Beta Oxidation

Beta Oxidation Definition

Beta oxidation (otherwise known as β-oxidation or fatty acid oxidation) is a crucial step in fat metabolism. When the body uses fat for fuel, it is first broken down into glycerol and fatty acid chains. Fatty acids are made of a carboxylic acid group with a chain of hydrocarbons that varies greatly in length. Beta oxidation acts specifically on the fatty acids, ultimately breaking them into several smaller components that can be used to create energy for a cell. These components include acetyl CoA, which enters the citric acid cycle, as well as NADH and FADH₂, which enter the electron transport chain. Both pathways produce ATP, which is a cell’s primary source of energy.

The term beta oxidation comes from the fact that the process involves an oxidation step starting at the β carbon. This is the carbon directly after the α carbon, which is the first carbon attached to the functional group.

Where Does Beta Oxidation Occur?

Beta oxidation in eukaryotic cells occurs primarily within mitochondria. Some types of fatty acids, especially those with very long chains, are processed in peroxisomes. In prokaryotes, which lack organelles, beta oxidation occurs in the cytosol.

After fats are broken down, the fatty acids must be moved into the cell by special transport proteins and converted into acyl-CoA form, before eventually making their way into the mitochondria (or peroxisome). The methods by which fatty acids are transported into the mitochondria depends on the length of the fatty acid chain, with longer chains requiring specialized carnitine shuttles.

Beta Oxidation Steps

Once in the mitochondria (or peroxisome), there are four main steps in the oxidation process:

Step 1

In the first step (see the image below), an enzyme known as Acyl-CoA-Dehyrogenase converts the acyl-CoA form of the fatty acid to a trans form with a double bond. This form is called trans-Δ²-Enoyl-CoA. Also during this step, an electron is donated to FAD to produce FADH₂, which goes into the electron transport chain to produce energy in the form of ATP.

Step 2

Next, a different enzyme hydrogenates the double bond, forming L-3-Hydroxyacyl-CoA.

Step 3

Yet another enzyme converts this to 3-Ketoacyl-CoA. Also during this step, an electron is donated to NAD⁺ to form NADH, which goes into the electron transport chain to produce energy.

Step 4

In the final step, the 3-Ketoacyl-CoA is cleaved by another enzyme to produce two products: a shorted version of the original acyl-CoA form and an Acetyl-CoA, which then goes into the citric acid cycle to produce energy in the form of ATP.

This 4-step process is repeated along the length of the remaining acyl-CoA until the entire fatty acid chain has been broken down. The exact biochemical steps and enzymes involved vary depending on the type of fatty acid. Different enzymes are needed, depending on whether the fatty acid is saturated or unsaturated, and whether it contains an odd or even numbered carbon chain.

Beta Oxidation Dysfunction

Dysfunctional beta oxidation leads to metabolic disorders in humans, which are generally referred to as fatty acid oxidation (FAO) disorders. Because there are several enzymes and transport proteins involved in normal beta oxidation, many different genetic mutations are responsible. Symptoms are highly variable between patients, but often manifest early in childhood. Many known FAO disorders are detected in neonatal screening.

Quiz

1. What is the purpose of beta oxidation?
A. To produce energy via metabolizing glucose
B. To produce energy via metabolizing fatty acids
C. To produce fatty acids for storage
D. To oxidize the alpha carbon to a beta carbon

Answer to Question #1

B is correct. Although energy can also be obtained from other food sources, such as glucose, beta oxidation specifically breaks down fatty acids into various products that can be used as energy by cells.

2. Where does beta oxidation occur in eukaryotes?
A. In the cytosol
B. In mitochondria
C. In peroxisomes
D. Both B and C

Answer to Question #2

D is correct. In eukaryotes, beta oxidation occurs predominately in mitochondria, but some forms of fatty acids are also processed in peroxisomes.

3. Which steps of beta oxidation provide products used to create ATP for energy?
A. Steps 1, 3, and 4
B. Steps 2 and 4 only
C. Steps 3 and 4 only
D. None of the above

Answer to Question #3

A is correct. Steps 1 and 3 generate FADH₂ and NADH, which go into the electron transport chain. In step 4, acetyl-CoA is produced, which enters the citric acid cycle.

References

• Eaton, S., Bartlett, K., & Pourfarzam, M. (1996). Mammalian mitochondrial β-oxidation. The Biochemical Journal, 320, 345-357.


• Houten, S.M., & Wanders, R.J.A. (2010). A general introduction to the biochemistry of mitochondrial fatty acid β-oxidation. Journal of Inherited Metabolic Disorders, 33, 469-477. DOI: 10.1007/s10545-010-9061-2


• Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V., & Jackson, R.B. (2014). Campbell Biology (10th ed.). Boston, MA: Pearson Education, Inc.

Beta Oxidation