Fatty acid biosynthesis, differences from

Fatty acid biosynthesis occurs by the sequential addition of acetyl groups and, on first inspection, appears to be a simple reversal of the (3-oxidation pathway. Although the biochemical reactions are similar, fatty acid synthesis differs from (3-oxidation in the following ways It occurs in the cytoplasm, utilizes acyl carrier protein and NADPH, and is carried out by a multienzyme complex, fatty acid synthase. 

As a rule, the anabolic pathway by which a substance is made is not the reverse of the catabolic pathway by which the same substance is degraded. The two paths must differ in some respects for both to be energetically favorable. Thus, the y3-oxidation pathway for converting fatty acids into acetyl CoA and the biosynthesis of fatty acids from acetyl CoA are related but are not exact opposites. Differences include the identity of the acvl-group carrier, the stereochemistry of the / -hydroxyacyl reaction intermediate, and the identity of the redox coenzyme. FAD is used to introduce a double bond in jS-oxidalion, while NADPH is used to reduce the double bond in fatty-acid biosynthesis. 

Although fatty acids are both oxidized to acetyl-CoA and synthesized from acetyl-CoA, fatty acid oxidation is not the simple reverse of fatty acid biosynthesis but an entirely different process taking place in a separate compartment of the cell. The separation of fatty acid oxidation in mitochondria from biosynthesis in the cytosol allows each process to be individually controlled and integrated with tissue requirements. Each step in fatty acid oxidation involves acyl-CoA derivatives catalyzed by separate enzymes, utihzes NAD and FAD as coenzymes, and generates ATP. It is an aerobic process, requiring the presence of oxygen. 

An alternative route to mevalonic acid is also possible, which differs from the former one in that the formation of P-hydroxy-P-methylglutaryl residue occurs on the surface of an acyl carrier protein (like in fatty acid biosynthesis). The intermediary product in this route, P-hydroxy-p-methylglutaryl-S-ACP, is re-duced by another enzyme to mevalonic acid. 

Two of the three attractant pheromones identified to date are very close structurally to those used in primary metabolism. The biosynthesis of the estolide 5 probably starts from 3-hydroxybutyric acid (4), an intermediate in fatty acid biosynthesis (Fig. 4.3). Condensation of two units furnishes the pheromone 5. The formation of cupilure (3 Fig. 4.2) can be easily explained by two methylations from ubiquitous citric acid. Both compounds are unlike any known insect pheromones, whereas the third known attractant pheromone (ketone 1 Fig. 4.1), bears some resemblance to some insect pheromones. A proper comparison of the differences and similarities between insect and arachnid pheromones will require the identification of representative compounds from most of the families of both groups of organisms. 

Cholesterol, like long-chain fatty acids, is made from acetyl-CoA, but the assembly plan is quite different. In early experiments, animals were fed acetate labeled with 14C in either the methyl carbon or the carboxyl carbon. The pattern of labeling in the cholesterol isolated from the two groups of animals (Fig. 21-32) provided the blueprint for working out the enzymatic steps in cholesterol biosynthesis. 

Ethanoic acid is activated for biosynthesis by combination with the thiol, coenzyme A (CoASH, Figure 18-7) to give the thioester, ethanoyl (acetyl) coenzyme A (CH3COSC0A). You may recall that the metabolic degradation of fats also involves this coenzyme (Section 18-8F) and it is tempting to assume that fatty acid biosynthesis is simply the reverse of fatty acid metabolism to CH3COSCoA. However, this is not quite the case. In fact, it is a general observation in biochemistry that primary metabolites are synthesized by different routes from those by which they are metabolized (for example, compare the pathways of carbon in photosynthesis and metabolism of carbohydrates, Sections 20-9,10). 

Fatty acid biosynthesis also occurs in steps of two carbon atoms. Biosynthesis takes place in the cytosol. In addition to occurring in a different cellular compartment from degradation, biosynthesis involves totally different enzymes and different coenzymes. 

Earlier, it was thought that fatty acid biosynthesis occurred by reversal of the P-oxidation pathway. On the contrary, it occurs by a separate pathway that differs from P-oxidation in several ways. 

