Fatty acids occur in the body mainly as esters in natural fats and oils, but are found in the unesterified form as free fatty acids, a transport form in the plasma. Fatty acids that occur in natural fats usually contain an even number of carbon atoms. The chain may be saturated (containing no double bonds) or unsaturated (containing one or more double bonds) (Figure 1).

Fig1. Fatty acids. Examples of a saturated (palmitic acid), monounsaturated (oleic acid), and a polyunsaturated (linoleic acid) fatty acid are shown.

Fatty Acids Are Named After Corresponding Hydrocarbons

The most frequently used systematic nomenclature names the fatty acid after the hydrocarbon with the same number and arrangement of carbon atoms, with-oic being substituted for the final-e (Genevan system). Saturated acids end in-anoic, for example, octanoic acid (C8) (from the hydrocar bon octane), and unsaturated acids with double bonds end in-enoic, for example, octadecenoic acid (oleic acid, C18) (from the hydrocarbon octadecane).

Carbon atoms are numbered from the carboxyl carbon (carbon no. 1). The carbon atoms adjacent to the carboxyl car bon (nos. 2, 3, and 4) are also known as the α, β, and γ carbons, respectively, and the terminal methyl carbon is known as the ω- or n-carbon.

Various conventions are used for indicating the number and position of the double bonds (Figure 2); for example, Δ⁹ indicates a double bond on the ninth carbon counting from carbon 1 (the α-carbon); ω9 or n-9 indicates a double bond on the ninth carbon counting from the ω- or n-carbon. In animals, additional double bonds can be introduced only between an existing double bond at the ω9, ω6, or ω3 position and the carboxyl carbon (carbon 1), leading to three series of fatty acids known as the ω9, ω6, and ω3 families, respectively.

Fig2. Nomenclature for number and position of double bonds in unsaturated fatty acids. Illustrated using oleic acid as an example. In this example, C1 is the Δ¹ carbon and C18 is the ω or n-1 carbon. Thus oleic acid may be termed 18:1 ω9 (or n-9) or 18:1 Δ⁹.

Saturated Fatty Acids Contain No Double Bonds

Saturated fatty acids may be envisaged as based on acetic acid (CH₃ —COOH) as the first member of the series in which CH₂ — is progressively added between the terminal CH₃ — and —COOH groups. Examples are shown in Table 1. Other higher members of the series are known to occur, particularly in waxes. A few branched-chain fatty acids have also been isolated from both plant and animal sources.

Table1. Saturated Fatty Acids

Unsaturated Fatty Acids Contain One or More Double Bonds

Unsaturated fatty acids (see Figure 1, Table 2, for examples) may be further subdivided as follows:

1. Monounsaturated (monoethenoid, monoenoic) acids, containing one double bond.

2. Polyunsaturated (polyethenoid, polyenoic) acids, containing two or more double bonds.

3. Eicosanoids: These compounds, derived from eicosa (20-carbon) polyenoic fatty acids , comprise the prostanoids, leukotrienes (LTs), and lipoxins (LXs). Prostanoids include prostaglandins (PGs), prostacyclins (PGIs), and thromboxanes (TXs).

Table2. Unsaturated Fatty Acids of Physiologic & Nutritional Significance

Prostaglandins exist in virtually every mammalian tissue, acting as local hormones; they have important physiologic and pharmacologic activities. They are synthesized in vivo by cyclization of the center of the carbon chain of 20-carbon (eicosanoic) polyunsaturated fatty acids (eg, arachidonic acid) to form a cyclopentane ring (Figure 3). A related series of compounds, the thromboxanes, have the cyclopentane ring interrupted with an oxygen atom (oxane ring) (Figure 4). Three different eicosanoic fatty acids give rise to three groups of eicosanoids characterized by the number of double bonds in the side chains , for example, PG1 , PG2 , and PG3 (PG, prostaglandin). Different substituent groups attached to the rings give rise to series of prostaglandins and thromboxanes (TX) labeled A, B, etc —for example, the “E” type of prostaglandin (as in PGE2 ) has a keto group in position 9, whereas the “F” type has a hydroxyl group in this position. Theleukotrienes and lipoxins (Figure 5) are a third group of eicosanoid derivatives formed via the lipoxygenase pathway . They are characterized by the presence of three or four conjugated double bonds, respectively. Leukotrienes cause bronchoconstriction as well as being potent proinflammatory agents, and play a part in asthma.

