Structure, Nomenclature, and Properties of Lipids

Structure, Nomenclature, and Properties of Lipids

J. Thomas Brenna, PhD and Gavin L. Sacks, PhD

Lipids are a diverse set of small molecules that are unified by their solubility in nonpolar solvents. They have many biological functions, and the amounts of lipids present in humans range from many kilograms for fatty acids to nanograms for docosanoids. The major biological functions of lipids include serving as structural components of cell membranes, serving as a form of energy storage, providing lubrication and conditioning for body surfaces, and functioning as signaling molecules of various types, including activators of nuclear receptors and G protein–coupled receptors and second messengers from phosphatidylinositol and sphingolipids. Lipids also function as receptors, antigens, sensors, electrical insulators, biological detergents, and membrane anchors for proteins.

Fats represent a major source of dietary and cellular energy. Although fat is sometimes used as a synonym for lipid, the subgroup of lipids called triacylglycerols (TAGs) is the dominant component of dietary fats and oils. In addition to TAGs, sterols (such as cholesterol and plant sterols) and membrane phospholipids (glycerophospholipids) are present in dietary lipids. TAGs and phospholipids all contain fatty acids esterified to glycerol and are important sources of fatty acids for oxidation to supply energy, essential fatty acids, and the gluconeogenic substrate glycerol. Most of the dietary lipid by far is TAG from fat droplets in plant and animal cells, and fat intake in the United States accounts, on average, for 32% to 37% of total caloric intake in adults.

Fat in animals is stored in specialized cells called adipocytes that contain very large fat droplets that push most of the cytoplasm toward the plasma membrane; fat is also found in smaller lipid droplets in other cell types. Fat in plants is found primarily as storage lipid droplets in the embryo or endosperm of seeds, where its purpose is to provide nutrition during germination. Important exceptions are olive and palm, where the lipids are mostly in the pulp. The fats and oils used in cooking are ones that have been separated from their animal and plant sources. Important animal fats are lard, tallow, and butterfat, and widely used plant oils include those extracted from soybeans, rapeseed/canola, cottonseed, peanuts, sunflower, corn, and olives.

The Chemical Classes of Lipids—Their Structure and Nomenclature

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 ( 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.

BOX 6-1   A Chemical Classification of Biologically Important Lipids

Nonesterified fatty acids

II Glycerolipids

III Glycerophospholipids

IV Glycoglycerolipids (including sulfates)


VI Isoprenoids (carotenoids, retinoids, prenols)

VII Steroids

VIII Biological waxes (long-chain ester waxes and related compounds)

IX Eicosanoids

Other lipids (acyl CoA, acylcarnitine, anandamide, lipopolysaccharides)

The chemical classification of lipids given in this table is necessarily incomplete and arbitrary. It progresses from hydrocarbons to more complex chemical structures. Simple esters and glycerol esters yield, on hydrolysis, alcohol and/or glycerol and fatty acid. The major membrane lipids, glycerophospholipids, yield fatty acid (or alcohol), glycerol, phosphate, and the appropriate base (choline, ethanolamine, etc.). Sphingolipids yield the base sphingosine and a fatty acid on hydrolysis. A comprehensive classification scheme can be found at and in abbreviated form at

Adapted from Small, D. M., & Zoeller, R. A. (1991). Lipids. In Encyclopedia of human biology (Vol. 4, pp. 725–748). Orlando, FL: Academic Press, Inc.

Fatty Acids

Fatty acyl chains are the basic units of glycerolipids that render these compounds nonpolar. They are referred to as fatty acids in part because many traditional analytical methods first hydrolyze all fatty esters into fatty acids. However, the level of free fatty acids (FFA), which are also called nonesterified fatty acids (NEFA), is very low compared to the amount of fatty acids present in esterifed forms, primarily as glycerolipids. The plasma concentration of FFA is about 0.3 to 0.6 mmol/L with about 99% of these FFA noncovalently bound to albumin. Higher concentrations of plasma FFA may occur locally at sites of high lipolytic activity (e.g., the capillary beds in adipose tissue, muscle, and heart) where FFA are liberated during lipolysis of TAG present in chylomicrons or very-low-density lipoproteins by lipoprotein lipase.

