metabolism of unsaturated fatty acids

  Regulation of Fuel Utilization in Response to Physical Activity, Structure, Nomenclature, and Properties of Carbohydrates, Metabolism of Fatty Acids, Acylglycerols, and Sphingolipids, Biochemical Physiological and Molecular Aspects of Human Nutrion, (FRACTION OF TOTAL FATTY ACIDS, % BY WEIGHT). The final three reactions in the n−6 PUFA metabolic pathway—(1) elongation to a 24-carbon intermediate, (2) Δ6-desaturation of this intermediate, and (3) retroconversion to the 22-carbon end-product—only become prominent when there is an n−3 PUFA deficiency. Many tissues are able to convert linoleic acid to ARA through the pathway illustrated in Figure 18-2, and linoleic acid (18:2n−6) and ARA (20:4n−6) are the main n−6 PUFAs that accumulate in the body. The levels of PUFAs present in the plasma lipids of human subjects who consumed western diets are shown in Table 18-1 (Edelstein, 1986). Fatty acids occur as saturated and unsaturated (with one or more double bonds) fatty acids. It is the most abundant fatty acid contained in the triacylglycerols of corn oil, sunflower seed oil, and safflower oil, and linoleic acid accounts for most of the n−6 PUFAs obtained from the diet. Lipid Metabolism   In every member of the n−6 class, the double bond closest to the methyl end is located 6 carbons from the methyl end. Conversion of the 24-carbon acyl-CoA intermediates to the 22-carbon end products is thought to occur through peroxisomal fatty acid oxidation, a β-oxidation system that shortens very-long-chain fatty acids. Likewise, the 20:4 and 22:5 fatty acids that occur in both pathways are isomeric pairs. A. Spector (Eds. A similar process can occur with n−6 PUFAs to produce 22:5n−6 from 24:5n−6 (see Figure 18-2). FIGURE 18-6 Fatty acid composition of the human erythrocyte as determined by gas liquid chromatography. The double bonds in all unsaturated fatty acids synthesized by plants and animals are in the cis configuration. Another designation for the n− notation is ω, and both ω and n− notations are used interchangeably for numbering double bonds from the methyl end of a fatty acid. Although a small amount of ARA is present in meat and other animal products in the diet, most of the ARA contained in the body is synthesized from linoleic acid. A supplement of fish oil and black currant seed oil, which contains γ-linolenic acid (18:3n−6), was prescribed. The expression of these two genes is coordinately regulated. A similar process can occur with n−6 PUFAs to produce 22:5n−6 from 24:5n−6 (see Figure 18-2). §The lipids contain only trace amounts (<0.5%) of 22:4n−6 and 22:5n−6. In contrast to the high n−6 PUFA content, n−3 PUFAs comprised only 1% to 3% of the total fatty acids in any of the plasma lipid fractions. Fatty acids are a family of molecules classified within the lipid macronutrient class. 15.60 ± 0.63 0.36 ± 0.05 Therefore both classes of essential fatty acids are necessary in the diet. Dietary PUFAs are incorporated into the lipids in chylomicrons produced by the small intestinal absorptive cells, and these lipoproteins are a major source of essential fatty acids for the tissues in the postprandial state. Figure 18-6 shows the fatty acid composition of normal human erythrocytes from a person consuming a typical western diet, as determined by gas-liquid chromatography. They cannot be interconverted. Essential Fatty Acid Composition of Normal Human Plasma Lipids The genes coding for FADS1 and FADS2 are located on human chromosome 11q12-q13.1 in reverse orientation, separated by about 10,000 bp (Marquardt et al., 2000). Therefore both classes of essential fatty acids are necessary in the diet. FREE FATTY ACID Although saturated fatty acid (FA) (SFA) and monounsaturated FA (MUFA) are synthesized in cancer cells from acetyl-CoA, polyunsaturated FAs (PUFAs) are necessarily obtained from diet. Therefore the 18:3 in the n−3 pathway is α-linolenic acid (9,12,15-18:3, or 18:3n−3), whereas the 18:3 in the n−6 pathway is γ-linolenic acid (6,9,12-18:3, or 18:3n−6). 