J. Thomas Brenna, PhD and Gavin L. Sacks, PhD Lipids do not share any overall characteristic chemical structural similarity, but they can nevertheless be categorized into subclasses according to structural similarities. One system for categorizing major classes of lipids is listed in Box 6-1, emphasizing those that are directly and indirectly of nutritional importance. A comprehensive system oriented toward detailed molecular studies is available at Lipidomics Gateway (www.lipidmaps.org/). The nomenclature of lipids is dominated by trivial names that are either historical or driven by systematics of metabolism. As with all organic compounds, systematic organic chemical naming rules have been established for lipids, but traditional lipid nomenclature persists for good reason. Most traditional lipid naming conventions are convenient when viewed in the context of mammalian lipid metabolism and, to a lesser extent, are logical extensions of the traditional methods used to analyze lipids. Thus study of lipid nomenclature also reveals structural and metabolic relationships among lipids, and familiarity with nomenclature makes the study of lipid metabolism much clearer. TABLE 6-1 Naturally Occurring Straight-Chain Saturated Carboxylic Acids and Their Melting Points MP, Melting point; n, normal, straight-chain structural isomer. TABLE 6-2 Naturally Occurring Straight-Chain Unsaturated Fatty Acids and Their Melting Points MP, Melting point. PUFA biosynthesis is discussed in depth in Chapter 18; however, a brief overview is necessary to rationalize PUFA nomenclature. Consider the pathway for synthesis of docosahexaenoic acid (C22, 6 double bonds) from α-linolenic acid (C18, 3 double bonds). This pathway begins with the insertion of a double bond between carbons 6 and 7 of α-linoleic acid by a Δ6-desaturase to make stearidonic acid (C18, 4 double bonds). Two carbons are then added to the carboxyl end of the molecule by an elongase, followed by insertion of a double bond to make eicosatetraenoic acid (C20, 5 double bonds). Additional desaturation, elongation, and oxidation steps finally result in docosahexaenoic acid. Figure 6-3 shows the systematic organic chemistry names and numbering for the first and last structures in this pathway. The systematic name of α-linoleic acid is 9,12,15-octadecadienoic acid, whereas the systematic name of the final product is 4,7,10,13,16,19-docosahexaenoic acid. Counting from the C1 position, as is required in systematic naming, the double bonds that were numbered 9, 12, and 15 in the precursor are now numbered 13, 16, and 19 in the product (9 → 13; 12 → 16; 15 → 19). Repeated many times for PUFAs, these changes in bond position number when counted from the carboxyl carbon make it difficult to track double bonds and, more importantly, fatty acids that are derived from one another. γ-Linolenic acid is also similarly designated as 9,12,15-18:3. Notations in which the individual double bonds are denoted, by counting from the carboxyl end, lend themselves to designation of isomers of γ-linolenic acid, often by adding leading “c” or “t” to specify double bond cis or trans geometry (e.g., c9,t11-octadecadienoic acid, or 18:2Δ c9,t11, for rumenic acid. Examples of structures and nomenclature for monoacylglycerols, diacylglycerols, and triacylglycerols are shown in Figure 6-6. Use of the IUPAC-IUB sn nomenclature is generally preferred over the use of the trivial names in most cases, although trivial names are commonly used for simple acylglycerols (e.g., the use of triolein for 1,2,3-tri-cis-9-octadecenoyl-sn-glycerol). A benefit of the sn nomenclature is that it clearly reveals the relationship between the precursor TAG and its DAG and MAG hydrolysis products. For example, the hydrolysis of 1-palmitoyl-2-stearoyl-3-myristoyl-sn-glycerol at the 1 position yields 2-stearoyl-3-myristoyl-sn-glycerol (Figure 6-7). The general structure of the diacylphospholipids is presented in Figure 6-8, and the several classes of common phospholipids and their structures are shown in Figure 6-9. In all cases, a phosphate group is esterified to the sn-3 position of glycerol, and fatty acids are esterified to the sn-1 and sn-2 positions. Most commonly, the sn-2 position is occupied by an unsaturated acyl chain, whereas the sn-1 position is occupied by a saturated chain. However, there are notable exceptions to this generality. For instance, the major surfactant lipid of the lung has 16:0 in both positions, and some of the phospholipids of the retinal photoreceptors have very high concentrations of unsaturated fatty acyl chains in both positions.
Structure, Nomenclature, and Properties of Lipids
The Chemical Classes of Lipids—Their Structure and Nomenclature
Fatty Acids
Saturated Fatty Acids
# OF CARBONS
SYSTEMATIC NAME
TRIVIAL NAME
MP (° C)
2
Ethanoic
Acetic
17
3
Propanoic
Propionic
−21
4
n-Butanoic
Butyric
−8
6
n-Hexanoic
Caproic
−3
8
n-Octanoic
Caprylic
17
10
n-Decanoic
Capric
32
12
n-Dodecanoic
Lauric
44
14
n-Tetradecanoic Acid
Myristic
54
16
n-Hexadecanoic Acid
Palmitic
63
18
n-Octadecanoic Acid
Stearic
70
20
n-Eicosanoic Acid
Arachidic
75
22
n-Docosanoic Acid
Behenic
80
24
n-Tetracosanoic Acid
Lignoceric
84
Unsaturated Fatty Acids
ABBREVIATED NOTATION
SYSTEMATIC NAME
TRIVIAL NAME
MP (° C)
14:1n−5
cis-9-tetradecenoic
Myristoleic
−4
16:1n−7
cis-9-hexadecenoic
Palmitoleic
0.5
18:1n−7
cis-11-octadecenoic
cis-Vaccenic
15
t-18:1n−7
trans-11-octadecenoic
trans-Vaccenic
44
18:1n−9
cis-9-octadecenoic
Oleic
16
t-18:1n−9
trans-9-octadecenoic
Elaidic
47
20:3n−9
All cis-5,8,11-eicosatrienoic
Mead
22:1n−9
All cis-13-docosenoic
Erucic
35
18:2n−6
All cis-9,12-octadecadienoic
Linoleic (LA)
−5
18:3n−6
All cis-6,9,12-octadecatrienoic
γ-Linolenic (GLA)
−11
20:4n−6
All cis-5,8,11,14-eicosatetraenoic
Arachidonic (AA)
−50
22:5n−6
All cis-4,7,10,13,16-docosapentaenoic
DPA
18:3n−3
All cis-9,12,15-octadecatrienoic
α-Linolenic (ALA)
−10
20:5n−3
All cis-5,8,11,14-eicosapentaenoic
EPA
−54
22:6n−3
All cis-4,7,10,13,16,19-docosahexaenoic
DHA
−44
Acylglycerols
Monoacylglycerols, Diacylglycerols, and Triacylglycerols
Glycerophospholipids
Diacylphospholipids, the Common Phospholipids
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