Biosynthesis of unsaturated fatty acids

Biosynthesis of Unsaturated Fatty Acids. In the mammalian tissues, the forma-tion of monoene fatty acids is only possible. Oleic acid is derived from stearic acid, and palmitooleic acid, from palmitic acid. This synthesis is carried out in the endoplasmic reticulum of the liver cells via the monooxigenase oxidation chain. Any other unsaturated fatty acids are not produced in the human organism and must be supplied in vegetable food (plants are capable of generating polyene fatty acids). Polyene fatty acids are essential food factors for mammals. 

As mentioned in chapter 17, unsaturated fatty acids are abundant in all living organisms. Alternative mechanisms for the biosynthesis of unsaturated fatty acids have evolved. Two chemically distinct pathways exist for the introduction of a cis double bond into saturated fatty acids The anaero-bic pathway as typified in E. coli, and the aerobic pathway found in eukaryotes. 

Jeffcoat R. (1979) The biosynthesis of unsaturated fatty acids and its control in mammalian liver. Essays Biochem. 15, 1-36. 

Fujii, D.K., and A.J. Fulco (1977). Biosynthesis of unsaturated fatty acids by bacilli—hyperinduction of desaturase synthesis. J. Biol. Chem. 252 3660-3670. 

Figure 1 Role of fatty acid desaturases in the biosynthesis of unsaturated fatty acids Typical fatty acids found as components of cell membranes and storage lipids (a) signaling agents (b) and plant-derived antifeedants (c).

Leesong M, Henderson BS, Gillig JR, Schwab JM, Smith JL. Structure of a dehydratase-isomerase from the bacterial pathway for biosynthesis of unsaturated fatty acids two catalytic activities in one active site. Structure 1996 4 253-264. 

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. 

Fulco, A.J. The biosynthesis of unsaturated fatty acids by bacilli. I. Temperature induction of the desaturation reaction. J Biol Chem, 244 (1969) 889-895. 

E. coli possesses two p-hydroxyacyl-ACP dehydratases. One is encoded by fabZ, and is active on all chain lengths of saturated and unsaturated intermediates. This enzyme is distinct from the dual-function P-hydroxydecanoyl-ACP dehydratase/isomerase (encoded by fabA) first described by Bloch and coworkers. The FabA enzyme dehydrates saturated, but not unsaturated, fatty acid intermediates and catalyzes a key isomerization reaction at the point where the biosynthesis of unsaturated fatty acids diverges from saturated fatty... 

This is the biosynthesis of arachidonic acid, biosynthesis of unsaturated fatty acids 18 ... 

Gas chromatography has been applied successfully to the separation and analysis of labeled compounds (chiefly those labeled with and in food samples. Studies have been made of the biosynthesis of unsaturated fatty acids, the effect of insulin on the biosynthesis of individual fatty acids, the biosynthesis of cholesterol, etc. Numerous studies deal with the incorporation of labeled compounds through foods into living organisms. 

Introduction Primary Fatty Acids Fatty Acids of Plant Vegetative Parts Biosynthesis Fatty Acid Biosynthesis The Two-Pathway Model of Lipid Biosynthesis The Second 3-Ketoacyl ACP Synthase Isozyme Biosynthesis of Unsaturated Fatty Acids The Prokaryotic Pathway The Eukaryotic Pathway Biosynthesis of Triacylglycerides Degradation of Fatty Acids Unusual Fatty Acids in Plants Fatty Acids from Unusual Starter Units Fatty Acids with Unusual Patterns of Unsaturation Hydroxy Fatty Acids Epoxy Fatty Acids... 

Gurr (1974) has thoroughly reviewed the literature on the biosynthesis of unsaturated fatty acids by both prokaryotic and eukaryotic organisms. 

Bloomfield, D. K., and K. Bloch The role of oxygen in the biosynthesis of unsaturated fatty acids. Biochim. biophys. Acta (Amst.) 30, 220 (1958). 

By 1950 biotin had been implicated in a number of seemingly unrelated enzymatic processes, including (1) the decarboxylation of oxaloacetate and succinate (2) the Wood-Werkman reaction , that is, the /5-carboxylation of pyruvate (3) the biosynthesis of aspartate (4) the biosynthesis of unsaturated fatty acids (5) the deamination of certain amino acids (6) the synthesis of certain biotin-independent enzymes. In each case the enzymatic or metabolic basis for these observations can now be explained in terms of the unequivocally established role of biotin as an enzymatic COg carrier or through indirect effects. 

The redox system of Fe /Fe covers a wide range of potentials (Table 2.5) depending on the attached ligands. Therefore, the system is exceptionally suitable for bridging large potential differences in a stepwise electron transport system. Such an example is encountered in the transfer of electrons by the cytochromes as members of the respiratory chain (cf. textbook of biochemistry) or in the biosynthesis of unsaturated fatty acids (cf. 3.2.4), and by some individual enzymes. 

Fig. 3.5. Biosynthesis of unsaturated fatty acids. Synthesis pathways a, c, g in higher plants a, c, g and a, c, d, f in algae a, b and d, f (main pathway for arachidonic acid) or e, f in mammals...

The biosynthesis of unsaturated fatty acids has been studied at the enzyme level only in yeast, rat liver, Mycobacterium phlei, and Escherichia coli. In all other cases the assignments to groups I or 11 are made on the basis of in vk>o experiments. Organisms are placed in column 1 if they synthesize unsaturated fatty acids from C -acetate anaerobically as well as in air, in column 2 if cells convert stearate or palmitate into octadecenoate or hexadecenoate, respectively. 

The biosynthesis of unsaturated fatty acids can use anaerobic or aerobic mechanisms... 

---