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Beta oxidation

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Beta oxidation

In biochemistry and metabolism, beta oxidation (also β-oxidation) is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA. Acetyl-CoA enters the citric acid cycle, generating NADH and FADH2, which are electron carriers used in the electron transport chain. It is named as such because the beta carbon of the fatty acid chain undergoes oxidation and is converted to a carbonyl group to start the cycle all over again. Beta-oxidation is primarily facilitated by the mitochondrial trifunctional protein, an enzyme complex associated with the inner mitochondrial membrane, although very long chain fatty acids are oxidized in peroxisomes.

The overall reaction for one cycle of beta oxidation is:

Free fatty acids cannot penetrate any biological membrane due to their negative charge. Free fatty acids must cross the cell membrane through specific transport proteins, such as the SLC27 family fatty acid transport protein. Once in the cytosol, the following processes bring fatty acids into the mitochondrial matrix so that beta-oxidation can take place.

Once the fatty acid is inside the mitochondrial matrix, beta-oxidation occurs by cleaving two carbons every cycle to form acetyl-CoA. The process consists of 4 steps.

This acetyl-CoA then enters the mitochondrial tricarboxylic acid cycle (TCA cycle). Both the fatty acid beta-oxidation and the TCA cycle produce NADH and FADH2, which are used by the electron transport chain to generate ATP.

Fatty acids are oxidized by most of the tissues in the body. However, some tissues such as the red blood cells of mammals (which do not contain mitochondria) and cells of the central nervous system do not use fatty acids for their energy requirements, but instead use carbohydrates (red blood cells and neurons) or ketone bodies (neurons only).

Because many fatty acids are not fully saturated or do not have an even number of carbons, several different mechanisms have evolved, described below.

Once inside the mitochondria, each cycle of β-oxidation, liberating a two carbon unit (acetyl-CoA), occurs in a sequence of four reactions:

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