Fatty acids, biosynthesis characteristics

The li t-emitting ludferase system from fireflies has been extensively studied and there is strong support for a functional dithioP. The extensive thiol involvement in fatty acid biosynthesis has already been indicated, and some enzyme components have characteristics expected of dithiol centres. There are many additional systems where there is some evidence for dithiol involvement, but proof for a clear functional role of a dithiol is lacking. 

Connected with the controversial views on the origin of the oil bodies is the question as to whether they are bounded by membranes. Evidence for and against the existence of some kind of peripheral membrane is presented in Section 3.3.8. We should note, in addition, favourable evidence for an oil body membrane raised by the freeze-fracture studies on barley aleurone cells [20 b] and by biochemical analyses of peanut and walnut oil bodies which reveal phospholipid and protein characteristic of membranes. Finally, we should be aware that in addition to the possible existence of membrane protein, enzyme protein may also occur in oil bodies. Enzymes for fatty acid biosynthesis have been reported in oil bodies of developing castor bean [35] and for triglyceride hydrolysis (acid lipases) in the mature endosperm of this species the latter enzymes may not occur in oil bodies of other species (Chap. 6). 

A more direct pathway for unsaturated fatty acid biosynthesis was first demonstrated in Saccharomyces cerevisiae by Bloomfield and Bloch (1960). Their studies revealed that the formation of unsaturated fatty acid CoA esters could proceed by a desaturation of the corresponding long-chain fatty acid CoA ester, as shown in Fig. 7. The particulate enzyme involved was found to have characteristics typical of mixed function oxidases, requiring molecular oxygen and ITNH. In contrast, the p,y dehydration mechanism outlined above is essentially... 

The sex pheromones of moths generally are mixtures of two or more chemical components, typically aldehydes, acetates, alcohols, or hydrocarbons, produced in specialized glands by biosynthesis and modification of fatty acids (34). Often, a species-specific blend of components is the message, and males of many moth species, including M. sexta, give their characteristic, qualitatively and quantitatively optimal behavioral responses only when stimulated by the correct blend of sex-pheromone components and not by individual components or partial blends lacking key components (43, 44). 

Guiet et al. (2003) demonstrated that deuterium (2H) distribution in fatty acids was non-statistical and could be related to isotopic discrimination during chain extension and desaturation. Petroselinic acid (C18 1A6) (Fig. 21.4), a fatty acid characteristic of the seeds of the Apiaceae, has been shown to be biosynthesized from palmitoyl-ACP (C16 0) by two steps, catalysed by a dedicated A4-desaturase and an elongase. The isotopic profile resulting from this pathway is similar to the classical plant fatty acid pathway, but the isotopic fingerprint from both the desaturase and elongase steps shows important differences relative to oleic and linoleic acid biosynthesis. 

Polyketide and fatty acid biosyntheses begin with condensation of the coenzyme A thioester of a short-chain carboxylic acid starter unit such as acetate or propionate with the coenzyme A thioester of a dicarboxylic acid extender unit such as malonate or methyl malonate. The driving force for the condensation is provided by the decarboxylation of the extender unit. In the case of fetty acid synthesis, the resulting -carbonyl is completely reduced to a methylene however, during the synthesis of complex poly-ketides, the -carbonyl may be left untouched or variably reduced to alcohol, olefinic, or methylene functionalities depending on the position that the extender unit will occupy in the final product. This cycle is repeated, and the number of elongation cycles is a characteristic of the enzyme catalyst. In polyketide biosynthesis, the full-length polyketide chain cyclizes in a specific manner, and is tailored by the action of additional enzymes in the pathway. 

This type of fatty acid is usually found at low levels (around 1%) in plant oils that contain cyclopropene acids. Cyclopropene acids are characteristic for oils from the Malvalaceae, Stercu-liaceae, Bombaceae, Tiliaceae, and Sapicidaceae families with sterculic acid (9,10-methyleneoctadec-9-enoic acid) and mal-valic acid (8,9-methyleneheptadec-8-enoic acid) as two major compounds. Sterculic acid is more abundant (about 50% of the total fatty acids in Sterculiafoetida oil). 2-Hydroxysterculic acid may also be present in this oil as a possible intermediate in the biosynthesis of malvalic acid (through a-oxidation of sterculic acid). 

The biosynthesis of fatty acids [7] and progestrone [8] are some characteristic examples of the domino type of reactions (Charts-5 to 6). 

It is unclear how the synthesis of predominantly short-chain fatty acids occurs, especially as the fatty acid synthase from the uropygial gland appears to have the same characteristics as in other tissues. During the biosynthesis of fatty acids from malonyl-CoA and methylmalonyl-CoA by the fatty add synthase, the growing chain is attached to the pantotheine group of the multienzyme polypeptide by a thioester link. One of the enzyme activities associated with a domain of the multienzyme polypeptide is the thioesterase (reaction 4.4), which has a specificity for cleaving thioesters of longer chain fatty acids esterified to the acyl carrier protein (ACP). 

The biosynthesis of a variety of biologically active peptides proceeds nucleic acid-free on protein templates (IK Peptide synthetases generally activate an acceptor amino acid by formation of amino-acyl adenylates or phosphates, which will be stabilized in an enzyne-aminoacylation step, similar as in tRNA-aminoacylation. Reaction with a donor peptide, which may be covalently bound, leads to a specific chain elongation. While small peptides like glutathione are formed by "one-step"-synthetases, more complex structures like gramicidin S are produced by multienzvme systems, which may contain multifunctional polypeptides. Characteristic features of such systems are 1.)activation as aminoacyl adenylates, 2.) aminoacylation of enzyme thiol-groups, 3.) covalently bound peptide intermediates and 4.) a specific intrinsic transport mechanism similar to the biosynthesis of fatty acids. 

U.9 Phospholipid biosynthesis (general).- A few only of the very many papers published in this area that appear to have particular mechanistic importance can be described here. Stable carbon isotope ratios ( C/ C) at natural abundance levels were determined for each of the major fatty acid components of the phospholipids of E. coli. The results were consistent with a model of lipid metabolism in which fatty acids were released from the fatty acid synthase in free form, and required re-activation to the acyl-acyl carrier protein prior to esterification. A close coupling of fatty acid and phospholipid synthesis was implied. The characteristics of fatty acid transfer from acyl-acyl carrier protein to sn-glyoerol- -phosphate in E. coli have been inves-... 

As mentioned earlier, many seed oils produce a fatty acid that is different from the usual leaf fatty acids and which is characteristic of the plant family (Table II) (Smith, 1970 Hitchcock and Nichols, 1971). In general this makes a useful marker for seed oil development (Fig. 7). Caution is needed, since the characteristic fatty acid is not always exclusive to the oil. Appelqvist (1975) has demonstrated the presence of the monoenoic acid in the hypocotyls and testa ofB. napus, as well as in the cotyledons. It should be noted, however, that the structure of the major C22 monoenoic acid in the testa is not that of erucic acid (cis-13-docosenoic acid) but of an isomer, cis-15-docosenoic acid (Table VIII). Thus, if whole seeds are used in tracer studies of erucic acid biosynthesis or compartmentation, care must be taken in the interpretation of results if anjdysis is made only of the whole C22 monoene fraction (Appelqvist, 1975). Again it is emphasized that, although the subcellular compartmentation of erucic acid may not be exclusive to the major sites of oil storage, the compartmentation within lipid classes is absolutely exclusive to the triacyigiycerols as opposed to the polar lipids (Table III). 

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