Methylmalonyl-CoA is present biologically in both the D and L isomers, which can be interconverted by the enzyme methylmalonyl-CoA epimerase (Figure 18.19). This reaction is important in the metabolism of propionyl-CoA, which arises from oxidation of odd-numbered chains of fatty acids. L-methylmalonyl-CoA is further acted upon by methylmalonyl-CoA mutase.