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Beta Oxidation of Acyl-CoA
The second step of fatty acid degradation is beta oxidation. Fats are broken down and converted to Acyl-CoA for beta oxidation to be used as energy sources during high energy demands such as exercise. When energy demands are high, hormone secretions release fatty acids from adipose tissues so they can be broken down for energy when alternate energy sources are low in concentration.

Beta oxidation occurs in the mitochondria of the cell. After activation of fatty acids in the cytosol, Acyl-CoA must be transported to the mitochondria so that beta oxidation can break down Acyl-CoA to provide energy. There are no transport proteins to bring Acyl-CoA into the mitochondria, so to get into the mitochondria carnitine palmitoyltransferase 1 (CPT1) converts Acyl-CoA into acylcarnitine which gets transported into the mitochondrial matrix. Once in the matrix, acylcarnitine is converted back to Acyl-CoA by CPT2. Beta oxidation may begin now that Acyl-CoA is in the mitochondria.

There are four steps to beta oxidation once the Acyl-CoA has entered the matrix.

1.      Acyl-CoA dehydrogenase removes 2 hydrogens from Acyl-CoA to create a double bond between the alpha and beta carbons. This also reduces FAD to FADH2.

2.      Enoyl-CoA hydrase adds water across the newly formed double bond to make an alcohol.

3.      3-hydroxyacyl-CoA dehydrogenase oxidizes the alcohol group to a ketone. This produces one molecule of NADH from NAD+.

4.      Thiolase cleaves between the alpha carbon and ketone to release one molecule of Acetyl-CoA and the Acyl-CoA which is now 2 carbons shorter.

This four step process repeats until Acyl-CoA has removed all carbons from the chain, leaving only Acetyl-CoA. During one cycle of beta oxidation, Acyl-CoA creates one molecule of Acetyl-CoA, FADH2, and NADH. Acetyl-CoA is then used in the citric acid cycle while FADH2 and NADH are sent to the electron transport chain. These intermediates all end up providing energy for the body as they are ultimately converted to ATP. Beta oxidation, as well as alpha-oxidation, also occurs in the peroxisome. The peroxisome handles beta oxidation of fatty acids that have more than 20 carbons in their chain because the peroxisome contains very-long-chain Acyl-CoA synthetases. These enzymes are better equipped to oxidize Acyl-CoA with long chains that the mitochondria cannot handle.

Example using Stearic Acid
Beta oxidation removes 2 carbons at a time, so in the oxidation of an 18 carbon fatty acid such as Stearic Acid 8 cycles will need to occur to completely break down Acyl-CoA. This will produce 9 Acetyl-CoA that have 2 carbons each, 8 FADH2, and 8 NADH.