Fatty alcohols biosynthesis

A COMPARISOH OF STEROL AND LONG CHAIN FATTY ALCOHOL BIOSYNTHESIS IN SORGHBM BICOLOR... 

The next three steps—reduction of the /3-carbonyl group to form a /3-alcohol, followed by dehydration and reduction to saturate the chain (Figure 25.7) — look very similar to the fatty acid degradation pathway in reverse. However, there are two crucial differences between fatty acid biosynthesis and fatty acid oxidation (besides the fact that different enzymes are involved) First, the alcohol formed in the first step has the D configuration rather than the L form seen in catabolism, and, second, the reducing coenzyme is NADPH, although NAD and FAD are the oxidants in the catabolic pathway. 

In step 6 of fatty-acid biosynthesis (Figure 29.5), acetoacetyl A CP is reduced steieospecifically by NADPH to yield an alcohol. Does hydride ion add to the Si face or the Re face of acetoacetyl ACP ... 

How the aliphatic monomers are incorporated into the suberin polymer is not known. Presumably, activated co-hydroxy acids and dicarboxylic acids are ester-ified to the hydroxyl groups as found in cutin biosynthesis. The long chain fatty alcohols might be incorporated into suberin via esterification with phenylpro-panoic acids such as ferulic acid, followed by peroxidase-catalyzed polymerization of the phenolic derivative. This suggestion is based on the finding that ferulic acid esters of very long chain fatty alcohols are frequently found in sub-erin-associated waxes. The recently cloned hydroxycinnamoyl-CoA tyramine N-(hydroxycinnamoyl) transferase [77] may produce a tyramide derivative of the phenolic compound that may then be incorporated into the polymer by a peroxidase. The glycerol triester composed of a fatty acid, caffeic acid and a>-hydroxy acid found in the suberin associated wax [40] may also be incorporated into the polymer by a peroxidase. 

Attractive Compounds. The female-produced sex pheromone of the yellow mealworm beetle, Tenebrio molitor, is (R)-4-methyl- 1-nonanol [316] 163 (Scheme 18). Careful investigations on the biosynthesis of this compound [317] revealed that it is produced through a modification of normal fatty acid biosynthesis (Fig. 1, Fig. 2) propanoate serves as the starter, while formal chain elongation with acetate, propanoate, and acetate (accompanied by removal of the oxygens) produces 4-methylnonanoate which yields the pheromone alcohol after reduction. The structures and role of proteins that are present in the hemolymph or secreted by the tubular accessory glands of T. molitor, and that may carry lipophilic chemical messengers (like pheromones) are under investigation [318,319]. 

Coenzyme A is used as the alcohol part of thioesters, which are more reactive than oxygen esters (see Section 7.9.3) and are thus exploited in biochemistry in a wide range of reactions, e.g. fatty acid biosynthesis and metabolism (see Section 15.5). 

A considerable amount of knowledge has accumulated about how pheromone components are produced in female moths since the first pathway was identified some 20 years ago. It appears that most female moths produce their pheromone through modifications of fatty acid biosynthesis pathways. For moths that utilize aldehydes, alcohols, or esters biosynthesis occurs in the pheromone gland. The exceptions are those that utilize linoleic or linolenic acids, which must be obtained from the diet. However, modifications of these fatty acids occur in the gland. For moths that utilize hydrocarbons or epoxides of hydrocarbons, the hydrocarbon is produced in oenocyte cells and then transported to the pheromone gland where the epoxidation step takes place. 

Morse D. and Meighen E. A. (1987a) Biosynthesis of the acetate ester precursors of the spruce budworm sex pheromone by an acetyl CoA fatty alcohol acetyltransferase. Insect Biochem. 17, 53-59. 

Several pheromones may be involved in mediating the mating behavior of the yellow mealworm, Tenebrio molitor L. (reviewed in Plarre and Vanderwel, 1999), but only one has been identified to date. Tanaka et al. (1986, 1989) identified the female-produced male attractantas(4/ )-(+)-4-methylnonan-l-ol(4-methylnonanol). Females produce the pheromone through a modification of normal fatty acid biosynthesis (Islam et al., 1999 Bacala, 2000). Initiation of the pathway with one unit of propionate results in the uneven number of carbons in the chain incorporation of another unit of propionate during elongation provides the methyl branch reduction of the fatty acyl intermediate produces the alcohol pheromone (Figure 6.8). 

NAD tends to be an electron acceptor in catabolic reactions involving the degradation of carbohydrates, fatty acids, ketone bodies, amino acids, and alcohol. NAD is used in energy-producing reactions. NADP, which is cytosolic, tends to be involved in biosynthetic reactions. Reduced NADP is generated by the pentose phosphate pathway (cytosolic) and used by cytosolic pathways, such as fatty acid biosynthesis and cholesterol synthesis, and by ribonucleotide reductase. The niacin coenzymes are used for two-electron transfer reactions. The oxidized form of NAD is NAD". There is a positive charge on the cofactor because the aromatic amino group is a quaternary amine. A quaternary amine participates in four... 

Figure 5. Biosynthetic pathways for diacyl, plasmalogen and alkyl-ether molecular subclasses of phospholipids. Monoacyl dihydroxyacetone phosphate is the key branch-point intermediate whose utilization determines the phospholipid subclass distribution of newly synthesized phospholipids. Reduction of monoacyl dihydroxyacetone phosphate leads to the biosynthesis of diacyl phospholipids. Fatty alcohol exchange, catalyzed by alkyl dihydroxyacetone phosphate synthase, is the first committed step in the biosynthesis of alkyl-ether and plasmalogen subclasses of phospholipids.

Like the acyl-CoA desaturases (Chapter 7), the 1 -alkyl desaturase exhibits the typical requirements of a microsomal mixed-function oxidase. Molecular oxygen, a reduced pyridine nucleotide, cytochrome b, cytochrome reductase, and a terminal desaturase protein that is sensitive to cyanide are all required. The precise reaction mechanism responsible for the biosynthesis of ethanolamine plasmalogens is unknown, but it is clear from an investigation with a tritiated fatty alcohol that only the 15 and 25 (erythro)-labeled hydrogens are lost during the formation of the alk-l -enyl moiety of ethanolamine plasmalogens. 

In other instances unsaturated fatty acids can strongly repress cerevlslae enzymic activity e.g. alcohol acetyltransferase (56), acyltransferases Involved In glycerollpld synthesis (57), fatty acid biosynthesis (In both cerevlslae and Candida llpolytlca, 58) and acetyl CoA carboxylase (59). Some enzymes are repressed by long chain fatty acids (57-59), while unsaturated compounds have specific effects in others (56). Defined mechanisms for these diverse responses are not clear, although nonspecific surfactant action Is probably rare (57) and fatty acid metabolites mediate some repressive fatty acid effects (59). 

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