0.71 ± 0.11 For example, a double bond located in the n−3 position of an 18-carbon fatty acid is at C15 in the Δ nomenclature (i.e., n−3, or 18−3=15), and an n−6 double bond in an 18-carbon fatty acid is at C12 in the Δ nomenclature (i.e., 18−6=12). Like their n−6 counterparts, n−3 PUFAs can be structurally modified but cannot be synthesized completely in the body and ultimately must be obtained from the diet. rumen fermentation running efficiently.) The desaturases act on the segment of the acyl-CoA chain between the carboxyl group and the first existing double bond. Metabolism of Unsaturated Fatty Acids. Fatty acid metabolism consists of catabolic processes that generate energy, and anabolic processes that create biologically important molecules (triglycerides, phospholipids, second messengers, local hormones and ketone bodies). However, all humans, even infants, can convert the 18-carbon members of each class to the corresponding 20- and 22-carbon products (Brenna et al., 2009). Fatty Acid Elongation Epidemiological evidence for the role of polyunsaturated fatty-acids (PUFA) in Crohn's disease (CD) is unclear, although the key metabolite leucotriene B4 (LTB4) is closely linked to the inflammatory process. Consequently, at least two different fatty acid elongation enzymes operating in sequence are needed to convert an 18-carbon polyunsaturated fatty acid to the 24-carbon intermediate, and the enzymes that act in one tissue may be different from those that act in another tissue. The PUFAs found in the body and in foods are mainly of the n−6 and n−3 classes. low-density lipoprotein Plants have the ability to synthesize the first 18-carbon member of each class, linoleic acid (n−6) and α-linolenic acid (n−3), and plant products are the ultimate sources of essential fatty acids in the human food chain. ARA is highly enriched in phosphatidylinositol, whereas linoleic acid and ARA are contained in large amounts in the choline glycerolphospholipids. For breakdown, unsaturated fatty acids are activated to form CoA derivatives as needed for ß-oxidation. These differences in fatty acid distribution are due primarily to the substrate specificities of the acyltransferases that incorporate acyl-CoA into the sn-2 position of phospholipids. Metabolism - Metabolism - Fate of fatty acids: As with sugars, the release of energy from fatty acids necessitates an initial investment of ATP. TABLE 18-1 It is the most abundant fatty acid contained in the triacylglycerols of corn oil, sunflower seed oil, and safflower oil, and linoleic acid accounts for most of the n−6 PUFAs obtained from the diet. Polyunsaturated Fatty Acids Therefore when 6,9,12-18:3 undergoes one elongation, the resulting 20-carbon fatty acid is 8,11,14-20:3. Fatty acids containing similar numbers of carbons and double bonds occur in the n−3 and n−6 classes (e.g., 18:3, 20:4, and 22:5). The expression of these two genes is coordinately regulated. eicosapentaenoic acid occurs to an appreciable extent only if there is a deficiency of n−3 PUFAs. that are present in essential fatty acids. Some terrestrial plants synthesize small amounts of this fatty acid, and α-linolenic is present in soybean oil and canola oil. The expression of these genes is tissue dependent. Therefore an n−6 PUFA can be converted only to another n−6 PUFA, and likewise, an n−3 PUFA can be converted only to another n−3 PUFA. They are positional isomers, not identical compounds. However, there is ongoing debate as to whether humans, especially infants, can synthesize enough 20- and 22-carbon n−3 PUFAs from α-linolenic acid for optimal growth and development of the neural and visual systems. 