Biosynthesis of methyl-branched

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

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. 

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)... 

General Conclusions on the Biosynthesis of Methyl-branched Sugars and on Sugars Having a Two-Carbon Branch... 

The foregoing results permit some general conclusions regarding the biosynthesis of methyl-branched sugars and of sugars having a two-carbon branch. In each example investigated, the acceptor for the... 

SCHEME 8.—General Scheme for the Biosynthesis of Methyl-branched Sugars, [(i) C-Methylation by AdoMet, with inversion of configuration (ii) C-methylation with retention of configuration.]... 

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

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. 

In support of Adelberg s scheme Willson and Adelbei (168) reported the isolation of citiamalic acid, which would be formed from the decarboxylation of Y-hydroxy-7-methyl-a-ketoglutaric acid, and the isolation of a,iS-dimethylmahc acid, the corresponding decarboxylation product of the postulated ramilar seven-carbon keto acid intermediate in the biosynthesis of isoleucine, from a strain of E. cdi blocked before the dihydroxy acid stage. Subsequently Adelberg observed that the malic acid compounds were inert for further utilization in the biosynthesis of the branched-chain amino acids and withdrew his scheme [see 164), footnote 7]. 

Methylmalonyl-CoA may participate in fatty acid biosynthesis in place of malonyl-CoA, with the formation of methyl-branched long-chain (C17) fatty acids, and these acids have been isolated from glycerolipids of brain, spinal cord and sciatic nerve of a patient with methylmalonic aciduria and homo-cystinuria (Kishimoto et al., 1973). Odd-carbon-number straight-chain fatty acids (Ci5 and C17), due presumably to the accumulation of propionyl-CoA and the utilization of this substrate in fatty acid biosynthesis (Section 11.1), have also been observed in the central nervous system tissues of this patient (Kishimoto et aL, 1973), and another (the fourth described. Section 11.2.3) also with combined methylmalonic aciduria and homocystinuria (Dayan and Ramsay, 1974). It is relevant that methylmalonic acid may be utilized for the... 

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