Cyclopropane fatty acid
Cyclopropane fatty acids are a subgroup of fatty acids that contain a cyclopropane group. Although they are usually rare, the seed oil from lychee contains nearly 40% CPAs in the form of triglycerides.
Biosynthesis
CPAs are derived from unsaturated fatty acids by cyclopropanation. The methylene donor is a methyl group on S-adenosylmethionine. The conversion is catalyzed by cyclopropane-fatty-acyl-phospholipid synthase. 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.Cyclopropene fatty acids
Cyclopropene fatty acids are even rarer molecules. The best-known examples are malvalic acid and sterculic acid. A triglyceride made of 3 sterculic acid molecules esterified with glycerol is present in Sterculia foetida seed oil and at low levels in the seed oil of other species of Malvaceae, cottonseed oil. 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, which catalyzes the biodesaturation of stearic acid to oleic acid.At least one review indicates that these compounds are carcinogenic.