22.94 ± 0.57 It is obviously dependent on the metabolism of carbohydrates and proteins. Peroxidation slows mitochondrial respiration, lowering the metabolic rate. This view gradually changed during the last 35 years because of increasing evidence that n−3 PUFAs are required for optimal visual and nervous system development (Innis, 2008). Although several genes may encode the FADS enzymes, in terms of PUFA metabolism, Both fatty acid desaturases can utilize either n−3 or n−6 polyunsaturated fatty acyl-CoA substrates, and they both require O. Positional differences in the double bonds inserted by the fatty acid Δ5- and Δ6-desaturases. The fatty acids are indicated as a ratio of the number of carbons to the number of double bonds. All the reactions in the PUFA metabolic pathway utilize fatty acids in the form of acyl-coenzyme A (CoA) derivatives. The classes of the unsaturated fatty acids detected in the erythrocyte lipids are n−9 (18:1), n−6 (18:2, 20:3, 20:4, 22:4), and n−3 (22:5, 22:6). The small amount of n−3 PUFAs are distributed almost equally between 22:5n−3 and DHA (22:6n−3). Although each of the seven reactions in PUFA metabolism can utilize either n−3 or n−6 PUFAs, the pathway functions differently with the two classes of essential fatty acids under normal physiological conditions. However, in chemistry the term long-chain fatty acid means any fatty acid greater than 12 carbons, thus leading to some confusion between the definitions of long-chain and very-long-chain fatty acids. 49.82 ± 1.79 n−6§ No well-defined disease occurred when experimental animals were fed a diet deficient in α-linolenic acid, the corresponding 18-carbon member of the n−3 PUFA class. Two unsaturated fatty acids were predictive of future diabetes risk and diabetes remission after metabolic surgery.   Metabolic Fate of Fatty Acids • Fatty acids are oxidized to acetyl CoA for energy production in the form of NADH.   The synthesis of omega-3 fatty acids, EPA and DHA, utilizes the other essential fatty acid, α-linolenic acid … Figure 1 Higher Unsaturated Lipid Profile of Ovarian Cancer Stem Cells Attributed to Increased SCD-1 Expression. This reaction forms the basis of the industrial production of hydrogenated oil (vegetable ghee). Thus, the location of a double bond in the Δ numbering system can be determined from the n− notation if the number of carbons that the fatty acid contains is known. It is the main substrate used for the synthesis of the eicosanoid biomediators, such as the prostaglandins and leukotrienes, and it is also a major fatty acid component of the inositol glycerolphospholipids. This process requires transport of the 24-carbon intermediate from the ER to the peroxisomes and, subsequently, transport of the 22-carbon product back to the ER where it is incorporated into tissue lipids. Furthermore, each ELOVL enzyme has different substrate specificity, although there is some overlap. ‡Cholesteryl esters contain 1.07 ± 0.07% 18:3n−6, but the other lipid fractions contain only trace amounts. Fatty Acid Metabolism • Other Fatty Acid Syntheses 1. There is only one fatty acid Δ6-desaturase, and this enzyme functions twice in n−3 PUFA metabolism, converting α-linolenic acid to 18:4n−3 and 24:5n−3 to 24:6n−3 (Sprecher, 2000). Fatty acid … The reverse occurs in the n− numbering system; the carbon at the methyl end of the hydrocarbon chain is designated as carbon 1. Fatty acids are long chain organic acids of the general formula CH3(CnHx)COOH. Cholesteryl esters contain 1.07 ± 0.07% 18:3n−6, but the other lipid fractions contain only trace amounts. Depending on concentrations and metabolism, these different FAs may support tumor proliferation but also exert growth inhibitory effects. Structures of the most prominent n−6 and n−3 essential PUFAs. Unsaturated Fatty Acids are Made by Desaturases Found in the Endoplasmic Reticulum 3. Linoleic acid and ARA comprised most of the n−6 PUFAs contained in these plasma lipids. In the condensation reaction, which is the rate-limiting step, the free carboxyl group of malonyl-CoA is released as CO. The fatty acid must be in the form of an acyl-CoA, and malonyl-CoA is the elongating agent. Some thought that the protective action was due entirely to the vitamin E present in the dietary fat. is at C15 in the Δ nomenclature (i.e., n−3, or 18−3=15), and an n−6 double bond in an 18-carbon fatty acid is at C12 in the Δ nomenclature (i.e., 18−6=12). These data show that n−6 PUFAs accounted for 17% of the fatty acids in the plasma free fatty acid fraction, 37% of the fatty acids in phospholipids, 22% of the fatty acids in triacylglycerols, and 59% of the fatty acids in cholesteryl esters. 