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In this lesson, you’ll discover what beta oxidation is and what it is used for. You’ll also learn a little bit about fatty acids and the steps involved in their breakdown.

What Is Beta-Oxidation?

Nowadays, many people worry about fat content in their food. Most nutrition labels will have something about the fat content found in a particular product. What does fat have to do with beta-oxidation?First you need to understand what fat, or a fatty acid, is. A fatty acid is composed of a carbon atom, double bonded to an oxygen with a string of carbons and hydrogens attached. A fatty acid is saturated when all the bonds between the atoms are single bonds.How does this affect you? Your body can make fatty acids or use fatty acids for energy. Your body makes fatty acids when energy needs are low and nutrients are abundant.

When you need energy (as in cases of severe starvation, intense exercise, or stress), your body sends fatty acids to be broken down for energy. Beta-oxidation is the process by which fatty acids, specifically saturated fatty acids, are broken down for use in energy production.

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Players in Beta-Oxidation

So beta-oxidation is a good thing, right? It breaks down fatty acids.

Imagine the process of beta-oxidation as a play. First, you need to know the setting, the stage if you will. Beta-oxidation most often takes place inside the mitochondrial matrix, although it can also occur in organelles called peroxisomes. Next up is the cast. The main roles are taken by saturated fatty acids, and a molecule called coenzyme A, also called CoA.

Every good play needs co-stars. The co-stars of the beta-oxidation process are called enzymes. Enzymes are proteins that speed up chemical reactions. Extras are also needed for a play. The extras here are the molecules of water, ATP, FAD+, and NAD+.

So now we know the cast, it’s time to get into the play.

Steps of Beta-Oxidation

Every good play has a plot. The main plot of beta-oxidation is that the fatty acids are looking to change themselves, or be broken down. Before we get to the main plot, a little bit of exposition is needed to set the scene. The enzyme, acyl-CoA ligase, uses adenosine triphosphate, or ATP, to join a fatty acid with CoA. They form a new happy molecule called acyl-CoA. It is as acyl-CoA that the fatty acids are able to broken down in the mitochondrial matrix.

There are four main steps,or acts, in beta-oxidation:

  1. Loss of hydrogens
  2. Addition of water
  3. Loss of another hydrogen
  4. Addition of another CoA

In the first step, two carbons in the long fatty acid chain, called the alpha and beta carbons, of acyl-CoA are exposed. Acyl-CoA then is attacked by the enzyme, acyl-CoA dehydrogenase, which steals two hydrogens from the alpha and beta carbons.The two hydrogens are passed off to FAD+, which changes it to FADH2. FADH2 continues on to aid in energy production. In the meantime, acyl-CoA changes its name to alpha beta-Enoyl-CoA in honor of the lost hydrogens.In the next step, after having been attacked, the enzyme, enoyl-CoA hydrase, adds water to alpha, beta-enoyl-CoA, giving a hydrogen back to the alpha carbon and adding an oxygen to the beta carbon. In response, alpha, beta-enoyl-CoA changes its name to 3-hydroxyacyl-CoA.

3-hydroxyacyl-CoA isn’t left alone for long. In the third step, another enzyme, 3-hydroxyacyl dehydrogenase, swipes the newly gained hydrogen from the oxygen attached to the beta carbon and gives one to NAD+, making it NADH. NADH then goes to the electron transport chain to produce energy. In lament of yet more hydrogen loss, the molecule is renamed beta-ketoacyl-CoA.

In the final step, beta-ketoacyl-CoA meets the enzyme thiolase. Thiolase separates the alpha and beta carbon, giving the alpha carbon a hydrogen and attaching it to its own CoA. The beta carbon group, having returned to looking like its old self, minus two carbons, renames itself acyl-CoA. Meanwhile the alpha carbon group with its new hydrogen and CoA becomes acetyl-CoA, a very desirable molecule.

Acetyl-CoA can go into the citric acid cycle where it is used by the cell to produce energy and ultimately ends up as CO2, hydrogen, and CoA, or it can be used as a basis to make fatty acids. The newly re-formed acyl-CoA minus the two carbons is doomed to return to the first step, losing two carbons each time and forming acetyl-CoA. Until ultimately, when only four carbons are left, two acetyl-CoAs are formed.

The ‘play’ beta-oxidation can be summed up by this equation:Acyl(n) – CoA + (FAD+) + (NAD+) + H2O + CoA = Acyl(n – 2) – CoA + FADH2 + NADH + (H+) + acetyl – CoA

Lesson Summary

Beta-oxidation is the breakdown of saturated fatty acids to aid in energy production. Fatty acids are strings of carbon and hydrogens attached to carboxyl groups (C=O).ATP attaches a fatty acid chain to coenzyme A (CoA), which then becomes acyl-CoA. Acyl-CoA is taken into the mitochondrial matrix, where it loses two carbons, and acetyl-CoA is produced.

The steps in beta-oxidation are as follows:

  1. Acyl-CoA dehydrogenase removes a hydrogen from the alpha and beta carbons. FAD+ takes the hydrogens to become FADH2 and uses them to make energy.
  2. Water is added to alpha beta-Enoyl-CoA by Enoyl-CoA hydrase.
  3. 3-Hydroxylacyl deydrogenase removes a hydrogen.

    NAD+ takes the hydrogen to become NADH and uses it to make energy.

  4. Thiolase adds a CoA to make acetyl-CoA and an acyl-CoA minus two carbons.

The process repeats with the new acyl-CoA until only acetyl-CoA is formed in step 4.

Acetyl-CoA is used by the cell for energy, becoming CO2, hydrogen, and CoA or to create fatty acids.

Learning Outcomes

When you’re through with the video, you should be able to:

  • Define fatty acid and understand what it means when a fatty acid is saturated
  • Explain what beta-oxidation is
  • Describe the components of beta-oxidation
  • List and describe the steps of beta-oxidation
  • Recall the equation that sums up beta-oxidation

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