Endoplasmic reticulum fatty acid synthesis

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 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). 

The rate limiting step in fatty acid synthesis is catalyzed by acetyl-CoA carboxylase to produce malonyl-CoA at the expense of one ATP.31 Malonate and acetate are transferred from CoA to acyl carrier protein in the cytosolic fatty acid synthetase complex, where chain extension leads to the production of palmitate. Palmitate can then be transferred back to CoA, and the chain can be extended two carbons at a time through the action of a fatty acid elongase system located in the endoplasmic reticulum. The >-hydroxylation that produces the >-hydroxyacids of the acylceramides is thought to be mediated by a cytochrome p450 just when the fatty acid is long enough to span the endoplasmic reticular membrane. 

In eucaryotic cells, glycolysis, gluconeogenesis and fatty acid synthesis takes place in the cytosol, while the Krebs cycle is isolated within mitochondria glycogen is made in glycogen granules, lipid is synthesized in the endoplasmic reticulum and lysosomes carry on a variety of hydrolytic activities. As in procaryotic cells, ribosomes in the cytosol are the site of protein synthesis. 

Regulation is also accomplished by compartmentaliza-tion of enzyme systems involved in anabolic and catabolic pathways into different cell organelles. For example, fatty acid synthesis occurs in the soluble fraction of the cytoplasm, whereas fatty acid oxidation takes place in mitochondria. Heme synthesis begins and is completed in mitochondria, but some of the intermediate reactions take place in the cytosol. Heme catabolism is initiated in the smooth endoplasmic reticulum. Transport of key metabolites across an organelle membrane system is also a form of regulation. 

The answer is d. (Murray, pp 230-267. Scriver, pp 2297-2326. Sack, pp 121-138. Wihon, pp 287-320.) In humans, the end product of fatty acid synthesis in the cytosol is palmitic acid. The specilicity of cytosolic multienzyme, single-protein fatty acid synthetase is such that once the C16 chain length is reached, a thioesterase clips off the fatty acid. Elongation as well as desaturation of de novo palmitate and fatty acids obtained from the diet occur by the action of enzymes in the membranes of the endoplasmic reticulum. 

After fatty acid synthesis, downstream enzymes can further modify palmi-tate for various cellular functions. In the endoplasmic reticulum, the 16 carbon fatty acid can be modified to fatty acids with eighteen or more carbons known as very long chain fatty acids (VLCFA), such as stearate (18 0) by a family of elongase enzymes called elongation of very long chain fatty acids (ELOVLl-6) (Jakobsson et ah, 2006). Palmitate and stearate can also be desaturated by stearoyl-CoA desaturase-1 (SCDl) at the cis-9 carbon to palmitoleate (16 1) and oleate (18 1), respectively (Sampath and Ntambi,... 

The studies on fatty acid synthesis in higher plants over the last 25 years have led to a consensus about the individual reactions and their localization in the cell. This consensus is that the enzyme system for fatty acid synthesis is procaryotic in nature, that is the enzymes are soluble and separable, and that the system is localized entirely in the plastld. Thus the membranes of the mitochondria, the endoplasmic reticulum, the plasmalemma, the tonoplast, the nuclear membrane, and the Golgi apparatus all depend for their fatty acid components on the activities of the plastids. In outline the reactions of fatty acid synthesis may be summarized ... 

As the 26S mRNA is translated, the C protein is produced first and catalyzes its own cleavage from the nascent polypeptide soon after the ribosome transits into the sequences that encode E3. After release of the C protein, the free amino terminus of E3 is bound to the membranes of the rough endoplasmic reticulum. As the synthesis of nascent E3 and E2 continues, the polypeptide is translocated into the lumen of the endoplasmic reticulum, where oligosaccharides and fatty acids are added.78 A domain of hydrophobic amino acids near the carboxyl terminus of E2 inhibits further transmembranal movement so that the last 30 to 40 amino acids of the E2 polypeptide remain exposed on the cytoplasmic side of the membrane. It is thought that the 6K... 

There is presumptive evidence that the proplastid and the ofl storage organelles differ in function. Thus, Yamada and his colleagues, as well as Simcox et al., have shown that proplastids isolated from developing castor bean endosperm synthesize only oleic acid. However, the major product of fatty acid synthesis in these seeds is triricinoleoylglycerol. Galliard and Stumpf (1966) have shown that the synthesis of ricinoleic acid involves a microsomal preparation (presumably endoplasmic reticulum) which hydroxylates oleoyl-... 