0.50 ± 0.06 Essential fatty acids are polyunsaturated fatty acids (PUFAs) that are necessary for growth and normal physiological function but cannot be completely synthesized in the body. Why was black currant seed oil prescribed instead of corn oil as a source of n−6 PUFAs for this patient? Therefore the dietary fat intake must contain both of these classes of PUFAs to maintain good health and prevent an eventual deficiency. For example, ELOVL5 acts on 18- and 20-carbon fatty acids, whereas ELOVL2 and ELOVL4 act on 20- and 22-carbon fatty acids. Essential Fatty Acid Composition of Normal Human Plasma Lipids. The complete pathway involves three elongation reactions, three desaturation reactions, and one retroconversion reaction. When necessary, adrenic acid can be converted back to ARA by removal of two carbons from its carboxyl end. Fatty acids are elongated in the endoplasmic reticulum (ER) through the mechanism illustrated in Figure 18-3. For example, the notation for a PUFA that contains 18 carbons and two double bonds that are present at C9 and C12 is 9,12-18:2. G protein All the reactions in the PUFA metabolic pathway utilize fatty acids in the form of acyl-coenzyme A (CoA) derivatives. 1.25 ± 0.17 Humans and other mammals do not have the enzymes necessary to form either the n−3 or the n−6 double bonds that are present in essential fatty acids. Tags: Biochemical Physiological and Molecular Aspects of Human Nutrion KB can be used as fuel in extrahepatic tissues. DHA These factors make elongation a complicated process that still is not fully understood. A problem unique to fats is a consequence of the low solubility in water of most fatty acids. In contrast to the high n−6 PUFA content, n−3 PUFAs comprised only 1% to 3% of the total fatty acids in any of the plasma lipid fractions. †Phospholipids contain 0.65 ± 0.08% 20:5n−3 and 0.77 ± 0.03% 22:5n−3. Fatty acids contain a hydrocarbon chain and a carboxyl group. The carbon atoms of fatty acids are numbered in two different ways. Larger amounts of α-linolenic acid are produced by vegetation that grows in cold water, and it is a prominent component in the food chain of fish and other marine animals. Although not evident from the figure, this enzymatic pathway only uses fatty acids in the form of fatty acyl-CoAs. Humans cannot completely synthesize either n−3 or n−6 PUFAs. It is generally agreed that the human requirement for n−6 PUFAs can be fully satisfied by synthesis from dietary linoleic acid. Caloric restriction slows the accumulation of the highly unsaturated fatty acids in mitochondria, and reduces peroxidation. Fatty acids are often abbreviated as a ratio of the number of carbons to the number of double bonds (e.g., 18:0 for stearic acid). For example, the notation for a PUFA that contains 18 carbons and two double bonds that are present at C9 and C12 is 9,12-18:2. The syndrome produced in rats by a lack of PUFAs, called essential fatty acid deficiency, causes a cessation of growth, dermatitis, loss of water through the skin, loss of blood in the urine, fatty liver, and loss of reproductive capacity. C14-24 saturated and mono-unsaturated fatty acids formed from released acetate either by synthesis de novo or by elongation of endogenous fatty acids, fatty acids formed by 2-6-carbon elongation of added substrates, and a number of water-soluble compounds, some of which were tentatively identified as the amino acids glutamine, glutamic acid and asparagine. Three types of reactions are involved: fatty acid chain elongation, desaturation, and β-oxidation (Sprecher, 2000). However, the data presented herein strongly suggest that fatty acid desaturation is a metabolic marker of ovarian CSC populations. Fatty acid composition of the human erythrocyte as determined by gas liquid chromatography. No well-defined disease occurred when experimental animals were fed a diet deficient in α-linolenic acid, the corresponding 18-carbon member of the n−3 PUFA class. The small amount of n−3 PUFAs are distributed almost equally between 22:5n−3 and DHA (22:6n−3). The n−6 PUFAs present are 18:2n−6, 20:3n−6, 20:4n−6, and 22:4n−6, with linoleic acid (18:2n−6) and. Would you expect to find an elevation in 20:3n−9 in the