Most fatty acids in the body contain an even number of carbons. The normal end-products of -oxidation of these compounds are all acetyl-CoA. -oxidation of fatty acids containing an odd number of carbons yields a propionyl-CoA in the last step. Propionyl-CoA cannot be used directly in the citric acid cycle. Instead, it is converted, in the following reactions (Figure 18.19), to succinyl-CoA.

1. Propionyl-CoA + ATP + HCO3^- <=> D-Methylmalonyl-CoA + ADP + Pi (catalyzed by Propionyl-CoA Carboxylase)

2. D-Methylmalonyl-CoA <=> L-Methylmalonyl-CoA (catalyzed by Methylmalonyl-CoA Epimerase)

3. D-Methylmalonyl-CoA <=> Succinyl-CoA (catalyzed by Methylmalonyl-CoA Mutase)

Methylmalonyl-CoA mutase, the catalyst for the third reaction in this process, requires vitamin B12 as a cofactor. The inability to catabolize propionyl-CoA properly has severe consequences in humans. Severe acidosis results, lowering blood pH and damaging the central nervous system.

-Oxidation