Oxidation of fatty acids

Topic: Metabolism Of Fatty Acids And Proteins

β-oxidation is the catabolic breakdown of fatty acids to produce energy; this process can completely degrade saturated fatty acids but requires the input of the enzymes enoyl-CoA isomerase and 2,4-dienoyl CoA, to complete degradation of unsaturated fatty acids.

To generate energy from fatty acids, they must be oxidized. Fatty acid oxidation is also referred to as β-oxidation because two carbon units are cleaved off at the β-carbon position (second carbon from the acid end) of an activated fatty acid. β-oxidation that takes place in the matrix of the mitochondria and converts their fatty acid chains into two carbon units of acetyl groups, while producing NADH and FADH2. The acetyl groups are picked up by CoA to form acetyl-CoA that proceeds into the citric acid cycle as it combines with oxaloacetate. The NADH and FADH2 are then used by the electron transport chain.

Since saturated fatty acids contain an even number of carbon atoms, the β-oxidation pathway accomplishes the complete degradation of saturated fatty acids. On the other hand, the oxidation of unsaturated fatty acids, which contain double bonds, requires additional steps. Because double bonds can disturb the stereochemistry needed for oxidative enzymes to act on the fatty acid, two additional enzymes (enoyl CoA isomerase and 2,4 dienoyl CoA) are needed.

β-oxidation can be broken down into a series of steps. First, the fatty acid is activated. This involves the addition of a coenzyme A (CoA) molecule to the end of a long-chain fatty acid, after which the activated fatty acyl-CoA enters the β-oxidation pathway. Second, an oxidation step occurs. This initial step of β-oxidation involves the oxidation of the fatty acyl-CoA to yield enoyl-CoA. As a result, a trans double bond is introduced into the acyl chain. Take a look at the diagram below to see where this double bond is formed. Third, the hydration of enoyl-CoA yields an alcohol (-C-OH). Fourth, another oxidation step occurs and the alcohol (-C-OH) is oxidized into a carbonyl (-C=O). Fifth, a thiolase enzyme cleaves off acetyl-CoA from the oxidized molecule, which also yields an acyl-CoA that is two carbons shorter than the original molecule that entered the β-oxidation pathway. This cycle repeats until the fatty acid has been completely reduced to acetyl-CoA, which is fed through the citric acid cycle to yield cellular energy in the form of ATP.

 


Practice Questions

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MCAT Official Prep (AAMC)

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Practice Exam 4 B/B Section Passage 2 Question 6

 


Key Points

• Fatty acids are broken down to produce cellular energy through β-oxidation.

• During β-oxidation, two-carbon fragments are removed sequentially from the carboxyl end of the fatty acid after dehydrogenation, hydration, and oxidation to form a keto acid, which is then cleaved by thiolysis.

• The acetyl-CoA molecules released during β-oxidation are eventually converted into ATP through the citric acid cycle.

• The β-oxidation pathway accomplishes the complete degradation of saturated fatty acids because they contain an even number of carbon atoms; unsaturated fatty acids need two additional enzymes (enoyl-CoA isomerase and 2,4 dienoyl CoA) because double bonds can disturb the stereochemistry needed for oxidative enzymes to act on the fatty acid.


Key Terms

Beta (β)-oxidation: A process that takes place in the matrix of the mitochondria and catabolizes fatty acids by converting them to acetyl groups while producing NADH and FADH2.

Catabolism: A series of metabolic pathways that break molecules down into smaller forms, which can be oxidized to release energy or be used as reactants in other reactions.

Fatty acids: Lipids that contain a carboxylic acid functional group attached to a long-chain hydrocarbon tail.

Saturated fatty acids: Fatty acids with hydrocarbon chains containing only single bonds (C-C).

Unsaturated fatty acids: Fatty acids with hydrocarbon chains containing at least one double bond (C=C).

Oxidation: The loss of electrons from a molecule to oxygen; in the context of lipid metabolism, electrons are transferred from a fatty acid to oxygen, oxidizing the fatty acid.

β-carbon: In a fatty acid, the second carbon from the carboxylic acid end.

Activated fatty acid: The addition of a coenzyme A (CoA) molecule to the end of a long-chain fatty acid; this allows the fatty acyl-CoA to enter the β-oxidation pathway.

Stereochemistry: In chemistry, the spatial arrangement of atoms in a molecule.

Coenzyme A: A coenzyme (protein) that is necessary for fatty acid synthesis and oxidation.

Hydration: A chemical reaction in which a molecule reacts with water.

Acetyl-CoA: A molecule that is involved in protein, carbohydrate and lipid metabolism by delivering an acetyl group to the citric acid cycle, which will be oxidized for energy production.

Citric acid cycle: Also known as the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle, a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA.

Adenosine triphosphate (ATP): An organic compound that provides energy to cellular organisms.



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