Cyclopropane fatty acid

Cyclopropane fatty acids (CPA) are a subgroup of fatty acids that contain a cyclopropane group.^[1] Although they are usually rare, the seed oil from lychee contains nearly 40% CPAs in the form of triglycerides.^[2]

Examples of mycolic acids, a class of mono- and dicyclopropane fatty acids found in the cell wall of the bacterium Mycobacterium tuberculosis

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.^[3] 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.^[4]

Cyclopropene fatty acids

malvalic acid, a cyclopropene fatty acid

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 (in amounts greater than 60%) and at low levels in the seed oil of other species of Malvaceae^[a] (~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.^[7]

At least one review indicates that these compounds are carcinogenic.^[8]

Notes

1. A number of species in several genera of the Malvaceae family are known as 'Kapok trees'. For example: Ceiba pentandra, Bombax ceiba and B. costatum are all widely and ambiguously called "the kapok tree".^[5][6]