
Now, β-hydroxyacyl-CoA dehydrogenase steps in to oxidize the hydroxyl group on the β-carbon, converting it into a keto group. This reaction also reduces NAD⁺ to NADH, and the product is β-ketoacyl-CoA — a key intermediate primed for cleavage.
Thiolytic Cleavage (Thiolase)
In the last step, thiolase (β-ketoacyl-CoA thiolase) catalyzes a thiolytic cleavage, slicing off a two-carbon acetyl-CoA unit from the molecule. The remaining acyl-CoA, now two carbons shorter, re-enters the β-oxidation cycle to continue the breakdown. Until the fatty acid is fully oxidized, the procedure is repeated.
Oxidation Process of Unsaturated Fatty Acids:
Unsaturated fatty acids, due to their characteristic double bonds, undergo oxidation differently compared to their saturated counterparts. These double bonds prevent full reduction, which means that unsaturated fatty acids yield slightly less energy during their breakdown.
Oxidation of Monounsaturated Fatty Acids (MUFA)
- In the case of monounsaturated fatty acids, only one auxiliary enzyme is required: enoyl-CoA isomerase.
- Taking oleate, an 18-carbon fatty acid with a cis double bond between carbons 9 and 10 (cis-∆9), as an example. The breakdown of oleate begins with its conversion into oleoyl-CoA, which is then shuttled into the mitochondria as oleoyl carnitine. Once inside the mitochondrial matrix, it reverts to oleoyl-CoA.
- This molecule undergoes three complete cycles of β-oxidation, generating three acetyl-CoA molecules and leaving behind a 12-carbon unsaturated intermediate: cis-∆3-dodecenoyl-CoA. However, this intermediate presents a problem: the enzyme enoyl-CoA hydratase, which usually acts next in the sequence, is only effective on trans double bonds—not cis.
- To bypass this bottleneck, enoyl-CoA isomerase steps in. It converts the cis-∆3 configuration into a trans-∆2 configuration, producing trans-∆2-enoyl-CoA, a substrate that can seamlessly re-enter the β-oxidation cycle and continue the degradation process.
- The latter undergoes 4 more passes through the pathway to yield altogether 9 acetyl-CoAs from one mole of the C–18 oleate.
Oxidation of Polyunsaturated Fatty Acids (PUFA)
Breaking down polyunsaturated fatty acids—those with multiple double bonds—requires more than just the standard β-oxidation machinery. This pathway enlists the help of two specialized enzymes:
- Enoyl-CoA isomerase
- 2,4-dienoyl-CoA reductase
Taking linoleate, an 18-carbon fatty acid as an example, Linoleoyl-CoA enters the standard β-oxidation cycle and completes three full rounds, resulting in:
3 acetyl-CoA molecules
A 12-carbon intermediate with double bonds at positions ∆3 and ∆6, both still in the cis form
This intermediate cannot proceed through regular β-oxidation. The double bonds are not only in the wrong positions but also in the wrong configuration (cis instead of trans), which renders them incompatible with the standard enzymes.
The combined action of Enoyl-CoA isomerase (which rearranges the double bonds), and 2,4-dienoyl-CoA reductase (which reduces conjugated dienes into a usable intermediate), transforms the molecule into a structure that can rejoin the regular β-oxidation pathway.