Table of Contents
- What is Beta Oxidation?
- Where does Beta Oxidation Occur?
- Beta Oxidation Cycle
- Beta Oxidation Products
- Lesson Summary
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.
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.
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.
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.
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.
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.
The third step of beta-oxidation results in the conversion of L-3-Hydroxyacyl-CoA to 3-Ketoacyl-CoA. This reaction is catalyzed by Hydroxyacyl-CoA-Dehydrogenase. The coenzyme NAD+ oxidizes the hydroxyl group by removing two electrons and a hydrogen to form NADH. An additional hydrogen atom is removed to form a double bond between C3 and O. Similar to the FADH2 from the first step, NADH and H+ go to the electron transport chain to fuel energy production.
The final step of beta-oxidation is facilitated by the enzyme Thiolase and results in the formation of an acetyl-CoA molecule and an acyl-CoA molecule. This reaction is a cleavage reaction carried out by CoA-SH, a thiol group with a CoA molecule attached to it. The thiol group cleaves the C2-C3 bond (hence beta-oxidation), releasing the acetyl-CoA molecule and donating its own CoA molecule to reform acyl-CoA. The acetyl-CoA molecule enters the citric acid cycle where it is utilized for further energy production. The acyl-CoA molecule enters the beta-oxidation cycle again, and the steps are repeated until the fatty acid molecule has been completely broken down.
Beta-oxidation reaches its final cycle when the starting fatty acid has gone through enough rounds to produce a four-carbon (in the case of a starting fatty acid with an even number of carbons) or five-carbon (in the case of a starting fatty acid with an odd number of carbons) acyl-CoA. When a four-carbon acyl-CoA enters its final round of beta-oxidation, two acetyl-CoA molecules are produced. In the case of a five-carbon acyl-CoA, the final round of beta-oxidation produces an acetyl-CoA and a propionyl-CoA molecule. The propionyl-CoA is converted to succinyl-CoA for entry into the citric acid cycle. At this point, the fatty acid has been completely broken down.
The products of beta-oxidation can be broken up into two groups: those that go to the electron transport chain immediately, and those that need further processing. FADH2, NADH, and H+ fall into the former group. These molecules go to the electron transport chain where they power ATP production. Acetyl-CoA (and succinyl-CoA in the case of odd-numbered fatty acids) must first enter the citric acid cycle for further energy harvesting. The reactions of the citric acid cycle also result in the formation of NADH and H+ and further production of ATP via the electron transport chain.
Beta-oxidation is the process by which prokaryotes and eukaryotes break down fatty acids. In prokaryotes, this process occurs in the cytosol, while eukaryotes carry out beta-oxidation in the mitochondria and peroxisomes. Beta-oxidation in the peroxisomes is primarily performed to shorten very long-chain fatty acids so that they can be transported into the mitochondria.
Beta-oxidation is a four-step process, which repeats until the fatty acid has been completely broken down. The four steps are dehydrogenation, hydration, oxidation, and thiolysis. Dehydrogenation is catalyzed by Acyl-CoA-dehydrogenase and coverts FAD to FADH2 to form a double bond between C2 and C3. Hydration results in a hydroxyl group on C3 as a result of the double bond being attacked by a water molecule. This reaction is catalyzed by Enoyl-CoA-Hydrase. Oxidation converts the hydroxyl group to a carbonyl group via the formation of NADH and H+ from NAD+. This step is catalyzed by Hydroxylacyl-CoA-Dehydrogenase. Thiolysis cleaves off an acetyl-CoA molecule and replaces the CoA to form an acyl-CoA that is two carbons shorter than the one that entered the process.
The cycle repeats until a four-carbon acyl-CoA is cleaved into two acetyl-CoA molecules or a five-carbon acyl-CoA is cleaved into an acetyl-CoA and propionyl-CoA molecule. A propionyl-CoA molecule is subsequently converted to succinyl-CoA. FADH2, NADH, and H+ donate electrons and protons to the electron transport chain to make ATP. Acetyl-CoA and succinyl-CoA (when applicable) enter the citric acid cycle.
Each round of beta-oxidation can be summed up using the following equation:
Acyl(n)-CoA + FAD + H2O + (NAD+) + SH-CoA = Acyl(n-2) + FADH2 + NADH + (H+) + acetyl-CoA
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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.
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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