patient’s plasma? The most prominent member of the n−6 class from a functional standpoint is arachidonic acid (20:4n−6; ARA). In cows fed most typical diets, more than 90% of the unsaturated fatty acids will be biohydrogenated to produce saturated fatty acids that flow to the small intestine. The 24-carbon fatty acids present in each class are metabolic intermediates that normally do not accumulate in either the plasma or the tissues. The fatty acids are abbreviated as number of carbons:number of double bonds, followed by the location of the first double bond counting from the methyl end. Double bonds are inserted into fatty acids by desaturation, a process that also occurs in the ER. On the other hand, n−3 PUFA metabolism does lead to formation of the final 22:6n−3 product, DHA. The PUFAs found in the body and in foods are mainly of the n−6 and n−3 classes. They include saturated fatty acids such as palmitate and stearate, and omega-9 unsaturated fats, such as oleic acid and omega-9 polyunsaturated fatty acids. A consensus now exists that, like the n−6 class, the n−3 PUFAs are essential nutrients for humans. 3. Thinking Critically The fatty acids are abbreviated as number of carbons:number of double bonds, followed by the location of the first double bond counting from the methyl end. The position of the double bonds does not shift relative to the methyl end when a PUFA is elongated, and their numbering remains the same in the n− or omega nomenclature. CONTENTS. Essential Fatty Acid Metabolism In an unsaturated fatty acid, what group lies between two double bonds? Finally, the carbonyl group, which is C3 in the elongated product, is reduced in a three-step process that utilizes two NADPH molecules. For example, ω3 indicates that the first double bond is the third carbon, counting from the methyl end of the fatty acid. ³Ý|YýwüþûæÅìÇj‡FûكÇWÅñ×b¶ZÎËÙv½šïf^\½ûÁ{¾¾Ú¾5þ².ïO¿©Êë®þ\×_¾¿Dý×(G“açü9&ËÁ9WÎúÖõw;ö÷CýýT|„Ï„ÝS;vÏå½Dû'¨ÿ3`Íåýµ®/ÁÃñæ\;ƒû¼[ÿÐÂsÿx/Äö3Ü+,ÿzsÓ-Ð~‹ö\N‰òu8Ù±­Q¾“çºB÷üþIUÞΗ§ò—u÷ʼnԎ¤°œoËLØr­S&&Ùߤòu¹_US¾.Õʪß? 3.11 ± 0.12 A consensus now exists that, like the n−6 class, the n−3 PUFAs are essential nutrients for humans. The lipids contain only trace amounts (<0.5%) of 22:4n−6 and 22:5n−6. Conversion of the 24-carbon acyl-CoA intermediates to the 22-carbon end products is thought to occur through peroxisomal fatty acid oxidation, a β-oxidation system that shortens very-long-chain fatty acids. The other lipid fractions contain only trace amounts (<0.3%) of these n−3 fatty acids. Others believed that, in addition to vitamin E, some component of the fat itself was an essential nutrient. The small amount of n−3 PUFAs are distributed almost equally between 22:5n−3 and DHA (22:6n−3). A supplement of fish oil and black currant seed oil, which contains γ-linolenic acid (18:3n−6), was prescribed. Linoleic acid is the most abundant PUFA in the diet.   3. All the elongation enzymes that have been studied effectively utilize both n−3 and n−6 PUFAs. With this designation, for example, 18:3n−3 would be the same as 9,12,15-18:3. Log In or. These factors make elongation a complicated process that still is not fully understood. Therefore the 18:3 in the n−3 pathway is α-linolenic acid (9,12,15-18:3, or 18:3n−3), whereas the 18:3 in the n−6 pathway is γ-linolenic acid (6,9,12-18:3, or 18:3n−6). Humans and other mammals do not have the enzymes necessary to form either the n−3 or the n−6 double bonds, Nomenclature of Polyunsaturated Fatty Acids, Fatty acids are often abbreviated as a ratio of the number of carbons to the number of double bonds (e.g., 18:0 for stearic acid). α-Linolenic acid (18:3n−3), the 18-carbon member, is structurally similar to linoleic acid except for the presence of an additional double bond at C15. The numbering of the carbons in the Δ nomenclature changes when retroconversion occurs because the carbons that were numbered 1 and 2 in the original fatty acid are removed. Thus elongation, desaturation, and retroconversion together may enable the body to utilize whichever n−3 and n−6 PUFAs are available in the diet to produce all of the necessary members of these essential fatty acid classes. 