It is generally considered that there are three systems of fatty acid synthesis. The first, which is highly active, is centred in the cell cytoplasm and results mainly in the production of palmitate from acetyl-coenzyme A or butyryl-coenzyme A. Nearly all other fatty acids are produced by modification of this acid. The second system occurs chiefly in the endoplasmic reticulum and to a minor extent in the mitochondria. It involves elongation of fatty acid chains by two-carbon addition, with malonyl-CoA as donor. The third system, confined to the endoplasmic reticulum, brings about desaturation of preformed fatty acids. 

Fatty acid synthesis occurs in the cell cytoplasm and produces palmitic acid. The addition of two-carbon units to preformed long-and medium-chain fatty acids takes place at the endoplasmic reticulum, as does desaturation. However, mammals are incapable of introducing double bonds beyond carbon atom 9 (see Chapter 3). 

The end-product of cytosolic fatty acid synthesis is palmitate (C16 0) longer-chain fatty acids (up to C24) and unsaturated fatty acids are synthesized from palmitate in the endoplasmic reticulum and mitochondria. 

These different developments must be reconsidered since, in vivo, at least in the liver, fatty acid synthesis seems to take place in the endoplasmic reticulum, not in the cytoplasm. Moreover, the endoplasmic reticulum is known to contain enzyme systems that permit the utilization of the hydrogen of the oxidation-reduction coenzymes by mechanisms different from those found in the cytoplasm. 

In order to examine the apparently selective inhibition of very long chain fatty acid synthesis in vivo, experiments have been carried out in our laboratories using microsomal fractions these contain endoplasmic reticulum, which is the site of fatty acid elongation (Section 3.2). 

In the endoplasmic reticulum of eukaryotic cells, the oxidation of the terminal carbon of a normal fatty acid—a process termed ch-oxidation—can lead to the synthesis of small amounts of dicarboxylic acids (Figure 24.27). Cytochrome P-450, a monooxygenase enzyme that requires NADPH as a coenzyme and uses O, as a substrate, places a hydroxyl group at the terminal carbon. Subsequent oxidation to a carboxyl group produces a dicarboxylic acid. Either end can form an ester linkage to CoA and be subjected to /3-oxidation, producing a... 

Pathways are compartmentalized within the cell. Glycolysis, glycogenesis, glycogenolysis, the pentose phosphate pathway, and fipogenesis occur in the cytosol. The mitochondrion contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and of oxidative phosphorylation. The endoplasmic reticulum also contains the enzymes for many other processes, including protein synthesis, glycerofipid formation, and dmg metabolism. 

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. 

The endoplasmic reticulum is composed of a convoluted network of channels and so has a large surface area. Apart from cytochromes P-450, the endoplasmic reticulum has many enzymes and functions, besides the metabolism of foreign compounds. These include the synthesis of proteins and triglycerides and other aspects of lipid metabolism and fatty acid metabolism. Specific enzymes present on the endoplasmic reticulum include cholesterol esterase, azo reductase, glucuronosyl transferase, NADPH cytochromes P-450 reductase and NADH cytochrome b5 reductase and cytochrome b5. A FAD-containing monooxygenase is also found in the endoplasmic reticulum, and this is discussed later in this chapter. 

Although palmitate, a 16-carbon, fully saturated LCFA (16 0), is to primary end-product of fatty acid synthase activity, it can be further I elongated by the addition of two-carbon units in the endoplasmic] reticulum (ER) and the mitochondria. These organelles use separate enzymic processes. The brain has additional elongation capabilities, allowing it to produce the very-long-chain fatty acids (up to 24 car bons) that are required for synthesis of brain lipids. 

The esterification of fatty acids in the mammary cell has been reported as a function of the microsomes and mitochondria (Bauman and Davis 1974 Moore and Christie 1978). While both microsomes and mitochondria may have acyltransferase activity, it has been observed to be 10 times greater in the microsomal fraction of the rat mammary cell (Tanioka et al. 1974). Based on autoradiographic studies, it appears that most synthesis of milk TG occurs in the rough endoplasmic reticulum of mouse mammary tissue (Stein and Stein 1971). 

Synthesis in mammalian tissues of arachidonic acid from linoleic acid. The A5 and A6 desaturases are separate enzymes and are also different from the A9 desaturase (fig. 18.16). The mechanisms, however, seem to be the same, involving cytochrome b5 and cytochrome reductase. The enzymes for elongation of unsaturated fatty acid such as 18 3 to 20 3 occur on the endoplasmic reticulum. 

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