What is Beta-Oxidation?

Beta-oxidation is a metabolic process whereby fatty acids are broken down into acetyl-CoA. There are four reactions in the process, and these reactions repeat until the entire fatty acid chain has been converted into individual acetyl-CoA molecules. Each of these molecules is then processed for energy. This process occurs in the mitochondria of the cell.

The molecules involved in beta-oxidation include the fatty acid chain and coenzyme A. Coenzyme A reacts with the fatty acid chain to produce fatty acyl-CoA. The fatty acid chain is now activated and ready to be processed.

Beta oxidation is regulated by malonyl-CoA. This is an enzyme which prevents fatty acyl-CoA from entering the mitochondria. By controlling the molecules that transport fatty acyl-coA into the mitochondria, beta oxidation can be initiated or stopped. When there is a need for more energy, malonyl-CoA usually will allow fatty acyl-CoA to be transported into the mitochondria. The molecule which malonyl-CoA targets is called carnitine acyltranferase.

The first reaction of beta-oxidation utilizes the enzyme acyl-CoA dehydrogenase. This is actually made up of three enzymes that are embedded in the mitochondrial matrix and each has an FAD molecule. The enzymes oxidize the activated fatty acid chain, while FAD is reduced. This means that an hydrogen molecule is transferred to FAD to produce FADH2.

Afterward, another reaction takes place that is catalyzed by the enzyme enoyl-CoA hydratase. In this reaction, water is added to the fatty acid chain to produce hydroacyl-CoA. This is the second reaction of beta-oxidation.

The third reaction involves the enzyme L-hydroxyacyl-CoA dehydrogenase. It contains the NAD+ coenzyme. In this step, a beta-ketoacyl-CoA derivative is produced from the oxidation of the hydroacyl-CoA. During the process, the NAD+ is converted to NADH.

In the last step of this process, thiolase is the enzyme that is utilized. It cuts the beta-ketoacyl-CoA into an acetyl-CoA molecule and a fatty acid chain that is reduced by two carbons. The acetyl-CoA molecule is the final product of beta oxidation, and the remaining fatty acid chain undergoes the same four reactions again. The process will repeat until the final two products are two acetyl-CoA molecules.

For unsaturated fatty acids, i.e, fatty acids with a single double bond, beta oxidation occurs normally until the enzymes reach the double bond. Enoyl-CoA isomerase then converts the double bond from cis to trans, and beta oxidation continues. Cis and trans describes the orientation of the double bond.