Many aroma compounds in fruits and plant materials are derived from lipid metabolism. Fatty acid biosynthesis and degradation and their connections with glycolysis, gluconeogenesis, TCA cycle, glyoxylate cycle and terpene metabolism have been described by Lynen (2) and Stumpf ( ). During fatty acid biosynthesis in the cytoplasm acetyl-CoA is transformed into malonyl-CoA. The de novo synthesis of palmitic acid by palmitoyl-ACP synthetase involves the sequential addition of C2-units by a series of reactions which have been well characterized. Palmitoyl-ACP is transformed into stearoyl-ACP and oleoyl-CoA in chloroplasts and plastides. During B-oxi-dation in mitochondria and microsomes the fatty acids are bound to CoASH. The B-oxidation pathway shows a similar reaction sequence compared to that of de novo synthesis. B-Oxidation and de novo synthesis possess differences in activation, coenzymes, enzymes and the intermediates (SM+)-3-hydroxyacyl-S-CoA (B-oxidation) and (R)-(-)-3-hydroxyacyl-ACP (de novo synthesis). The key enzyme for de novo synthesis (acetyl-CoA carboxylase) is inhibited by palmitoyl-S-CoA and plays an important role in fatty acid metabolism. 

Fatty acid synthesis in plants differs from that in animals in the following ways location (plant fatty acid synthesis occurs mainly in the chloroplasts, whereas in animals fatty acid biosynthesis occurs in the cytoplasm), metabolic control (in animals the rate-limiting step is catalyzed by acetyl-CoA carboxylase, whereas in plants, this does not appear to be the case), enzyme structure (the structures of plant acetyl-CoA carboxylase and fatty acid synthetase are more closely related to similar enzymes in E. coli than to those in animals). 

PHA biosynthesis has been well studied over the past many years. Acetyl-CoA is the key component to supply the 3-hydroxyaIkanoyl-CoA of different lengths as substrates for PHA synthases of various specificities (Fig. 4, Table 1). In addition, 3-hydroxyaIkanoyl-CoA can also be supplied from f)-oxidation of fatty acids of different chain lengths (Fig. 4). Many genes encoding various enzymes are direetly or indirectly involved in PHA synthesis (Table 1). 

Although the number of fatty acids detected in plant tissues approaches 300, most of them only occur in a few plant species (Hitchcock and Nichols, 1971). The major fatty acids are all saturated or unsaturated monocarboxylic acids with an unbranched even-numbered carbon chain. The saturated fatty acids, lauric (dodecanoic), myristic (tetradecanoic), palmitic (hexadeca-noic), and stearic (octadecanoic), and the unsaturated fatty acids, oleic (cis-9-octadecenoic), linoleic (c/5 -9,cw-12-octadecadienoic), and linolenic (all-cij-9,12,15-octadecatrienoic (Table I), together account for almost all of the fatty acid content of higher plants. For example, about 94% of the total fatty acids of commercial oils and 89-97% of leaf fatty acids consist of these seven structures alone. It will be noted that the unsaturated acids all contain a cis-9 double bond and that the polyunsaturated acids contain a methylene-interrupted structure. The four saturated fatty acids differ from each other by two carbons. These structural relationships are due to the principal pathways of fatty acid biosynthesis in plants (see Stumpf, this volume. Chapter 7). 

The oil bodies of C. abyssinica synthesized fatty acids from [ Cjmalonyl-CoA and triacylglycerols from [ C]palmitoyl-CoA or [ C]glycerol-3-P (Gurr et al., 1974). Evidence that this was not due to contamination was, first, that the fat fraction synthesized a pattern of fatty acids (predominantly erucic acid) totally different from that of other subcellular fractions and, second, that of all the fractions tested only the fat fraction had an appreciable specific activity for triacylglycerol biosynthesis. In ultrastructural studies, no inclusions could be seen in Crambe oil bodies, and it was concluded that the enzymes were contained in a bounding membrane or in granular material that was always associated with the oil body fraction (Fig. 12). 

A different developmental trend in fatty acid composition is observed during culture of muscle cells (Table 3). Stearate steadily increases from 2 to 13 days of culture while linoleate decreases. Simultaneously, there is an increase in arachidonic acid. The standard medium used to grow muscle cells in vitro includes horse serum which is rich in linoleate (42 % of total fatty acids) and contains low amounts of arachidonate (5%). Therefore, the changes observed in fatty acids from muscle developed in vitro might be related to enzymes involved in fatty acid biosynthesis or uptake. If horse serum is replaced in the medium by fetal calf serum with a low content of linoleate (5%), this fatty acid is almost totally replaced in cell phospholipids by oleate. The linoleate content of the cells can be replenished by supplementation of the medium with free linoleate (Fig. 3). This shows a remarkable plasticity of cultured muscle cells to modify their fatty acid composition. The in vitro system could... 

Northern analysis of RNA isolated from different segments (I+II, III+IV, and VI+VII) of the leaf showed no significant change in the level of ICAS III transcript (data not shown). This indicated that the steady state level of KASHI transcript was not increased eventhough there may be an increased demand for fatty acid biosynthesis due to wax production in the epidermal cells. 

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