β-Oxidation of Saturated Fatty Acid (Palmitic Acid)

In the process of beta-oxidation, fatty acid molecules are broken down into energy through multiple steps. In particular, beta-oxidation occurs when long fatty acids that are converted to acyl-CoA chains are broken down into increasingly shorter chains. A further metabolic process, the citric acid cycle, or the Krebs cycle, produces ATP as a by-product of this reaction, which releases Acetyl-CoA, FADH2, and NADH. The acetyl-CoA chain is completely broken during beta-oxidation after two acetyl-CoA molecules have been formed. Cells of prokaryotic origin undergo beta-oxidation in the cytosol, but cells of eukaryotic origin undergo it in the mitochondria. It is essential that fatty acids first enter the cell membrane, bind to the coenzyme A (CoA), and form fatty acyl CoA before entering the mitochondria, where beta-oxidation takes place.

The beta-oxidation process occurs in mitochondria of prokaryotic and eukaryotic cells alike. The action of breaking down fat takes place after it has entered the cell and in the mitochondria of eukaryotes. The oxidation of beta fatty acids in peroxisomes can also occur when fatty acid chains are too long for mitochondria to transport. In the first place, fatty acids can cross the cell membrane and enter the cytosol through fatty acid protein transporters, since negatively charged chains cannot pass through the membrane otherwise. Then acyl-CoA is formed by adding a CoA group to the chain by fatty acyl-CoA synthase (FACS).

There are two ways in which the acyl-CoA chain enters the mitochondria, depending on its length:

• As long as the acyl-CoA chain is short, the chain can pass through mitochondrial membranes freely.
• The carnitine shuttle is needed to transport long acyl-CoA chains across the membrane. Once CPT1 binds to the outer mitochondrial membrane, the acylcarnitine translocase (CAT) transports the chain across the mitochondrial membrane. The inner mitochondrial membrane of CPT2 converts the acylcarnitine back into acyl-CoA once it enters the mitochondria. Rather than undergoing beta-oxidation at this point, acyl-CoA is already inside the mitochondria.

Peroxisomes can break down excessive chain lengths by beta-oxidation if the mitochondria can't handle it. A large number of acyl-CoA chains are broken down before entering the beta-oxidation cycle so that they can be transported to the mitochondria. In the peroxisome, beta-oxidation produces H2O2 instead of FADH2 and NADH, resulting in heat production.

Steps in beta-oxidation

There are four steps in beta-oxidation: dehydrogenation, oxidation, hydration, and thyolisis. Various enzymes catalyze each step.

In a nutshell, this process results in one acetyl-CoA chain being shortened to two carbons, one FADH2 and one NADH, and one water being generated in each cycle. A total of 17 ATP molecules are generated per cycle (see below for the breakdown). Two acyl-CoA molecules are formed every time a cycle of two is completed, but two acetyl-CoA molecules are instead formed.

Dehydrogenation

During the first step of acyl-CoA oxidation, the enzyme acyl CoA dehydrogenase oxidizes it. Trans-Δ2enoyl-CoA is formed when the second and third carbons of the acyl-CoA chain undergo a second and third double bond during the beta-oxidation cycle. The citric acid cycle produces ATP when FADH2 is formed due to the use of FAD.

Hydration

This step results in a hydroxyl group (OH) replacing the double bond in C2 in Trans-Δ2-enoyl-CoA, which is converted into L-β-hydroxyacyl CoA. In another enzyme called enoyl CoA hydratase, this reaction is catalyzed. Water is needed for this process.

Oxidation

3-hydroxyacyl-CoA dehydrogenase catalyzes the oxidation of L-β-hydroxyacyl CoA's C2 hydroxyl group by NAD+ and 3-hydroxyacyl-CoA. Ultimately, β-ketoacyl CoA and NADH + H will be produced. After entering the citric acid cycle, NADH produces ATP, which is used for energy.

Thiolysis

Finally, CoA-SH of another molecule (CoA) cleaves β-ketoacyl CoA in the fourth step. β -ketothiolase is the enzyme responsible for this reaction. C2 and C3 are cleaved. Two acyl-CoA chains are resulting from this: the original acyl-CoA chain that was entered into the beta-oxidation cycle and the shorter acyl-CoA chain.

Completion of β oxidation

When even-numbered acyl-CoA chains are beta oxidized to two acetyl-CoA units, a four-carbon chain is broken down. The citric acid cycle produces the molecules of acetyl-CoA. Typical beta-oxidation takes place in the same manner for odd-numbered acyl-CoA chains except for the last step, in which a five-carbon chain is divided into a propionyl-CoA and an acetyl-CoA. After the propionyl-CoA has been converted to succinyl-CoA, it enters the citric acid cycle to create ATP.

End products and energy yield

In the beta oxidation process, we yield 1 FADH2, 1 NADH, and 1 acetyl-CoA, which is about equal to 17 ATP molecules:

• 1 FADH2 (x 2 ATP) = 2 ATP
• 1 acetyl-CoA (x 12 ATP) = 12 ATP
• 1 NADH (x 3 ATP) = 3 ATP
• 2 + 3 + 12 = 17 ATP – total

Although the theoretical ATP yield is higher than the actual yield, it is still higher than the real yield. During each beta-oxidation cycle, about 12 to 16 ATPs are generated. During each cycle, the chain of fatty acyl-CoA gains two carbons, in addition to energy yield. Furthermore, beta-oxidation yields large quantities of water, which, given the difficulty of accessing drinkable water by eukaryotes such as camels, is of benefit.

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