Beta oxidation of fatty acids

The process of beta oxidation is named after the carbon atom in the beta position of the fatty acyl-CoA which becomes the most oxidized during the cyclic redox reactions that remove C2 units in form of acetyl-CoA from the fatty acyl chain. The beta carbon becomes the new carboxyl end of the shortened (n-2) fatty acyl-CoA. The oxidation steps are strictly analogous to the reaction steps in the citric acid cycle converting succinyl-CoA to oxaloacetate involving an initial oxidation by acyl-CoA dehydrogenase (EC 1.3.99.3; driven by FAD reduction), an hydration by enoyl-CoA hydratase (EC 4.2.1.17), and a second oxidation by hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35 driven by NAD+ reduction). A C2 unit is released by beta-ketothiolase (EC 2.3.1.16) to produce acetyl-CoA and a shortened acyl(n-2)-CoA. The latter is recycled until the acyl chain is shortened to its acetyl-CoA end product and oxidized by the citric acid cycle enzymes.

The acyl-CoA dehydrogenase is specific for the length of the acyl chain being oxidized. Three types of the dehydrogenase exist in mitochondria; type I (EC 1.3.99.12; long chain) which oxidizes C12-C18 fatty acids, type II (EC 1.3.99.3) which oxidizes C4-C14 fatty acids, and type III (EC 1.3.99.2; butyryl dehydrogenase) which oxidizes C4 and C6 acyl-CoA substrates. The energy yield per cycle is 5 mols of ATP for each round, 2 mols per FADH2 (goes into complex II) and 3 mols per NADH/H+ (goes into complex I). The energy balance for palmitic acid (sixteen carbon atoms)