Stearic acid biosynthesis

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. 

Specific chain length fatty acids could be produced in two ways. One is through the action of a thioester hydrolase that interacts with fatty acid synthetase to produce fatty acids shorter in length. Aphids produce myristic acid (14 carbons) and a specific thioester hydrolase releases the fatty acid from fatty acid synthetase after 6 additions of malonyl-CoA. If the hydrolase is not present then the fatty acid synthetase produces stearic acid [27]. A specific thioester hydrolase was ruled out in the biosynthesis of moth sex pheromones because labeling studies showed that longer chain length fatty acids were incorporated into shorter chain length pheromone components [22,28]. 

The common fatty acids have a linear chain containing an even number of carbon atoms, which reflects that the fatty acid chain is built up two carbon atoms at a time during biosynthesis. The structures and common names for several common fatty acids are provided in table 18.1. Fatty acids such as palmitic and stearic acids contain only carbon-carbon single bonds and are termed saturated. Other fatty acids such as oleic acid contain a single carbon-carbon double bond and are termed monounsaturated. Note that the geometry around this bond is cis, not trans. Oleic acid is found in high concentration in olive oil, which is low in saturated fatty acids. In fact, about 83% of all fatty acids in olive oil is oleic acid. Another 7% is linoleic acid. The remainder, only 10%, is saturated fatty acids. Butter, in contrast, contains about 25% oleic acid and more than 35% saturated fatty acids. 

Butyric acid is one of the simplest fatty acids. Fatty acids, which are the building units of fats and oils, are natural compounds of carbon chains with a carboxyl group (-COOH) at one end. Most natural fatty acids have an unbranched carbon chain and contain an even number of carbon atoms because during biosynthesis they are built in two carbon units from acetyl coenzyme A (CoA). Butyric acid is an unsaturated fatty acid, which means all carbon-carbon bonds are single bonds. Common names for fatty acids stem from their natural sources. In addition to butyric acid, some other common saturated fatty acids include lauric acid, palmitic acid, and stearic acid. Lauric acid was first discovered in Lauraceae (Laurus nobilis) seeds, palmitic oil was prepared from palm oil, and stearic acid was discovered in animal fat and gets its name from the Greek word stear for tallow. 

Fatty acids usually have an even number of carbons because they are biosynthesized from acetate ion, which has two carbons. Those with 14, 16, 18, and 20 carbons are most common. Their biosynthesis is outlined in Figure 28.10. They may be saturated, like stearic acid and palmitic acid, or they may have one or more CC double bonds, like... 

Figure 9.18 Structures of sphinogosine (A), a sphingomyelin (B), a glucocerebroside (C), and a ganglioside (D). In the last, stearic acid is attached to the -NH2 residue of sphingosine. The structure shown represents a GMj ganglioside. (Parts A, B, C reproduced by permission from Diem K, Lentner C. Scientific Tables. Basel Ciba-Geigy, 1971, p. 376 part D reproduced with permission from Fishman PH, Brady RO. Biosynthesis and function of gangliosides. Science 194 906-915, 1976.

The above data suggest that pheromone component biosynthesis is related to fatty acid biosynthesis, but that some unusual reactions need to be added. Consideration of a large number of pheromone components (2) suggests that these reactions include chain-shortening by two carbons and desaturation at the 11-12 position. Fig. 1 shows the variety of potential intermediates that can be produced from palmitic or stearic acids employing these reactions. 

The biosynthesis of FA occurs predominantly in the two subcellular compartments, chloroplasts and endoplasmic reticulum (ER) plant mitochondria also contribute to FA synthesis, but only in a very minor way. De novo synthesis of palmitic, stearic and FAs with shorter chain length and also the first desaturation step of saturated FAs e.g. palmitic acid to palmitoleic or stearic acid to oleic one occur in plastids, whereas the next desaturation steps occurs in the ER. [4,6],... 

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). 

This section will define the reactions involved in the formation of palmitic and stearic acids. A review by Stumpf (1977) examines the biosynthesis of saturated fatty acids in plants. More specialized reviews are those of Givan and Harwood (1976) and those found in Recent Advances in the Chemistry and Biochemistry of Plant Lipids, edited by Galliard and Mercer (1975). An excellent although somewhat out-of-date monograph on plant lipids by Hitchcock and Nichols (1971) is a source of much information. 

In plants, de novo fatty acid biosynthesis occurs exclusively in the stroma of plastids, whereas, with the exception of plastidial desaturation, modification of fatty acid residues including further desaturation and triacylglrycerol (TAG) assembly are localized in the cytosol/endoplasmic reticulum (ER). The primary fatty acids formed in the plastid (palmitic, stearic, and oleic acid) are used in the plastidic prokaryotic pathway for membrane lipid synthesis or diverted to the cytoplasmic eukaryotic pathway for the synthesis of membrane lipids or storage TAGs (1). Movement of glycerolipids is believed to occur in the reverse direction between the cytosol/ER and the plastids in the highly regulated manner (2). 

When nystatin was added to the growing mycelium in the intensive growing phase, the effect of the antibiotic on the lipid metabolism can better be seen. Sensitivity is followed by the relative decrease of linoleic and stearic acids and the relative increase of palmitic and linolenic acids. These changes and the Increased anrx)unt of the shorter fatty acids suggest that the Inhibition in the fatty acid biosynthesis is responsible for the sensitivity of the nrK)uids to saponin or polyene antibiotics. 

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