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The beta-oxidation of fatty acids in mitochondria progressively shortens fatty acids two-carbons at a time as acetyl-CoA units are removed with each round of the cycle. Fatty acids that enter beta-oxidation with an even number of carbons are converted entirely to acetyl-CoA, with the last round producing two acetyl-CoA molecules from one four carbon fatty acid. A fatty acid that enters beta-oxidation with 18 carbons, for example, will produce 9 acetyl-CoA units. Fatty acids that have an odd number of carbons present a special challenge. The last round of beta-oxidation with a five-carbon chain releases acetyl-CoA and the 3-carbon chain propionyl-CoA. Propionyl-CoA is not a common metabolic intermediate in other pathways, so it must be converted into something else to be effectively metabolized. In addition to production from fatty acids, propionyl-CoA is derived from the metabolism of several fatty acids, including isoleucine, valine and methionine. Propionyl-CoA is converted to succinyl-CoA, an intermediate in the Kreb\'s cycle. Propionyl-CoA has three carbons and succinyl-CoA has four carbons, so one of the first steps in this pathway is the carboxylation of propionyl-CoA with an input of energy from ATP. Mutation of propionyl-CoA carboxylase has been observed to cause a genetic condition called propionic acidemia. The rearrangement of this molecule, first between two stereoisomers, and then two structural isomers, results in succinyl-CoA. (This definition may be outdated - see the DesignNote.)
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