Fatty Acids Are Aliphatic Carboxylic Acids

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

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). Thus, saturated acids end in -anoic, eg, octanoic acid, and unsaturated acids with double bonds end in -enoic, eg, octadecenoic acid (oleic acid).

Carbon atoms are numbered from the carboxyl carbon (carbon No. 1). The carbon atoms adjacent to the carboxyl carbon (Nos. 2, 3, and 4) are also known as the a, P, and Y carbons, respectively, and the terminal methyl carbon is known as the ffl or n-carbon.

Various conventions use A for indicating the number and position of the double bonds (Figure 14-1); eg, A9 indicates a double bond between carbons 9 and 10 of the fatty acid; ffl9 indicates a double bond on the ninth carbon counting from the ffl- carbon. In animals, additional double bonds are introduced only between the existing double bond (eg, ffl9, ffl6, or ffl3) and the

CH3(CH2)7CH = CH(CH2)7COOH or o9,C18:1 or n-9, 18:1

rn 23456789 10 18

CH3CH2CH2CH2CH2CH2CH2CH2CH CH(CH2)7COOH

Figure 14-1. Oleic acid. n - 9 (n minus 9) is equivalent to 0)9.

carboxyl carbon, leading to three series of fatty acids known as the 09, 06, and 0)3 families, respectively.

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 14-1. 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.

Table 14-1. Saturated fatty acids.

Common Name

Number of C Atoms

Acetic

2

Major end product of carbohydrate fermentation by rumen organisms1

Propionic

3

An end product of carbohydrate fermentation by rumen organisms1

Butyric

4

In certain fats in small amounts (especially butter). An end product of carbohydrate fermentation by rumen organisms1

Valeric

5

Caproic

6

Lauric

12

Spermaceti, cinnamon, palm kernel, coconut oils, laurels, butter

Myristic

14

Nutmeg, palm kernel, coconut oils, myrtles, butter

Palmitic

16

Common in all animal and plant fats

Stearic

18

Also formed in the cecum of herbivores and to a lesser extent in the colon of humans.

Also formed in the cecum of herbivores and to a lesser extent in the colon of humans.

Unsaturated Fatty Acids Contain One or More Double Bonds (Table 14-2)

Fatty acids 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).

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 14-2). A related series of compounds, the thromboxanes, have the cyclopentane ring interrupted with an oxygen atom (oxane ring) (Figure 14-3). Three different eicosanoic fatty acids give rise to three groups of eicosanoids characterized by the number of double bonds in the side chains, eg, PG1, PG2, PG3. Different substituent groups attached to the rings give rise to series of prostaglandins and thromboxanes, labeled A, B, etc—eg, 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. The leukotrienes and lipoxins are a third group of eicosanoid derivatives formed via the lipoxygenase pathway (Figure 14-4). 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.

Most Naturally Occurring Unsaturated Fatty Acids Have cis Double Bonds

The carbon chains of saturated fatty acids form a zigzag pattern when extended, as at low temperatures. At higher temperatures, some bonds rotate, causing chain shortening, which explains why biomembranes become thinner with increases in temperature. A type of geometric isomerism occurs in unsaturated fatty acids, depending on the orientation of atoms or groups around the axes of double bonds, which do not allow rotation. If the acyl chains are on the same side of the bond, it is cis-, as in oleic acid; if on opposite sides, it is trans-, as in elaidic acid, the trans isomer of oleic acid (Fig-

Table 14-2. Unsaturated fatty acids of physiologic and nutritional significance.

Number of C

Atoms and Number

and Position of

Common

Double Bonds Family

Name

Systematic Name

Occurrence

Monoenoic acids (one double bond)

16:1;9

m7

Palmitoleic

c/s-9-Hexadecenoic

In nearly all fats.

18:1;9

0)9

Oleic

c/s-9-Octadecenoic

Possibly the most common fatty acid in natural fats.

18:1;9

0)9

Elaidic

frans-9-Octadecenoic

Hydrogenated and ruminant fats.

Dienoic acids (two double bonds)

18:2;9,12

0)6

Linoleic

all-c/'s-9,12-Octadecadienoic

Corn, peanut, cottonseed, soybean, and many plant oils.

Trienoic acids (three double bonds)

18:3;6,9,12

0)6

y-Linolenic

all-c/'s-6,9,12-Octadecatrienoic

Some plants, eg, oil of evening primrose, borage oil; minor fatty acid in animals.

18:3;9,12,15

0)3

a-Linolenic

all-c/'s-9,12,15-Octadecatrienoic

Frequently found with linoleic acid but particularly in linseed oil.

Tetraenoic acids (four double bonds)

20:4;5,8,11,14

0)6

Arachidonic

all-c/'s-5,8,11,14-Eicosatetraenoic

Found in animal fats and in peanut oil; important component of phospho-lipids in animals.

Pentaenoic acids (five double bonds)

20:5;5,8,11,14,17

0)3

Timnodonic

all-c/'s-5,8,11,14,17-Eicosapentaenoic

Important component of fish oils, eg, cod liver, mackerel, menhaden, salmon oils.

Hexaenoic acids (six double bonds)

22:6;4,7,10,13,16,19

0)3

Cervonic

all-c/'s-4,7,10,13,16,19-Docosahexaenoic

Fish oils, phospholipids in brain.

ure 14-5). Naturally occurring unsaturated long-chain fatty acids are nearly all of the cis configuration, the molecules being "bent" 120 degrees at the double bond. Thus, oleic acid has an L 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 configurations of the molecule—eg, arachi-donic acid, with four cis double bonds, has "kinks" or a

U shape. This has profound significance on molecular packing in membranes and on the positions occupied by fatty acids in more complex molecules such as phospholipids. Trans double bonds alter these spatial relationships. Trans fatty acids are present in certain foods, arising as a by-product of the saturation of fatty acids during hydrogenation, or "hardening," of natural oils in the manufacture of margarine. An additional small ure 14-5). Naturally occurring unsaturated long-chain fatty acids are nearly all of the cis configuration, the molecules being "bent" 120 degrees at the double bond. Thus, oleic acid has an L 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 configurations of the molecule—eg, arachi-donic acid, with four cis double bonds, has "kinks" or a

Figure 14-2. Prostaglandin E2 (PGE2).

Figure 14-2. Prostaglandin E2 (PGE2).

U shape. This has profound significance on molecular packing in membranes and on the positions occupied by fatty acids in more complex molecules such as phospholipids. Trans double bonds alter these spatial relationships. Trans fatty acids are present in certain foods, arising as a by-product of the saturation of fatty acids during hydrogenation, or "hardening," of natural oils in the manufacture of margarine. An additional small

Figure 14-3. Thromboxane A2 (TXA2).

Figure 14-3. Thromboxane A2 (TXA2).

COO-

COO-

Figure 14-4. Leukotriene A4 (LTA4).

contribution comes from the ingestion of ruminant fat that contains trans fatty acids arising from the action of microorganisms in the rumen.

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

The melting points of even-numbered-carbon fatty acids increase with chain length and decrease according to unsaturation. A triacylglycerol containing three saturated fatty acids of 12 carbons or more is solid at body temperature, whereas if the fatty acid residues are 18:2, it is liquid to below 0 °C. In practice, natural acylglyc-erols contain a mixture of fatty acids tailored to suit their functional roles. The membrane lipids, which must be fluid at all environmental temperatures, are more unsaturated than storage lipids. Lipids in tissues that are subject to cooling, eg, in hibernators or in the extremities of animals, are more unsaturated.

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