Fatty acid chain elongation pathway

Figure 1). These 8-keto and -hydroxy acids also could be viewed as Intermediates In a pathway toward the fully saturated extended-chain metabolite. Thus, while 6-hydroxy, -keto, and 0,0-unsaturated Intermediates are usually not abundant species in natural fatty acid chain elongation (I.e. the fully saturated product predominates), these intermediary metabolites may be quantitatively Important for xenobiotic acids.

The individual steps in the elongation of the fatty acid chain are quite similar in bacteria, fungi, plants, and animals. The ease of purification of the separate enzymes from bacteria and plants made it possible in the beginning to sort out each step in the pathway, and then by extension to see the pattern of biosynthesis in animals. The reactions are summarized in Figure 25.7. The elongation reactions begin with the formation of acetyl-ACP and malonyl-ACP, which... 

The growing fatty acid chain on the fatty acid synthase complex is elongated, two carbons at a time, by the addition of the three-carbon compound, malonyl CoA, which is subsequently decarboxylated. With each two-carbon addition, the growing chain, which initially contains a P-keto group, is reduced in a series of steps that require NADPH. NADPH is produced by the pentose phosphate pathway and by the reaction catalyzed by the malic enzyme. 

The growing fatty acid chain, attached to the fatty acid synthase complex in the cytosol, is elongated by the sequential addition of 2-carbon units provided by malonyl CoA. NADPH, produced by the pentose phosphate pathway and the malic enzyme, provides reducing equivalents When the growing fatty acid chain is 16 carbons in length, it is released as palmitate After activation to a CoA derivative, palmitate can be elongated and desaturated to produce a series of fatty acids. 

Several additional reactions are required for the elongation of fatty-acid chains and the introduction of double bonds. When mammals produce fatty acids with longer chains than that of pahnitate, the reaction does not involve cytosolic fatty-acid synthase. There are two sites for the chain-lengthening reactions the endoplasmic reticulum (ER) and the mitochondrion. In the chain-lengthening reactions in the mitochondrion, the intermediates are of the acyl-GoA type rather than the acyl-AGP type. In other words, the chainlengthening reactions in the mitochondrion are the reverse of the catabolic reactions of fatty acids, with acetyl-GoA as the source of added carbon atoms this is a difference between the main pathway of fatty-acid biosynthesis and these modification reactions. In the ER, the source of additional carbon atoms... 

Biosynthesis of MCL-PHAs involves three different pathways namely (1) de novo fatty acid biosynthesis, (2) P-oxidation pathway and (3) chain elongation pathway, which is elucidated in Fig. 8.4. 

Chain elongation pathway is another route via which MCL-PHA precursors can be generated from non-related carbon sources by extending the acetyl CoA to acyl CoA. MCL-PHA precursors generated by the elongation of acyl CoA derived from fatty acids in this pathway is significant however, it only forms a minute fraction of the pathway used for the total PHA accumulation within the bacteria. The final step which involves the polymerisation of the (/J)-3-hydroxyacyl CoA into poly-(ff)-3-hydroxyacyl CoA is catalysed by the PHA synthase enzyme with the concomitant release of CoA (Zinn et al., 2001). 

The discovery of a novel pathway for biosynthesis of medium and short chain fatty acids in plants (a-keto acid elongation pathway, 1) raises the possibility (however unlikely) that medium-chain fatty acids (mcFAs) of certain oil seeds producing them may be derived by this pathway. Alternatively, these may be formed after release of elongating fatty acid chains from fatty acid synthase mediated biosynthesis (FAS) by specific medium chain thioesterases [2, 3,4]. Thus far the aKAE pathway is only known to occur in trichome glands of plants in the family Solanaceae. In the aKAE pathway, iso-, anteiso- or straight-chain keto acid products of branched-chain amino acid metabolism are elongated by one carbon (via acetate) per cycle. The final step is predicted to be oxidative decarboxylation to yield CoA activated acids. The mechanism that determines the chain length of aKAE products is not understood [1]. 

Three metabolic pathways (Pathway I Chain elongation reaction Pathway II Fatty acid 3-oxidation Pathway III Fatty acid de novo biosynthesis) are used by organisms to produce the hydroxyacyl-CoA substrates (HACoA), which are then polymerized into PHAs (Figure 3.4). Pathway I is the best known among the PHA... 

Fig. 1. The biosynthetic pathvay of the major fatty acids in edible vegetable oilseeds. The chain elongation pathway from oleic to eruclc acid is unique to the Brassica oil crops.

Historically, many attempts have been made to systematize the arrangement of fatty acids in the glyceride molecule. The even (34), random (35), restricted random (36), and 1,3-random (37) hypotheses were developed to explain the methods nature utilized to arrange fatty acids in fats. Invariably, exceptions to these theories were encountered. Plants and animals were found to biosynthesize fats and oils very differently. This realization has led to closer examination of biosynthetic pathways, such as chain elongation and desaturation, in individual genera and species. 

