Cyclopropane fatty acids (CPA) are a subgroup of fatty acids that contain a cyclopropane group.
Occurrence Although cyclopropane fatty acids are usually rare, the seed oil from lychee contains nearly 40% CPAs in the form of their triglycerides.Gaydou, E. M.; Ralaimanarivo, A.; Bianchini, J. P. "Cyclopropanoic Fatty-Acids of Litchi (Litchi-Chinensis) Seed Oil - a Reinvestigation" J. Agr. Food Chem. 1993, vol. 41, pp. 886-890.
Olibanic acids are cyclopropane-containing undecylic acids that are implicated for their distinctive odor of frankincense.
Biosynthesis CPAs are derived from unsaturated fatty acids by cyclopropanation. The methylene donor is a methyl group on S-adenosylmethionine (SAM). The conversion is catalyzed by cyclopropane-fatty-acyl-phospholipid synthase.Grogan DW, Cronan JE Jr: Cyclopropane ring formation in membrane lipids of bacteria" Microbiol Mol Biol Rev 1997, vol. 61, pp. 429–441. The mechanism is proposed to involve transfer of a CH3+ group from SAM to the alkene, followed by deprotonation of the newly attached methyl group and ring closure.Ludger A. Wessjohann and Wolfgang Brandt, Thies Thiemann "Biosynthesis and Metabolism of Cyclopropane Rings in Natural Compounds" Chem. Rev., 2003, volume 103, pp 1625–1648.
Cyclopropene fatty acids
Cyclopropene fatty acids are even rarer molecules. The best-known examples of fatty acids containing cyclopropene rings are malvalic acid and sterculic acid. A triglyceride made of 3 sterculic acid molecules esterified with glycerol is present in Sterculia foetida seed oil (in amounts greater than 60%) and at low levels in the seed oil of other species of Malvaceae (~12%), cottonseed oil (~1%). The cyclopropene ring is destroyed during refining and hydrogenation of the oils. They have attracted interest because they reduce levels of the enzyme stearoyl-CoA 9-desaturase (SCD), which catalyzes the biodesaturation of stearic acid to oleic acid.
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