Biosynthesis of methyl-branched hydrocarbons

Biosynthesis of methyl branched hydrocarbons of the German cockroach Blattella germanica (L.) (Orthoptera, Blattellidae). Insect Biochemistry 20 149-156. 

Figure 8.5 Steps in the biosynthesis of methyl-branched hydrocarbons in the housefly.

Chase J., Jurenka R. J., Schal C., Halamkar P. P. and Blomquist G. J. (1990) Biosynthesis of methyl branched hydrocarbons in the German cockroach Blattella germanica (L.) (Qrthoptera, Blattellidae). Insect Biochem. 20, 149-156. 

Propionate serves several unique and important roles in insects. It is used by some insects, in very small amounts, as a precursor to homomevalonate which is an intermediate in the biosynthesis of juvenile hormone (JH) II (1,2) and probably JH I and JH 0 as well. Much larger amounts of propionate and methylmalonate are needed for the biosynthesis of methyl branched hydrocarbons which are major cuticular components in most of the approximately 100 insect species whose cuticular lipids have been examined (3-7). Until recently, there was little information available on either the source of propionate or its metabolism in insects. In mammals vitamin B 2 Is a key cofactor in propionate and methylmalonate metabolism (B—9). Recent observations that some insect species lack or contain low levels of vitamin B 2 (10)... 

A microsomal FAS was implicated in the biosynthesis of methyl-branched fatty acids and methyl-branched hydrocarbon precursors of the German cockroach contact sex pheromone (Juarez et al., 1992 Gu et al., 1993). A microsomal FAS present in the epidermal tissues of the housefly is responsible for methyl-branched fatty acid production (Blomquist et al., 1994). The housefly microsomal and soluble FASs were purified to homogeneity (Gu et al., 1997) and the microsomal FAS was shown to preferentially use methylmalonyl-CoA in comparison to the soluble FAS. GC-MS analyses showed that the methyl-branching positions of the methyl-branched fatty acids of the housefly were in positions consistent with their role as precursors of the methyl-branched hydrocarbons. 

A 13C NMR study of methyl-branched hydrocarbon biosynthesis in the housefly. 

Charlton R. E. and Roelofs W. L. (1991) Biosynthesis of a volatile, methyl-branched hydrocarbon sex pheromone from leucine by Arctiid moths (Holomelina spp.). Arch. Insect Biochem. Physiol. 18, 81-97. 

Insects utilize propionate and methylmalonate in the biosynthesis of ethyl branched juvenile hormones and methyl branched cuticular hydrocarbons. The sources of propionate and methylmalonate in some insects appear to differ from those in mammals. Succinate is the precursor of propionate and methylmalonate in a termite, whereas valine and probably other amino acids are the sources of propionate and methylmalonate in several other species. An unusual pathway for propionate metabolism has been shown to occur in insects and it may be related to the absence or low levels of vitamin B found in many species. Propionate is converted directly to acetate with carbon 1 of propionate lost as C02> carbon 2 of propionate becoming the methyl carbon of acetate and carbon 3 of propionate becoming the carboxyl carbon of acetate. This pathway suggested the possibility that 2-fluoropropionate might be selectively metabolized in insects to the toxic 2-fluoro-acetate. However, preliminary data indicate that 2-fluoropropionate is not toxic to the housefly or the American cockroach. 

Propionate is a key intermediate in JH and hydrocarbon biosynthesis In insects. It serves as a precursor for methyl branched hydrocarbons which in many insects are important compounds for communication and cuticular protection, and it is a precursor for juvenile hormone biosynthesis (JH 0, JH I and JH II). Sources of propionate have been shown to be succinate in a termite and certain amino acids such as valine in other species. 

The assembly of the carbon skeletons of these unusual hydrocarbons was first studied in Carpophilus freemani Dobson, through careful GC-MS and Nuclear Magnetic Resonance (NMR) studies of the incorporation of 2H or 13C-labeled precursors (Petroski et al., 1994). Assembly of the carbon skeleton of the aggregation pheromone of C. freemani, (2 , 4 , 6ii)-5-ethyl-3-methyl-2,4,6-nonatriene, involves initiation with acetate elongation with first propionate (to provide the methyl branch), then butyrate (to provide the ethyl branch) and chain termination with a second butyrate (Figure 6.7). At some point, loss of C02 from one of the butyrate units occurs to yield the appropriate hydrocarbon, but Petroski et al. (1994) were unable to determine which of the butyrate units loses its carboxyl group. Bartelt and Weisleder (1996) studied the biosynthesis of 15 additional methyl- and/or ethyl-branched, tri- and tetraenes in the related... 

Chu A. J. and Blomquist G. J. (1980) Biosynthesis of hydrocarbons in insects succinate is a precursor of the methyl branched alkanes. Arch. Biochem. Biophys. 201, 304-312. 

The biosynthesis of hydrocarbons occurs by the microsomal elongation of straight chain, methyl-branched and unsaturated fatty acids to produce very long-chain fatty acyl-CoAs (Figure 11.1). The very long chain fatty acids are then reduced to aldehydes and converted to hydrocarbon by loss of the carboxyl carbon. The mechanism of hydrocarbon formation has been controversial. Kolattukudy and coworkers have reported that for a plant, an algae, a vertebrate and an insect, the aliphatic aldehyde is decarbonylated to the hydrocarbon and carbon monoxide, and that this process does not require cofactors (Cheesbrough and Kolattukudy, 1984 1988 Dennis and Kolattukudy, 1991,1992 Yoder et al., 1992). In contrast, the Blomquist laboratory has presented evidence that the aldehyde is converted to hydrocarbon and carbon dioxide in a process that... 

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