What is Beta-Oxidation?

Brittany Stork, Darla Reed
  • Author
    Brittany Stork

    Brittany taught high school mathematics for two years. They have a B.S. in Biological Sciences and Secondary Mathematics Education from the University of Nebraska-Lincoln and a Ph.D. in Cellular and Molecular Biology from Baylor College of Medicine. They tutored student-athletes at University of Nebraska-Lincoln for 5 years in various math and science classes. Brittany has served as a TA for various undergraduate and graduate level biology classes. They also are a CLRA Level II certified tutor.

  • Instructor
    Darla Reed

    Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

Learn what Beta Oxidation is. Understand where Beta Oxidation occurs and the whole pathway and mechanism of Beta Oxidation. Discover what the Beta Oxidation products are. Updated: 01/31/2022

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What is Beta Oxidation?

Eating good food is one of the simple pleasures in life. A piece of decadent chocolate cake, a perfectly cooked steak, or a slice of freshly baked bread can add a touch of extravagance to an otherwise ordinary day. However, food is not eaten simply for enjoyment. Humans and all other living organisms need to ingest food to make energy.

Living organisms consume or produce energy-storing molecules including carbohydrates, proteins, and fats. These molecules are subsequently broken down when energy levels are low. Carbohydrates, proteins, and fats each have specific processes to carry out their digestion. This lesson will focus on beta-oxidation, the catabolic process which breaks down fatty acids molecules to harvest ATP.

Beta-oxidation occurs in both prokaryotes and eukaryotes. In prokaryotes, fatty acids are broken down in the cytosol. In eukaryotes, beta-oxidation occurs in both mitochondria and peroxisomes. Through the reactions in beta-oxidation, acetyl-CoA, NADH, H+, and FADH2 are produced. NADH and FADH2 are coenzymes that transport electrons to the electron transport chain to produce ATP. The acetyl-CoA enters the citric acid cycle, where it is oxidized to harvest even more energy. The rest of this lesson will explore the location, steps, and products of beta-oxidation more in-depth.

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Where does Beta Oxidation Occur?

Prokaryotes are single-celled organisms that lack membrane-bound organelles. Bacteria and archaea are examples of prokaryotic organisms. Because prokaryotes do not have organelles, all of their reactions occur in the cytosol, including beta-oxidation. Fatty acids are transported across the plasma membrane into the cytosol and modified to form fatty acyl-CoAs by the addition of coenzyme A (CoA). This modification allows for the cytosolic enzymes involved in beta-oxidation to identify the fatty acids and begin the cyclic process of breaking them down.


Depiction of a standard prokaryotic cell. As prokaryotes lack organelles, beta-oxidation occurs in the cytosol.

Image of an average prokaryotic cell. Note that there are no organelles. Beta-oxidation occurs in the cytosol of prokaryotes.


As expected, beta-oxidation in eukaryotes is more complicated. Unlike prokaryotes, eukaryotes have membrane-bound organelles. Two of these, the mitochondria and the peroxisomes, contain the specialized enzymes necessary for beta-oxidation. Importantly, ATP is produced during beta-oxidation in the mitochondria, but not in the peroxisomes. Like the process in prokaryotes, fatty acids that are transported into the cell are converted to fatty acyl-CoAs. However, these fatty acyl-CoAs are transported into the mitochondria or peroxisomes instead of remaining in the cytoplasm, as is the case in prokaryotes. Fatty acyl-CoAs that are transported to the peroxisome immediately enter beta-oxidation.

Conversely, fatty acids destined for the mitochondria must undergo additional modifications. Fatty acyl-CoAs are converted to acyl-carnitines at the outer mitochondrial membrane via an enzyme called CptI. The acyl-carnitine then enters the inter-membrane space. Next, the acyl-carnitine is transported across the inner mitochondrial membrane into the matrix via an enzyme called translocase. Once inside the matrix, the acyl-carnitine is converted back to acyl-CoA by the enzyme CptII. At this point, the acyl-CoA is ready to enter beta-oxidation. While the mitochondria are responsible for breaking down most fatty acids, they are unable to transport very long-chain fatty acids into the matrix. The peroxisomes, however, can uptake very long-chain fatty acids, and thus are charged with the pre-processing of very long-chain fatty acids to shorten them for transport into the mitochondria.


