Beta-oxidation of fatty acids: saturated, unsaturated, and odd-chain

Localization of the process and fate of fatty acids

Beta-oxidation is a catabolic pathway for the breakdown of higher fatty acids that occurs in almost all cells requiring energy. During starvation or increased energy demand, adipocyte triacylglycerols are hydrolyzed, and the released fatty acids are transported via the bloodstream to consumer tissues.

Upon entering the cell, fatty acids first end up in the cytoplasm, whereas beta-oxidation enzymes are localized in the mitochondrial matrix. The inner mitochondrial membrane is impermeable to long-chain fatty acids (14 carbons or more); short-chain ones (up to 12 C atoms) pass freely.

Activation and the carnitine shuttle

Before transport, the fatty acid must be activated. The reaction is catalyzed by acyl-CoA synthetase with the expenditure of ATP and the attachment of coenzyme A; the product is acyl-CoA. It is important not to confuse acyl-CoA (the fatty acid residue) and acetyl-CoA (the two-carbon fragment).

The transfer of the acyl residue into the matrix is carried out in three stages:

• Carnitine acyltransferase I on the outer membrane transfers the acyl from CoA to carnitine (at the OH group), forming acyl-carnitine.
• The inner membrane translocase transports acyl-carnitine into the matrix down its concentration gradient.
• Carnitine acyltransferase II in the matrix returns the acyl to the matrix coenzyme A; free carnitine is shuttled back.

Beta-oxidation of saturated fatty acids

The name of the process stems from the fact that chemical transformations affect the beta-carbon atom (the second from the carboxyl end; the first is alpha). In one cycle, a two-carbon fragment is cleaved from acyl-CoA in the form of acetyl-CoA. The cycle includes four sequential reactions:

1. Dehydrogenation — acyl-CoA dehydrogenase forms a double bond between the alpha- and beta-carbons; the coenzyme FAD is reduced to FADH₂.
2. Hydration — enoyl-CoA hydratase adds water across the double bond: H to the alpha-carbon, OH to the beta-carbon; beta-hydroxyacyl-CoA is formed.
3. Dehydrogenation at the beta-carbon — beta-hydroxyacyl-CoA dehydrogenase oxidizes the OH group to a keto group; the coenzyme NAD⁺ is reduced to NADH.
4. Thiolytic cleavage — beta-ketoacyl-CoA thiolase, with the participation of a second molecule of coenzyme A, cleaves off acetyl-CoA; the acyl-CoA, shortened by two carbons, enters a new cycle.

The process is cyclical: palmitic acid (16 C) yields 8 molecules of acetyl-CoA in 7 cycles. The resulting acetyl-CoA enters the tricarboxylic acid cycle, while NADH and FADH₂ enter the electron transport chain.

Oxidation of unsaturated fatty acids

In naturally occurring unsaturated fatty acids, double bonds have the cis configuration, whereas enoyl-CoA hydratase only works with the trans isomer. When oxidation reaches the position of the double bond, an additional enzyme comes into play — enoyl-CoA isomerase, which converts the cis bond into the trans form.

Since a double bond is already present in the substrate, the first reaction of the cycle (dehydrogenation with FAD) is bypassed: the process starts immediately with hydration. After passing the double bond, oxidation proceeds according to the standard four-reaction scheme.

Oxidation of odd-chain fatty acids

In the penultimate cycle of such an acid, a five-carbon residue remains; after another cleavage of acetyl-CoA, a three-carbon propionyl-CoA is formed at the end. Its further fate:

• Propionyl-CoA carboxylase (coenzyme — biotin) attaches CO₂, forming D-methylmalonyl-CoA.
• Methylmalonyl-CoA epimerase converts the D-form to L-methylmalonyl-CoA.
• Methylmalonyl-CoA mutase (coenzyme — vitamin B₁₂, cobalamin) rearranges the molecule into succinyl-CoA.

Succinyl-CoA is a metabolite of the Krebs cycle, so the propionyl residue is incorporated into the general catabolic pathway and used for energy production.

Key points

• Beta-oxidation takes place in the mitochondrial matrix and provides the cell with energy during the breakdown of fatty acids.
• The activation of a fatty acid to acyl-CoA is catalyzed by acyl-CoA synthetase with the expenditure of ATP.
• Long-chain acyl-CoAs are transported into the matrix via the carnitine shuttle (CAT-I, translocase, CAT-II).
• One cycle includes dehydrogenation (FAD), hydration, dehydrogenation (NAD⁺), and thiolytic cleavage with the release of acetyl-CoA.
• During the oxidation of unsaturated acids, enoyl-CoA isomerase converts the cis bond to trans, and the FAD reaction is bypassed.
• Odd-chain acids yield propionyl-CoA, which is converted into succinyl-CoA via carboxylation and a B₁₂-dependent mutase.
• The end products are acetyl-CoA for the Krebs cycle, as well as NADH and FADH₂ for the electron transport chain.