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Fatty Acids: Acetylenic and Allenic

Aliphatic compounds containing acetylenic (triple) and allenic bonds, including alcohols, aldehydes, ketones, hydrocarbons and fatty acids, are widespread in nature. Innumerable different structures (more than 1,000) have been characterized, and such compounds appear to have many different biological functions. In particular, fatty acids with acetylenic bonds are found in seed oils of higher plants, and they are present in the lipids of mosses, fungi and algae. They have also been detected in primitive marine animals, such as sponges, and in some insects. Discussion of all of these natural lipids would be a daunting task, so a few only of the more important of the fatty acids with acetylenic bonds have been selected for discussion below. While their primary functions are in the organisms in which they are produced, often as defense compounds, some natural acetylenic fatty acids appear to have useful pharmaceutical properties, for example as anti-cancer agents, and synthetic analogues are under development for clinical testing. They are chemically unstable and highly reactive compounds, especially in the presence of oxygen.

Monoynoic Fatty Acids from Plants

Tariric (octadec-6-ynoic) acid was identified as long ago as 1892 by Arnaud in seed oils of Picramnia species, where it can amount to as much as 95% of the total fatty acids. It was much later before stearolic (octadec-9-ynoic) acid was detected in a seed oil, although it was well known as a product of chemical synthesis; it is only encountered at low levels in seed oils of the Santalaceae and Olacaceae. These fatty acids have been detected together with an isomer with the acetylenic bond in position 12 in moss and liverwort species

Tariric and stearolic acids

Non-Conjugated Enynoic Fatty Acids from Plants

Crepenynic acid or cis-9-octadecen-12-ynoic acid, sometimes abbreviated to 9c,12a-18:2, was first found in 1964 as a major component of the seed oil of Crepis foetida and subsequently in other seed oils of this and some other plant families.

Crepenynic acid

It is now known to be derived biosynthetically from linoleic acid (see below), and is itself the precursor of a substantial number of acetylenic metabolites with diverse biological functions in plants. Other isomers or related acids found in seed oils include 6Z-octadecen-9-ynoic (from Ongokea klaineana), hexadec-15-en-12-ynoic (‘scleropyric’ from Scleropyrum wallichianum), and octadec-9-yn-17-enoic (from Alvardoa amorphoides) acids; sterculynic acid (cis-9,10-methylene-9-octadecen-17-ynoic acid) might also be considered here but see our web page on cyclic fatty acids. Crepenynic acid has also been detected in mosses together with the isomers 6a,9c-18:2 and 9a,12c-18:2, while related fatty acids with up to three hydroxyl groups have been found in Basidomycetes (fungi). Although it was first detected in the seed oils, crepenynic acid is now know to occur in other parts of higher plants such as the flowers and leaves of Afzelia cuanzensis.

Mosses, algae and fungi can contain polyunsaturated fatty acids similar to the conventional common range of (n-3) and (n-6) families, but in which the first double bond is instead acetylenic. These include 6a,9c,12c-18:3, 8a,11c,14c-20:3, 6a,9c,12c,15c-18:4 and 5a,8c,11c,14c-20:4, the last two of which are illustrated. These are precursors of a wide range of acetylenic oxylipins that are produced on wounding, with defensive activities against attack by fungi, bacteria and even herbivores, such as slugs.

Acetylenic fatty acids from mosses and fungi

Conjugated Enynoic Fatty Acids (One Acetylenic Bond) from Plants

Ximenynic or 11-trans-octadecen-9-ynoic acid was first isolated and identified from seed oils of Ximenia species, where it occurs together with a series of homologues up to C26 in chain-length, presumably produced by chain-elongation. ‘Santalbic’ acid (from Santalum spicatum) is an earlier name, but the structure was determined later. A structurally related acid, pyrulic (trans-10-heptadecen-8-ynoic) acid, has been characterized from a seed oil (Pyrularia pubera, Santalaceae).

Ximenynic and dehydrocrepenynic acids

Dehydrocrepenynic acid or (9-cis,14-cis-octadecadien-12-ynoic acid), also with a double and a triple bond in conjugation, is occasionally found with crepenynic acid in seed oils but more often in fungi, including some mushroom species. 10-trans,16-Heptadecadien-8-ynoic acid and a related C18 fatty acid (from Acanthosyris spinescens) also have an enyne system.

Similarly, fatty acids with an acetylenic bond in conjugation with two double bonds occur in seed oils, including trans-11,trans-13-octadecadien-9-ynoic (from Ximenia americana) acid, heisteric or cis-7,trans-11-octadecadien-9-ynoic acid from Heisteria silvanii, and trans-8,trans-10-octadecadien-12-ynoic acid from Tanacetum (Chrysanthemum) corymbosum; the last species also contains crepenynic acid.

Poly-Acetylenic Fatty Acids from Plants

Octadeca-9,11-diynoic acid is the simplest of a number of di- and polyacetylenic fatty acids present in isano seed oil (Onguekoa gore) and other species from the Olacaceae. Others include isanic (17-octadecen-9,11-diynoic), bolekic (13-cis-octadecen-9,11-diynoic) and 13-cis,17-octadecadien-9,11-diynoic acids. Similarly, several fatty acids with ene-diyne structures have been isolated from species from the Santalaceae, including exocarpic (13-trans-octadecen-9,11-diynoic) acid.

