Unsaturated hydrocarbons biosynthesis

This synthetic allylamine derivative inhibits the enzyme squalene epoxidase at an early stage in fungal sterol biosynthesis. Acting as a structural analogue of squalene, naffidine causes the accumulation of this unsaturated hydrocarbon, and a decrease in ergosterol in the fungal cell membrane. 

After excision of the transposon, the pheromone phenotype was reversed in 69 percent of the lines and the other excision lines showed generally decreased 7-unsaturated hydrocarbon levels (paralleled with similar increases in linear saturated hydrocarbons). These results clearly show the involvement of desatl in the biosynthesis of unsaturated hydrocarbons (Labeur et al., 2002). 

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... 

In the case of unsaturated fatty acids there is at least one carbon-to-carbon double bond. Because of the double bonds, the carbon atoms involved in these bonds are not "saturated" with hydrogen atoms. The double bonds found in almost all naturally occurring unsaturated fatty acids are in the cis configuration. In addition, the double bonds are not randomly located in the hydrocarbon chain. Both the placement and the geometric configuration of the double bonds are dictated by the en-z)unes that catalyze the biosynthesis of unsaturated fatty acids. Examples of common unsaturated fatty acids are also given in Table 18.1. 

Fatty acids are carboxylic acids with long hydrocarbon chains. The fatty acids most frequently found in nature are shown in Table 26.1. Because they are synthesized from acetate, a compound with two carbon atoms, most naturally occurring fatty acids contain an even number of carbon atoms and are unbranched. The mechanism for the biosynthesis of fatty acids is discussed in Section 19.21. Fatty acids can be saturated with hydrogen (and therefore have no carbon-carbon double bonds) or unsaturated (have carbon-carbon double bonds). Fatty acids with more than one double bond are called polyunsaturated fatty acids. Double bonds in naturally occurring unsaturated fatty acids are never conjugated—they are always separated by one methylene group. 

The hydrocarbons released into the water and in the sediments by biosynthesis from various life forms or issued from non-marine sources are subjected to physical, chemical and biological transformations, some of these affecting the molecular structure of the more labile compounds. There have been several studies of the modification of hydrocarbons in the ocean, initiated mainly by an attempt to evaluate the persistence of crude oil in the sea. The mechanisms affecting petroleum products are closely related to those for natural hydrocarbons with one important exception many biogenic hydrocarbons are unsaturated compounds and probably disappear rapidly in the water column. This is an important consideration in evaluating the persistent contribution of biogenic hydrocarbons to dissolved and particulate lipids of sea water and sediments. 

Fatty acids and their derivatives. Fatty acids are characterized by the presence of a carboxylic acid function attached to a hydrocarbon chain. Because the biosynthesis of fatty acids involves the combination of a series of two-carbon fragments, the comnon fatty acids are unbranched chains with even numbers of carbon atoms. Many are saturated, but biochemical interest centers principally around the unsaturated fatty acids containing up to five double bonds. Comnon unsaturated acids have their double... 

In relation of their biosynthesis, it is interesting to note that all the hydrocarbons from the A race possess an odd carbon number and a terminal double bond moreover all the dienes and the two trienes identified up to date exhibit C((o9)-C(a)10) unsaturation. 

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. 

Related to the long-chain fatty adds, several linear derivatives with very long chains (n-G37-n-C39), olefinic hydrocarbons, methylketones and esters, were charader-ized in the species Emiliania huxieyi, for instance in sediments of the Quaternary period. For the unsaturated derivatives, it was established by synthesis that all the double bonds had the configuration ( ), suggesting original biosynthesis mechanisms (Volkman et al., 1980,1998 ... 

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