Monounsaturated fatty acids nomenclature

Three major families of unsaturated fatty acids are seen in warm-blooded animals, that is, the n-9, monounsaturated fatty acids (e.g. oleic acid, OA), and the n-6 and n-3, both polyunsaturated fatty acids (PUFAs). However, only the n-6 and n-3 families, derived from LA and ALA, respectively, are EFA. These must be obtained from the diet since mammals lack the desaturase enzymes necessary for the insertion of a double bond in the n-6 and n-3 positions of the fatty acid carbon chain. Fatty acid nomenclature is as follows The first number denotes the number of carbon atoms in the acyl chain and the second refers to the number of unsaturated (double) bonds. This is followed by a symbol n or co and a number that denotes the number of carbon atoms from the methyl terminal of the molecule to the first double bond. Hence, LA is 18 2(n-6), while the more unsaturated ALA is denoted as 18 3(n-3) (Figure 26.1). These fatty acids must be metabolized to their longer chain derivatives before carrying out many of their activities. 

The most common dietary fatty acids are the saturated long-chain fatty acids palmitate (C16) and stearate (C18), the monounsaturated fatty acid oleate (C18 l), and the polyunsaturated essential fatty acid, linoleate (C18 2) (To review fatty acid nomenclature, consult Chapter 5). Animal fat contains principally saturated and monounsaturated long-chain fatty acids, whereas vegetable oils contain linoleate and some longer-chain and polyunsaturated fatty acids. They also contain smaller amounts of branched-chain and odd-chain-length fatty acids. Medium-chain-length fatty acids are present principally in dairy fat (e.g., milk and butter), maternal milk, and vegetable oils. 

Lipids with saturated fatty acids are called saturated fats and are commonly solids at room temperature (such as butter and shortening). Unsaturated fats contain one or more double bonds in their carbon-carbon chains. The cis and trans nomenclature we learned for alkenes applies Trans fats have H atoms on the opposite sides of the C=C double bond, and cis fats have H atoms on the same sides of the C=C double bond. Unsaturated fats (such as olive oil and peanut oil) are usually Hquid at room temperature and are more often found in plants. For example, the major component (approximately 60 to 80%) of olive oil is oleic acid, ds-CH3(CH2)7CH = CH(CH2)7COOH. Oleic acid is an example of a monounsaturated fatty acid, meaning it has only one carbon-carbon double bond in the chain. In contrast, polyunsaturated fatty acids have more than one carbon-carbon double bond in the chain. 

Figure 1 Fatty-acid structure and nomenclature. (A) Chemical formula and carbon atom numbering system for a 16-carbon saturated fatty acid (16 0). (B) Schematic representation of 16 0. (C) A monounsaturated fatty add, 18 1n-9, showing the double bond nine carbon atoms from the methyl end (carbon 18). (D) The essential n-6 fatty acid 18 2n-6, where the first double bond is found six carbon atoms from the methyl end. The two double bonds are separated by a methylene (-CH2-) group. (E) The essential n-3 fatty acid 18 3n-3, where the first double bond is found three carbon atoms from the methyl end. (F) Phytanic acid, a dietary / -methyl-branched-chain fatty acid (3,7,11,15-tetramethyl 16 0). The melhyl group on carbon 3 prevents this fatty acid from degradation by /3-oxidation. (G) Pristanic acid (2,6,10,14-tetramethyl 15 0) is the product of phytanic acid o-oxidation, in which a single carbon (carbon 1) is lost. The methyl group on carbon 2 does not preclude subsequent degradation by /3-oxidation.

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