Fig. 3 Proposed biosynthetic pathways for the production of the sex pheromone components in the indicated insects. Common mechanisms include fatty acid synthesis, desaturation, chain elongation, and decarboxylation...

P. putida grown with hexanoic acid contained approximately 75, 11, and 10 mol% of 3HHx,3HO, and 3HD units and also small amounts of four unsaturated repeating units. The mechanism for the formation of 3HO unit was investigated by 13C NMR study, which showed that the most of 3HO units found in this PHA were formed by the reaction of hexanoic acid with acetyl-CoA [53]. These results confirmed that P. putida produces 3HA units by fatty acid synthesis pathway, through a -oxidation and chain elongation process. 

FIGURE 3-7 Pathways for the interconversion of brain fatty acids. Palmitic acid (16 0) is the main end product of brain fatty acid synthesis. It may then be elongated, desaturated, and/or P-oxidized to form different long chain fatty acids. The monoenes (18 1 A7, 18 1 A9, 24 1 A15) are the main unsaturated fatty acids formed de novo by A9 desaturation and chain elongation. As shown, the very long chain fatty acids are a-oxidized to form a-hydroxy and odd numbered fatty acids. The polyunsaturated fatty acids are formed mainly from exogenous dietary fatty acids, such as linoleic (18 2, n-6) and a-linoleic (18 2, n-3) acids by chain elongation and desaturation at A5 and A6, as shown. A A4 desaturase has also been proposed, but its existence has been questioned. Instead, it has been shown that unsaturation at the A4 position is effected by retroconversion i.e. A6 unsaturation in the endoplasmic reticulum, followed by one cycle of P-oxidation (-C2) in peroxisomes [11], This is illustrated in the biosynthesis of DHA (22 6, n-3) above. In severe essential fatty acid deficiency, the abnormal polyenes, such as 20 3, n-9 are also synthesized de novo to substitute for the normal polyunsaturated acids. 

The fatty acid synthesis pathway can be seen to occur in two parts. An initial priming stage in which acetyl-CoA is converted to malonyl-CoA by a carboxylation reaction (Figure 6.9) is followed by a series of reactions which occur on a multi-enzyme complex (MEC), which achieves chain elongation forming C16 palmitoyl-CoA. The whole process occurs in the cytosol. 

In the ruminant mammary tissue, it appears that acetate and /3-hydroxybutyrate contribute almost equally as primers for fatty acid synthesis (Palmquist et al. 1969 Smith and McCarthy 1969 Luick and Kameoka 1966). In nonruminant mammary tissue there is a preference for butyryl-CoA over acetyl-CoA as a primer. This preference increases with the length of the fatty acid being synthesized (Lin and Kumar 1972 Smith and Abraham 1971). The primary source of carbons for elongation is malonyl-CoA synthesized from acetate. The acetate is derived from blood acetate or from catabolism of glucose and is activated to acetyl-CoA by the action of acetyl-CoA synthetase and then converted to malonyl-CoA via the action of acetyl-CoA carboxylase (Moore and Christie, 1978). Acetyl-CoA carboxylase requires biotin to function. While this pathway is the primary source of carbons for synthesis of fatty acids, there also appears to be a nonbiotin pathway for synthesis of fatty acids C4, C6, and C8 in ruminant mammary-tissue (Kumar et al. 1965 McCarthy and Smith 1972). This nonmalonyl pathway for short chain fatty acid synthesis may be a reversal of the /3-oxidation pathway (Lin and Kumar 1972). 

Figure 6.7 Biosynthesis of (2 ,4 ,6 )-5-ethyl-3-methyl-2,4,6-nonatriene, pheromone component of Carpophilus freemani Dobson (Nitidulidae). The pathway is a modification of normal fatty acid biosynthesis, involving initiation with acetate elongation with first propionate (to provide the methyl branch), then butyrate (to provide the ethyl branch) and chain termination with a second butyrate. The final butyrate to add suffers a loss of C02, after adding in a unique head-to-head reaction. Intermediates of the pathway likely occur as activated (e.g. CoA) derivatives (Petroski et al., 1994 Bartelt and Weisleder, 1996).

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). 

The pathway The first committed step in fatty acid biosynthesis is the carboxylation of acetyl CoA to form malonyl CoA which is catalyzed by the biotin-containing enzyme acetyl CoA carboxylase. Acetyl CoA and malonyl CoA are then converted into their ACP derivatives. The elongation cycle in fatty acid synthesis involves four reactions condensation of acetyl-ACP and malonyl-ACP to form acetoacetyl-ACP releasing free ACP and C02, then reduction by NADPH to form D-3-hydroxybutyryl-ACP, followed by dehydration to crotonyl-ACP, and finally reduction by NADPH to form butyryl-ACP. Further rounds of elongation add more two-carbon units from malonyl-ACP on to the growing hydrocarbon chain, until the C16 palmitate is formed. Further elongation of fatty acids takes place on the cytosolic surface of the smooth endoplasmic reticulum (SER). 

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