Odd-Numbered Chain and Branched Fatty Acids

Although the fatty acid oxidation scheme works neatly for even-numbered chain lengths, it can t work completely for fatty acids that contain an odd number of carbons. P-oxidation of these compounds leads to propionyl-CoA and acetyl-CoA, rather than to two acetyl-CoA at the final step. The propionyl-CoA is not a substrate for the TCA cycle or other simple pathways. Propionyl-CoA undergoes a carboxylation reaction to form methylmalonyl-CoA. This reaction requires biotin as a cofactor, and is similar to an essential step in fatty acid biosynthesis. Methylmalonyl-CoA is then isomerized by an epimerase and then by methylmalonyl-CoA mutase—an enzyme that uses Vitamin Bi2 as a cofactor—to form succinyl-CoA, which is a TCA-cycle intermediate. 

Branched-chain fatty acids present a problem of a similar kind. For example, phytanic acid, found in animal milk, can t be oxidized directly by P-oxidation because the addition of water is a problem at the branched P-carbon. 

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