Pheromone biosynthesis desaturases

Wolf W. A. and Roelofs W. L. (1986) Properties of the All-desaturase enzyme used in cabbage looper moth sex pheromone biosynthesis. Arch. Insect Biochem. Physiol. 3, 45-52. 

Gosalbo L., Fabrias G. and Camps F. (1994) Inhibitory effect of 10,11-methylenetetradec-10-enoic acid on a Z9-desaturase in the sex pheromone biosynthesis of Spodoptera littoralis. Arch. Insect Biochem. Physiol. 26, 279-286. 

After the first three molecular characterizations of animal desaturases from rat liver (Thiede et al., 1986), mouse adipose tissue (Ntambi et al., 1988) and carp (Tiku et al., 1996), a Drosophila desaturase was isolated in 1997 (Wicker-Thomas et al., 1997). Since then, numerous studies have been made on lepidopteran desaturases involved in nonhydrocarbon short-chain pheromones (Knipple et al., 1998 reviewed in Knipple and Roelofs, 2003). In Drosophila, there are seven fatty acyl-CoA desaturase genes, which are all located on chromosome III (Figure 4.1), but only three desaturases appear to be involved in hydrocarbon synthesis. In the cricket, a desaturase has been characterized but there is no evidence that this desaturase is involved in pheromone biosynthesis (Riddervold et al., 2002). On the other hand, the desaturase isolated from the housefly is probably involved in both lipid and pheromone biosynthesis (Eigenheer et al., 2002). 

Can a general pathway be drawn for HC synthesis in Drosophila In mature D. mela-nogaster, only three desaturases seem to participate in pheromone biosynthesis, but as reported in the previous paragraphs, at least four elongases are involved (in cytological positions 55, 68, 85). 

The next example is used to demonstrate how different pathways could produce the same pheromone component. Helicoverpa zea and Helicoverpa assulta are closely related species that use aldehydes as the major pheromone. Helicoverpa zea uses a blend of components with Z11-16 Aid as the major component, and minor components include 16 Ald, Z9-16 Aid, and Z7-16 Aid (Klun et al., 1980). H. assulta uses Z9-16 Ald as the major component and Z11-16 Aid as a minor component (Cork et al., 1992 Sugie et al., 1991). The biosynthesis of Zll-16 Aid occurs by Al 1 desaturation of 16 CoA to produce Z1 l-16 CoA, which is reduced to the aldehyde. This probably occurs in both species, but Z9-16 Ald could be produced by the action of a A9 desaturase using 16 CoA as a substrate or by the Al 1 desaturation of 18 CoA to produce Zll-18 CoA that is then chain shortened to Z9-16 CoA. To determine between these two pathways, deuterium-labeled precursors were applied topically to the glands in dimethyl sulfoxide and females injected with PBAN 1 h later the glands were extracted and analyzed for incorporation using GC/MS (Choi et al., 2002). Figure 3.4 shows the data and biosynthetic pathways. 

Legendre, A., Miao, X.-X., Da Lage, J.-L. and Wicker-Thomas, C. (2008). Evolution of a desaturase involved in female pheromonal cuticular hydrocarbon biosynthesis and courtship behavior in Drosophila. Insect Biochem. Mol. Biol., 38, 244-253. 

First, the use of two specific reactions — All desaturation and controlled 2 carbon chain shortening of fatty acid precursors to account for the biosynthesis of a large number of pheromones — has been an extremely fruitful approach. Even in a case where it seemed uncertain if this approach was appropriate (22)r it turned out that it was (23.). Other reactions should now be added to increase the range of products accounted for. Examples already mentioned include the A10 desaturase and the chain elongation of branched-chain starting materials. Other functional groups that appear in sex pheromones should also be accounted for, such as epoxides. 

The intensive study of lepidopteran sex pheromones and their biosynthesis has made possible a scheme showing how many of them are produced by a small number of reactions, summarized in Figure 3.22. While the A9-desaturase enzyme for making unsaturated acids is common to plants and animals the All-desaturase used here is unique to insects. The optimum chain lengths are apparently 12 or 14 carbon atoms, less than this is too volatile. There are fewer identified pheromone compounds with 16 carbons and still fewer with 18. With still larger molecules volatility is perhaps too low (except for hydrocarbons) for efficient detection by the males. 

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