Nomenclature fatty acids

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. 

Fatty acid nomenclatures as follows The number before the colon represents the number of carbon atoms. The number after the colon represents the number of ois double bonds. The number following the omega represents the position of the first double bond measured from the methyl end of the molecule. It is assumed that all of the double bonds are methylene interrupted. 

Figure 1. Fatty acid nomenclature - A. Arachidonic acid 20 4a>6 5-c, 8-c , 11-cis, 14-c -eicosatetraenoic acid. B. 10, ll(12)-H pH 10-hydroxy-ll(12)-epoxy-8-lriiifs-heptadecanoic acid. C. Prostaglandin B2 (PGB2) 15a-hydroxy-9-ketoprosta-5 tf-8(12), 13-lra t5-trienoic acid.

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 nomenclature for associating individual fatty acid groups with particular phosphodig-lyceride derivatives is straightforward. For instance, a phosphatidic acid (PA) derivative which contains two myristic acid chains is commonly called dimyristoyl phosphatidic acid (DMPA). Likewise, a PC derivative containing two palmitate chains is called dipalmitoyl phosphatidyl choline (DPPC). Other phosphodiglyceride derivatives are similarly named. 

FIGURE B-1 Structures of some fatty acids of neurochemical interest (see also Fig. 3-7 and text). The n minus nomenclature for the position of the double bond(s) is given here. Note that the position of the double bond from the carboxyl end can be indicated by the symbol A, so that linoleic acid may be also be designated as 18 2A9,12. The linolenic acid shown is the a isomer. 

The different phosphoglycerides are often named by placing the constituent attached to the phosphate group after phosphatidyl , e.g. phosphatidyl choline (3-in-phosphatidylcholine or l,2-diacyl-sn-glycero-3-phosphoryl-choline). There are many phosphoglycerides because of the possible variation in the fatty acid chains, and when the full chemical structure is known, it should be used (e.g. l-palmitoyl-2-oleoyl-phosphatidylcholine). Nomenclature that entails the use of the DL system should be avoided. 

There are two general systems of nomenclature for fatty acids, as well as a few useful shorthand designations. Common names have their origins in history and are frequently used. Several examples are provided in table 18.1. Systematic names derive from the number of carbon atoms in the fatty acid for example, lauric acid is systematically known as dodecanoic acid. The parent hydrocarbon dodecane has 12 carbon atoms as does lauric acid. Several other examples are provided in table 18.1. 

There is one nomenclature that is commonly used, particularly in news articles intended for the lay audience. It is useful to understand it. Here families of fatty acids are collected by designating where the first double bond occurs, starting at the methyl group (CH3) end of the molecule, frequently known as the omega (co) carbon atom. Thus, the omega-3 fatty acids look hke ... 

As noted several times in this text, one of the problems for a student entering any biochemical field for the first time is nomenclature this is very much the case for fatty acids. 

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). 

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