Fatty Acid Chain Length and Double Bond Positions

The common names and structures of some fatty acids of physiologic importance are listed in Figure 1.

In humans, fatty acids with an even number of carbon atoms (16, 18, or 20) predominate, with longer fatty acids (>22 carbons) being found in the brain. The carbon atoms are numbered, beginning with the carbonyl carbon as carbon 1. The number before the colon indicates the number of carbons in the chain, and those after the colon indicate the numbers and positions (relative to the carboxyl end) of double bonds. For example, as denoted in Figure 1, arachidonic acid, 20:4(5,8,11,14), is 20 carbons long and has four double bonds (between carbons 5–6, 8–9, 11–12, and 14–15). [Note: Carbon 2, the carbon to which the carboxyl group is attached, is also called the α-carbon, carbon 3 is the β- carbon, and carbon 4 is the γ-carbon. The carbon of the terminal methyl group is called the ω-carbon regardless of the chain length.] The double bonds in a fatty acid can also be referenced relative to the ω (methyl) end of the chain. Arachidonic acid is referred to as an ω-6 fatty acid because the terminal double bond is six bonds from the ω end (Fig. 2A). [Note: The equivalent designation of n-6 may also be used (Fig. 3B).] Another ω-6 fatty acid is the essential linoleic acid 18:2(9,12). In contrast, α-linolenic acid, 18:3(9,12,15), is an essential ω-3 fatty acid.

Figure 1: Some fatty acids of physiologic importance. [Note: A fatty acid containing 2–4 carbons is considered short; 6–12, medium; 14–20, long; and ≥22, very long.]

Figure 2: Arachidonic acid, 20:4(5,8,11,14), illustrating the position of the double bonds. A. Arachidonic acid is an ω-6 fatty acid because the first double bond from the ω end is 6 carbons from that end. B. It is also referred to as an n-6 fatty acid because the last double bond from the carboxyl end is 14 carbons from that end: 20 − 14 = 6 = n. Thus, the “ω” and “n” designations are equivalent .