Fig3. Prostaglandin E₂ (PGE₂ ).

Fig4. Thromboxane A₂ (TXA₂ ).

Fig5. Leukotriene and lipoxin structure. Examples shown are leukotriene A₄ (LTA₄ ) and lipoxin A₄ (LXA₄ ).

Most Naturally Occurring Unsaturated Fatty Acids Have cis Double Bonds

The carbon chains of saturated fatty acids form a zigzag pat tern when extended at low temperatures (see Figure 1). At higher temperatures, some bonds rotate, causing chain shortening, which explains why biomembranes become thinner with increases in temperature. Since carbon–carbon double bonds do not allow rotation, a type of geometric isomerism occurs in unsaturated fatty acids, termed cis–trans isomerism, which depends on the orientation of atoms or groups around the axes of their double bonds. If the acyl chains are on the same side of the bond, it is cis-, as in oleic acid; if on oppo site sides, it is trans-, as in elaidic acid, the trans isomer of oleic acid (Figure 6). Double bonds in naturally occur ring unsaturated long-chain fatty acids are nearly all in the cis configuration, the molecules being “bent” 120° at the double bond. Thus, oleic acid has a V shape, whereas elaidic acid remains “straight.” Increase in the number of cis double bonds in a fatty acid leads to a variety of possible spatial con figurations of the molecule—for example, arachidonic acid, with four cis double bonds, is bent into a U shape (Figure 7). This has profound significance for molecular packing in cell membranes  and on the positions occupied by fatty acids in complex lipids such as phospholipids. Trans double bonds alter these spatial relationships. Although double bonds in naturally occurring unsaturated fatty acids are almost always in the cis configuration, trans-fatty acids are present in foods such as those derived from ruminant fat (caused by the action of microorganisms in the rumen) and those containing partially hydrogenated vegetable oils (industrial trans fats), which are a by-product of the saturation of fatty acids during hydrogenation, or “hardening,” of natural oils in the manufacture of margarine. Dietary trans-fatty acids, however, are now known to be detrimental to health, being associated with increased risk of diseases including cardiovascular disease, diabetes mellitus, and cancer. For this reason the con tent of industrial trans fats in foods is currently limited to a low level by law in many countries. A plan to eliminate these industrially generated fats completely from the world’s food supply was announced by the World Health Organization in 2018.

Fig6. Geometric isomerism of Δ⁹, 18:1 fatty acids (oleic and elaidic acids). There is no rotation around carbon–carbon double bonds. In the cis configuration, the acyl chains are on the same side of the bond, while in trans form they are on opposite sides.

Fig7. Arachidonic acid.Four double bonds in thecis configuration bend the molecule into a U shape.

Physical and Physiologic Properties of Fatty Acids Reflect Chain Length & Degree of Unsaturation

 The melting points of even-numbered carbon fatty acids increase with chain length and decrease according to unsaturation. Thus, a triacylglycerol containing three saturated fatty acids of 12 or more carbons is solid at body temperature, whereas if the fatty acid residues are polyunsaturated, it is liquid to below 0°C. In practice, natural acylglycerols contain a mixture of fatty acids tailored to suit their functional roles. For example, membrane lipids, which must be fluid at all environ mental temperatures, are more unsaturated than storage lip ids. Lipids in tissues that are subject to cooling, for example, during hibernation or in the extremities of animals, are also more unsaturated.

ω3 Fatty Acids Are Anti-Inflammatory & Have Health Benefits

Long-chain ω3 fatty acids such as α-linolenic (ALA) (found in plant oils), eicosapentaenoic (EPA) (found in fish oil), and docosahexaenoic (DHA) (found in fish and algal oils) (see Table 21–2) have anti-inflammatory effects, perhaps due to their promotion of the synthesis of less inflammatory prostaglandins and leukotrienes as compared to ω6 fatty acids . In view of this, their potential use as a therapy in severe chronic disease where inflammation is a contributory cause is under intensive investigation. Current evidence suggests that diets rich in ω3 fatty acids are beneficial, particularly for cardiovascular disease, but also for other chronic degenerative diseases such as cancer, rheumatoid arthritis, and Alzheimer disease.

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