Acetylenic fatty acids from isano oil

One of the first polyacetylenic fatty acids to be isolated was dehydromatricaria (trans-2-decen-4,6,8-triynoic) acid in roots of Solidago species (Compositae) as long ago as 1826, although it was very much later before it was properly characterized, together with some related compounds (see dihydromatricaria acid below). Triynoic acids have also been detected in species of Santalaceae, while 17-octadecen-9,11,13-triynoic (oropheic) acid has been found in leaves of Orophea enneandra (Annonaceae).

Dehydromatricaria acid and a phomoallenic acid

Complex polyacetylenic fatty acids occur in fungi, including a C9 triynoic fatty acid with a terminal triple bond from Basidiomycetes, and three phomallenic acids with an allenyldiyne structure from a Phoma species (one of which is illustrated). Similarly, the bacterial family Actinomycetes produce mycomycin, one of the first polyacetylenic fatty acids to be characterized with two triple bonds and an allenic structure - tridecatetra-3,5,7,8-en-10,12-diynoic acid. Many similar compounds are now known. Fungi and algae may also contain complex acetylenic fatty acids with additional hydroxyl, keto and epoxyl groups or bromine atoms.

Allenic Fatty Acids from Plants

Laballenic acidThe first allenic fatty acid to be described from a higher plant was 8-hydroxy-5,6-octadienoic acid, which is found in estolide linkage with 2,4-decadienoic acid in the seed oil of Sapium sebiferum. Laballenic or (R)-5,6-octadecadienoic acid is a major constituent of the seed oil of Leonotis nepetaefolia (Labiateae). It is noteworthy that the allenic group is responsible for the marked optical activity ([α]D = −47°) of the fatty acid, a much greater effect than with other common substituents.

Subsequently, a C20 homologue of laballenic (i.e. phlomic) acid and lamenallenic acid (5,6,16-18:3) were isolated from other seed oils of the Labiateae. Fatty acids with allene and cumulene groups are now known to be widespread if minor components in higher plants and fungi (as oxylipins), and even in primitive animals, c.f. the phomallenic acids discussed above.

Acetylenic Fatty Acids from the Animal Kingdom

A number of different fatty acids containing triple bonds have been isolated from sponges and corals. These can be highly complex molecules with very long chains and one to four triple bonds. For example, haliclonyne is a C47 oxo-octahydroxy-dientetraynoic acid from a marine sponge Haliclona species. Others are known with bromine and thiophene substituents in addition to hydroxyl groups, and 18-bromooctadeca-(9Z,17E)-diene-7,15-diynoic and 18-bromooctadeca-(9E,17E)-diene-5,7,15-triynoic acids were isolated from the sponge Xestospongia testudinaria. A few of the large number of such fatty acids found in sponges are illustrated, i.e. from the sponges Xestospongia testudinaria (1), Oceanapia species (2) and Stellata species (3).

Acetylenic fatty acids from a sponge

Dihydromatricaria acid8Z-Dihydromatricaria acid is a defense compound that has long been known from plants of the family Asteraceae and from fungi, and it is also produced in insects such as the soldier beetle (Chauliognathus lecontei). It is of interest that the enzymes responsible for its biosynthesis differ between plants and beetles, although both utilize the same precursors, first linoleate and eventually 9Z,16Z-octadecadiene-12,14-diynoic acid, suggesting that the relevant genes developed independently. Beetles of the family Lycidae produce lycidic or 5E,7E-octadecadien-9-ynoic acid as a defense against predators. In addition, the moth Thaumetopoea pityocampa produces 13-cis-hexadecen-11-ynyl acetate as a sex pheromone, with hexadec-11-ynoic acid as a biosynthetic intermediate.

Biosynthesis of Acetylenic Fatty Acids in Plants

The biosynthesis of crepenynic acid and its subsequent metabolism has received intensive study as this fatty acid is the primary precursor of bioactive polyacetylenic compounds in plants of which nearly 2,000 have been characterized; some of these have pharmaceutical potential. While many aspects of the mechanism remain to be ascertained, the general pathway is as illustrated.

Biosynthesis of acetylenic fatty acids

Linoleic acid (1) is converted by a Δ12 acetylenase, which abstracts two hydrogens from position 12/13, to form crepenynic acid (2); a further double bond is inserted at position 14 by a Δ14 desaturase to produce a conjugated ynene system in dehydrocrepenynic acid (3), followed by the action of a Δ14 acetylenase to form 9-cis-octadecen-12,14-dynoic acid (4). The last can be further desaturated or can enter into the complex secondary metabolic pathways that lead to the polyacetylenes. It has been suggested that as a triple bond in a substrate imparts rotational and rigidity constraints to a molecule as well as reducing the carbon-carbon bond length of part of the acyl chain, sequential introduction of adjacent functional groups by the same or similar enzymes is facilitated, leading to the formation of the innumerable secondary metabolites with acetylenic bonds in conjugation.

It appears that the acetylenases involved in the biosynthesis of tariric and stearolic acids and for front-end acetylenation of polyunsaturated fatty acids in mosses and algae are structurally distinct from the enzymes with similar functions in crepenynic acid biosynthesis and metabolism in higher plants. For example, the Δ6 fatty acyl acetylenase from the moss Ceratodon purpureus is a bifunctional enzyme that is also capable of inserting a Δ6 double bond.

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Lipid listings Credits/disclaimer Updated: June 27th, 2019 Author: William W. Christie LipidWeb icon