Fatty Acids

The common FAs are long hydrocarbon chains, i.e., a chain of carbon atoms with accompanying hydrogen atoms and with a carboxyl group (an organic acid) at one end (Fig. 1).[1-5] The simplest FA is formic acid with one carbon atom. Acetic acid has two carbons, propionic acid has three carbons, etc. The FA components of complex lipids are long-chain FAs usually with 12 or more carbons. The most common mammalian saturated FAs (no double bonds) are palmitic acid or C16:0 (C16=number of carbons and number after the colon, i.e., 0=number of double bonds) and stearic acid or C18:0. These two FAs are concentrated in animal fats, e.g., lard and tallow.

Unsaturated FAs, containing one to six double bonds, are the other common type of animal FA (Fig. 1). Monounsaturated FAs (MUFAs) have one double bond and polyunsaturated FAs (PUFAs) have two or more double bonds. The most common MUFA is oleic acid, with 18 carbons and one double bond at the ninth carbon

(C18:1). Double bonds are numbered either from the end opposite the carboxyl group (oleic acid=C18:1, n-9) or from the carboxyl group (oleic acid=C18:1A9). Oleic acid is predominant in many feedstuffs and animal cells; it is concentrated in olive and canola oils making up approximately 70% of the total FA.

The common PUFAs (Fig. 1) are linoleic acid (C18:2, A9,12) and a-linolenic acid (C18:3, A9,12,15). Linoleic acid is present at high concentration in many plant oils (e.g., corn oil with >50%) and animal tissues. Linolenic acid is present at high concentration in select plant oils (e.g., linseed oil) and is at low concentration in most animal tissues. Both of these PUFAs are also important because they give rise to other FAs and to various FA derivatives (mostly oxidation products, e.g., eicosanoids) that have important regulatory functions in animal cells.

Many FAs are supplied by the diet as components of animal and plant feedstuffs. However, some animal cells synthesize FAs de novo. This complex process consists of sequential addition of 2-carbon units (C2+C2=C4+C2= C6, etc.), with the final product being C16:0. Mammalian cells synthesize FAs from glucose or acetate, predominantly in the gut, liver, and adipose tissue; in ruminants, FAs with an odd number of carbons arise from use of propionate to initiate FA synthesis. The prevalent site for de novo FA synthesis varies with species; the liver in chickens and humans, the adipose tissue in pigs, and both liver and adipose tissue in rats. Mammalian cells can also transform FAs by elongation and desaturation (Fig. 2). Elongation proceeds by sequential addition of two carbons, e.g., C16:0+C2= C18:0. Because the C2 are added at the carboxyl end, the position of the double bonds shifts, e.g., C18:2, A9,12+C2=C20:2, A11,14. There are three common mammalian desaturase enzymes that insert double bonds into specific positions in the carbon chain (A9, A6, and A5). Linoleic acid (C18:2, n-6) is elongated and desaturated to arachidonic acid (C20:4, n-6), which is an important structural lipid, but also a precursor to many eicosanoids, e.g., prostaglandins. a-Linolenic acid (C18:3, n-3) is elongated and desaturated to other eicosanoid precursors and to eicosapentaenoic acid (C20:5, n-3) and docosahexaenoic acid (C22:6, n-3), both important structural elements in cell membranes, particularly in nervous tissue. Eicosapentaenoic acid and docosahexaenoic

Common Position of

Structure name Shorthand double bond Source

Saturated _

CH3-(CH2)14-C-0- acid C160 synthesis

CH3-(CH2)16-C-0- acid C180 synthesis

Unsaturated m Oleic n-9 Diet

CH3-(CH2)7-CH=CH-(CH2)^C-0- acid C18:1 Aa synthesis

O cn ii Linoleic C18- n-6.9

CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7-C-0- acid C18Z Ag>12 Dlet

CH3-CH2-CH=CH-CH2-CH=CH-CH2-CH=CH-(CH2)7-C-0 acid C18"3 A9,12,15 Dlet

Fig. 1 Types and source of fatty acids.

acid are at relatively high concentration in fish oils. The positions for mammalian FA desaturation are limited; thus, linoleic and a-linolenic acid are required dietary components.

volume. The metabolism of this cell revolves around TAG synthesis and degradation (mobilization of FA). Phospholipids (PLs; Fig. 3), another major class of FA-containing complex lipids, are principal components of cellular membranes. Compared to TAG, PLs are less hydrophobic

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