Image showing how fatty acids are transported into the mitochondrial matrix. Fatty acyl-CoA is first converted to fatty acyl-carnitine, which enters the intermembrane space. The acyl-carnitine is then transported across the inner membrane into the matrix. Here, the acyl-carnitine is converted back to an acyl-CoA and beta-oxidation begins.

Depiction of the process of transporting fatty acids into the mitochondrial matrix. In eukaryotes, beta-oxidation occurs in the mitochondrial matrix and peroxisomes.


Beta Oxidation Cycle

Now that we have discussed the location of and preparatory steps for beta-oxidation, let us now look at the actual process of beta-oxidation. In both prokaryotes and eukaryotes, a round of beta-oxidation consists of four reactions. Briefly, the steps are as follows:

1. Dehydrogenation - oxidation of the acyl-CoA via the removal of two hydrogen atoms

2. Hydration - addition of a water molecule and formation of a hydroxyl (OH) group

3. Oxidation - oxidation of the hydroxyl group via the removal of two hydrogen atoms

4. Thiolysis - cleavage to release an acetyl-CoA

As each round of beta-oxidation produces a two-carbon acetyl-CoA, it takes several rounds to completely break down a fatty acid. Given this, it is useful to think of beta-oxidation as a cycle. The cycle continues until a four or five-carbon fatty acyl-CoA remains. Let us look at each of these steps in a little more detail.

Dehydrogenation

In the first step of the beta-oxidation cycle, the acyl-CoA is oxidized to form trans-delta 2-Enoyl-CoA. This reaction is carried out by Acyl-CoA-Dehydrogenase and results in the formation of a double bond between C2 and C3 (the second and third carbons). The C2 carbon is also referred to as the alpha carbon and the C3 carbon is known as the beta carbon, hence the process being named beta-oxidation. The coenzyme FAD accepts two electrons and two hydrogen atoms during the oxidation of acyl-CoA and is converted to FADH2. The FADH2 then transports the electrons and hydrogens to the electron transport chain.


Dehydrogenation is the first step in beta-oxidation. FAD accepts two protons and two electrons from acyl-CoA to form FADH2. This results in a double bond between C2 and C3. This reaction is catalyzed by Acyl-CoA-Dehydrogenase.

Image depicting dehydrogenation, the first step in beta-oxidation.


Hydration

In the second step of beta-oxidation, a water molecule attacks the double bond that was formed in the first step. This results in the addition of a hydroxyl (OH) group to C3 (the beta carbon). The other hydrogen from the water molecule binds to C2, leading to the double bond between C2 and C3 being converted into a single bond. The enzyme Enoyl-CoA-Hydratase facilitates this reaction and leads to the formation of L-3-Hydroxyacyl-CoA.


Hydration is the second step of beta-oxidation. In this step, a water molecule attacks the double bond, resulting in a hydroxyl group at C3. This reaction is catalyzed by Enoyl-CoA-Hydratase.

Image depicting hydration, the second step of beta-oxidation.


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Frequently Asked Questions

What are the products of beta oxidation?

The products of beta-oxidation are FADH2, NADH, H+, and acetyl-CoA. One of each of these molecules is produced for each round of beta-oxidation a fatty acid goes through.

What are the steps of beta oxidation?

There are four steps in beta-oxidation: dehydrogenation, hydration, oxidation, and thiolysis. Dehydrogenation results in a double bond between C2 and C3 and produces FADH2. Hydration results in a hydroxyl group at C3. Oxidation converts the hydroxyl group to a carbonyl group and produces NADH and H+. Thiolysis cleaves the C2-C3 bond via SH-CoA, releasing an acetyl-CoA and forming another acyl-CoA, which can enter beta-oxidation again.

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