495–505). TRIACYLGLYCEROLS The fatty acid must be in the form of an acyl-CoA, and malonyl-CoA is the elongating agent. Larger amounts of α-linolenic acid are produced by vegetation that grows in cold water, and it is a prominent component in the food chain of fish and other marine animals. Discovery of Essential Fatty Acids Modified from data compiled by Edelstein, C. (1986). §The lipids contain only trace amounts (<0.5%) of 22:4n−6 and 22:5n−6. FIGURE 18-4 Positional differences in the double bonds inserted by the fatty acid Δ5- and Δ6-desaturases. Similarly, unsaturated fatty acids need special enzymes to provide the beta oxidation intermediate trans-D2-enoyl-CoA, the ... Fatty acid metabolism requires a balance between degradation and synthesis according to the energy need of cells and an organism as a whole. Western diets typically contain about 10 times more n−6 than n−3 PUFAs. To generate energy from fatty acids, they must be oxidized. Fatty Acid Desaturation In the delta (Δ) numbering system, the carboxyl carbon is designated as carbon 1. 0.35 ± 0.04 3. However, plants have the capacity to synthesize PUFAs containing these double bonds; terrestrial plants can form 18-carbon n−3 and n−6 PUFAs and marine plants up to 22-carbon n−3 and n−6 PUFAs. All the elongation enzymes that have been studied effectively utilize both n−3 and n−6 PUFAs. These terms generally are applied to ARA (20:4n−6) and adrenic acid (22:4n−6) of the n−6 class and to EPA (20:5n−3) and DHA (22:6n−3) of the n−3 class (see Figure 18-1). All the elongation enzymes that have been studied effectively utilize both n−3 and n−6 PUFAs. The main n−6 PUFA product normally is ARA, and the last n−6 product normally formed is 22:4. They are constituents of lipids and can be saturated or unsaturated. Many more n−6 than n−3 PUFAs are contained in the erythrocyte lipids. Linoleic acid (18:2n−6), the first member of the n−6 class, is the main PUFA synthesized by terrestrial plants. Methods and Principal Results Pratima Bajpai, Pramod K. Bajpai. Figure 18-1 illustrates the chemical structures of the major n−6 and n−3 PUFAs present in humans and animals. Although the intestinal mucosa can desaturate α-linolenic acid, most of the dietary intake is incorporated into the intestinal lipoproteins and absorbed by humans without structural modification. The terms highly unsaturated fatty acids, long-chain PUFAs, and very-long-chain PUFAs were introduced to distinguish between the 20- and 22-carbon PUFAs, which produce most of the functional effects of essential fatty acids, and their 18-carbon precursors, which serve primarily as substrates for the synthesis of these more highly unsaturated derivatives. Both fatty acid desaturases can utilize either n−3 or n−6 polyunsaturated fatty acyl-CoA substrates, and they both require O2, NADH, cytochrome b5, and cytochrome b5 reductase. Adrenic acid (22:4), the elongation product of ARA, accumulates in tissues that have a high content of ARA. The Δ5-desaturase acts on polyunsaturated acyl-CoAs that have the first double bond at C8, inserting the new double bond at C5. The desaturases act on the segment of the acyl-CoA chain between the carboxyl group and the first existing double bond. docosahexaenoic acid Although not evident from the figure, this enzymatic pathway only uses fatty acids in the form of fatty acyl-CoAs. Several genes modulating unsaturated fatty acids at both transcriptional and post-translational levels, as well as many key enzymes and regulated pathways involved in either the metabolism or the synthesis of fatty acid have been documented. Likewise, the 20:4 and 22:5 fatty acids that occur in both pathways are isomeric pairs. Journal of Biotechnology 1993, 30 (2) , 161-183. The Δ6-desaturase acts on polyunsaturated fatty acyl-CoA substrates that have the first double bond at C9, and inserts the new double bond at C6. For n−6 PUFAs can be saturated or unsaturated metabolize pre-existing double bonds also. That occur in both pathways are isomeric pairs of n−3 PUFAs are contained primarily in membrane phospholipids •... 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