Fatty aldehydes, decarbonylation

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... 

Early studies in a termite (Chu and Blomquist, 1980a), a cockroach (Major and Blomquist, 1978) and the housefly (Tillman-Wall et al., 1992) showed that tritium-labeled fatty acids were converted in vivo to hydrocarbons one carbon shorter. The mechanism of how this occurs has been controversial. Kolattukudy and co-workers have proposed a mechanism in which a fatty acyl-CoA is reduced to the aldehyde and, in the absence of cofactors, is decarbonylated to the hydrocarbon one carbon shorter and carbon monoxide. This has been demonstrated in plants, algae, vertebrates (Bognar et al, 1984 Cheesbrough and Kolattukudy, 1984, 1988 Dennis and Kolattukudy, 1991) and the flesh fly Sarcophaga crassipalpis (Yoder et al., 1992). 

Alkanes are a desirable fuel compound as they have fuel properties nearly identical to that of gasoline. Production of a range of alkanes including nonane and decane has been shown inf. coli [102]. Fatty acyl-ACPs (AGP = acyl carrier protein) were produced through the fatty acid biosynthetic pathway. They were then reduced to aldehydes and decarbonylated to alkanes by an exogenous enzyme, CERl from A. thaliana. 

Secondly, decarboxylation. The available routes to this involve decarboxylation or decarbonylation of the long-chain fatty acid or its aldehyde, respectively this would yield the predominant series of odd-carbon n-alkanes (Figure 2 route A). The occurrence of the alternative even-carbon alkanes would necessitate an additional a-oxidation step (route B). 

Extracts from tea leaves in the presence of oxygen and ascorbate converted fatty acids (n-Cig to n-C32) into n-alkanes containing 2 carbons less thus a-oxidation must have preceded final loss of carbon. However, the aldehydes prepared from the C g and C24 acids were straightforwardly decarbonylated to the acids with one carbon atom less. Tea leaves in vivo produced the odd-number series of n-alkanes and it was presumed the latter route here predominated Thus the implication was that the final step in n-alkane formation was a decarbonylation rather than a decarboxylation and studies using particulate preparations from peas have confirmed this. The mechanism is obscure, but tracer studies have shown that the conversion, RCHO RH 4- CO, involves retention of the aldehydic hydrogens, and it has also been demonstrated that a metal ion is implicated (effect of chelating agents) This type of mechanism is consistent with... 

Current views on the compartmentation of the enzyme systems that promote chain elongation and decarbonylation are represented in Figure 4. This scheme is based on electron microscopy and on studies of enzyme location and inhibitionThe chain length of alkanes formed probably depends on the aldehydes available rather than on the specificity of the decarbonylation thus crude extracts of leaves generated n-alkanes from n-Ci6 to n-C32 fatty acids with no particular preference for substrate. The restricted ranges of n-alkanes observed in vivo presumably result from the specificity of products from the fatty-acid elongation system that produce the substrate for decarbonylation. 

Besides isomerization of the olefinic substrates, some other side reactions may complicate the hydroformylation of fatty acids. Such reactions are decarboxylation and decarbonylation (see also Chapter 8) under formation of saturated or unsaturated compounds reduced by one carbon atom (Scheme 6.88) [37]. For example, at high temperatures and long reaction times, the formed aldehydes can undergo dehydrocarbonylation (a). Subsequent hydrogenation produces saturated fatty acids [25, 38]. This reaction sequence may lead to the false conclusion that hydrogenation of the starting olefin has taken place. The same products can suffer decarbonylation (b). On the other hand, decarboxylation of formyl carboxyl acids produces aldehydes (c). 

Wax analysis on the surface of the leek leaf revealed that a C31 ketone was a major component in the wax demonstrating that the decarbonylation pathway was dominant in the leek. The C31 ketone, therefore, was used as a marker to monitor wax content in subsequent experiments. Other detectable components included aldehydes and alkanes with chain length ranging from C26 to C31 as well as free fatty acids from Cl6 to C22. The analyses also showed that there was a differential accumulation of the total wax along the length of the leaf (Fig. 1). The lower section of the leaf has little wax compared to the upper portions of the leaf. Increased wax accumulation began in segment III (Fig. 1) and continued until... 

Waxes are synthesised by reduction of fatty acids to primary alcohols via aldehydes. Primary alcohols react with acyl-CoAs to form esters, aldehydes eliminate carbon monoxide (by decarbonylation) giving rise to hydrocarbons (alkanes). Oxidation of alkanes yields secondary alcohols, and oxidation of secondary alcohols gives rise to ketones. 

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