Unsaturated fatty acids nomenclature

Using the omega nomenclature, the major classes of unsaturated fatty acids found in mammalian tissue are co-3, co-6 and co-9. They are not interconvertible (Figure 11.9). 

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. 

Table 17.1. Nomenclature of common saturated and unsaturated fatty acids.

Figure 35.8 Eicosapentaenoic add (EPA). A C20 5 poiy-unsaturated fatty acid, i.e. it has 20 carbon atoms and five cis-unsaturated bonds at C5, C8, Cll, C14 and C17. Nomenclature NB There is an alternative system for identifying the carbon atoms of fatty acids which is popular with nutritionists and uses Greek letters. The carboxylic acid group is ignored and the next carbon is a-, then P-, y-, etc. until the last caibon which is the last letter of the Greek alphabet, CD-. The system then counts backwards from ffl, so we have 1, 2, 3, etc. Thus EPA, which is an essential fatty add found in fish oU, is classified as a 3 fatty add. (Chemists (who claim to be the prima donnas of chemical nomenclature) pr er to label the last carbon n , so chemists refer to nl, n2, n3, etc.)...

Additional nomenclature for the unsaturated fatty acids is used to identify the site of C=C units relative to the terminal methyl group (the omega end). Because the closest double bond to the methyl group in palmitoleic acid (96) is 7 carbon atoms away from the methyl, palmitoleic acid is called an omega-7 ((0-7 or n-7) fatty acid. For a-linolenic acid (99), the double bond closest to the methyl group is only 3 carbons away, so it is an omega-3 ((0-3 or n-3) fatty acid. Several other acids are suitably labeled. 

The hydroperoxidation of unsaturated fatty acids performed by lipoxygenases is an important process involved in pathways leading to the inflammatory response. The lipoxygenase nomenclature derives from their positional specificity with respect to arachidonic acid substrates in mammalian organisms. The well studied plant Hpoxygenase-1 from soybeans is, in this respect, a 15-lipoxygenase. Crystal structure data for the inactive Fe(II) form in soybeans... 

Saturated fatty acids under systematic nomenclature contain the syllable, -an-, while unsaturated fatty acids contain the syllable. ... 

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). 

The composition of seed oil triglycerides is well understood. Triglycerides are fatty acid esters of glycerin, and the composition depends on the source of the oil (Figure 2). The nomenclature used is standard in the fats and oils industry, with the number of carbons in the fatty acid indicated first, followed by the number of sites of unsaturation in parentheses. 

The nomenclature of carboxylic acids and their derivatives was discussed in Section 7-6. Many carboxylic acids have trivial names and often are referred to as fatty acids. This term applies best to the naturally occurring straight-chain saturated and unsaturated aliphatic acids, which, as esters, are constituents of the fats, waxes, and oils of plants and animals. The most abundant of these fatty acids are palmitic, stearic, oleic, and linoleic acids. 

The numbering of the carbon atoms is, according to the nomenclature rules used by the Chemical Abstracts7, 8, consistent with the numbering for fatty acids. The compounds are typed depending upon the substitution pattern of the cyclopentane ring and classified by the degree of unsaturation in the side chains see Scheme 1. 

Table 26-1). In addition, the carbon atoms may be labeled with Greek symbols, with a being adjacent to the carboxyl group and 0) being farthest away. In the A-system, fatty acids are abbreviated according to the number of carbon atoms, the number of double bonds, and the position(s) of double bond(s). For example, linoleic acid, which contains 18 carbons and two unsaturated bonds between carbons 9 and 10 and between carbons 12 and 13, could be written as Ci8 2 Using the T - or (D-system, linoleic acid would be abbreviated to Ci8 2n-6, where only the first carbon forming the unsaturated pair is written. The Geneva or systematic classification is a third system of nomenclature (Table 26-1).

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 liquid 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, as-CH3(CH2)7CH=CH(CH2)7COOH. 

Throughout this book, an abbreviated nomenclature is used to designate the structures of fatty acids. For example, y-linolenic acid (GLA), the commonly used term for all-cw-6,9,12-octadecatrienoic acid, can also be designated as 18 3n-6. This indicates a fatty acid with 18 carbon atoms and 3 double bonds, with the first double bond being found on the 6th carbon from the methyl group. It is understood, in the absence of other indicators, that all the double bonds of all fatty acids have the cis configuration and are methylene-interrupted i.e. the unsaturated centres are separated from each other by one CH group ... 

Aliphatic framework molecules most common in organic acids include alkanes (saturated hydrocarbons) and alkenes (unsaturated hydrocarbons). These saturated and unsaturated aliphatic carboxylic acids may be acyclic (straight or branched chains) or alicyclic (aliphatic rings). Acyclic aliphatic monocarboxylic acids are also referred to as fatty acids (Table 1). The first five saturated acids (formic to valeric) of this type are sometimes referred to as short-chain, low-molecular-weight, or volatile fatty acids. Although a nomenclature for these acids has been established by the International Union of Pure and Applied Chemistry (lUPAC), the convention of using the trivial names for the first five saturated acids has remained. Similarly, trivial names are used for the aliphatic dicarboxylic acids (Table 2) that are saturated with two to four carbon atoms (C2-C4) and unsaturated with four carbon atoms (C4). Alicyclic carboxylic acids contain one or more saturated or partially unsaturated rings. These acids most commonly occur... 

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