Fatty acids are characterized by their carboxylic acid head group and their hydrocarbon chain tail. Fatty acyl chains in glycerolipids are the result of condensation of the carboxylic acid head group with a hydroxyl (alcohol) group in the glycerol backbone via an ester bond. In mammals, fatty acids or fatty acyl chains may range from 2 carbons (C2) to as many as 40 carbons (C40) in length but primarily exist as C12 to C22 chains. Fatty acids are sometimes most conveniently classified according to chain length, although these categories are not rigidly defined. We can define short-chain fatty acids as those that have 6 or fewer carbon atoms. The chemistry of short-chain fatty acids is sufficiently dominated by the carboxyl group that they are soluble at least to some extent in water. Medium-chain fatty acids have 8 to 14 carbon atoms, and long-chain fatty acids have more than 14 carbons. Fatty acids are also classified according to their degree of unsaturation, which is the number of double bonds in their hydrocarbon chains.

Saturated Fatty Acids

The term saturated in the context of fatty acids refers specifically to fatty acyl chains that are exclusively made up of sp3 hybridized (tetrahedral geometry) carbon atoms arranged as linear −CH2− chains as shown in Figure 6-1. Such chains are said to be saturated with hydrogen because the chain has no double (or triple) bonds across which hydrogen may be added.

Fatty acids are synthesized predominantly through the successive addition of two-carbon units to a growing acyl chain by the fatty acid synthase enzyme. Chain-elongation ceases at 16 carbons to create palmitic acid in mammals, although small amounts of fatty acids with 14- (myristic acid) and 18- (stearic acid) carbons are also produced. Additional two-carbon units may be added to these fatty acids by elongation enzyme systems to yield longer saturated fatty acids. Table 6-1 presents the chain lengths, systematic names, trivial names, and melting points of the most abundant saturated fatty acids in mammalian tissues.

In mammals, the majority of saturated fatty acids are configured as straight chains, but in rare cases methyl groups extend from the main chain, usually near its terminal methyl. A fatty acid that terminates with two methyl groups (i.e., an isopropyl configuration) at the end of a hydrocarbon chain is referred to as iso. If the chain terminates with a methyl and an ethyl group (i.e., an isobutyl configuration), the fatty acid is said to be anteiso, as shown in Figure 6-2. In humans, branched-chain fatty acids appear in vernix, the protective waxy white substance that coats and protects the fetus during late gestation. They have also been detected as minor components of skin, blood, and hair, as well as in cancer cells. Iso and anteiso C15 and C17 saturated fatty acids are major components of some microorganisms such as gram-positive bacteria.

Unsaturated Fatty Acids

Unsaturated is used to describe fatty acids with at least one double bond, consisting of two adjacent sp2 hybridized carbon atoms, with a trigonal, approximately planar geometry. They are unsaturated with respect to hydrogen, because hydrogen can be covalently added across the double bond to yield sp3 carbon atoms. The overwhelming majority of unsaturated sites in mammalian fatty acids are double bonds configured in the cis geometry. When two or more double bonds are present in a molecule, the fatty acid is said to be polyunsaturated. Generally, double bonds in mammalian polyunsaturated fatty acids (PUFAs) are separated by a methylene (−CH2−) group (“methylene-interrupted”). This structural property of PUFAs confers special chemical properties. One important chemical property is that the double bonds are not conjugated, and thus there is free rotation about the −CH2− group. As with saturated fatty acids, use of trivial names is very common for unsaturated fatty acids. Table 6-2 is a compilation of the systematic and trivial names for the most common unsaturated fatty acids.

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 →

image Nutrition Insight

Fat Matters: Quality as well as Quantity

The rise of obesity in America in the last decade of the twentieth century has led to an explosion of research on metabolic consequences of excess adipose tissue. Diet fads that focus on either low-fat foods or low-carbohydrate (and thus high-fat) foods have prompted many studies of the metabolic consequences of dietary fat levels. Remarkably, much of this research has completely ignored the composition of fat and focused only on dietary amount, implying that the composition is unimportant. Many papers, including ones published in highly ranked research journals, report animal studies in which one group is fed a high-fat diet of unspecified composition that induces overeating and obesity and a control group is fed a standard rodent diet that contains a lower amount of fat, again of unspecified and almost certainly different composition.

That so little attention would be given to the composition of the dietary fat is remarkable in light of the large number of human and animal studies showing that dietary fatty acid composition has a profound influence on many aspects of metabolism. Detailed studies in humans and experimental animals, conducted in the 1950s and since, have shown that each fatty acid has unique but overlapping sets of metabolic properties (Warden and Fisler, 2008).

Could changes in dietary fatty acid composition have a role in the obesity epidemic? Do some fats adversely affect human health more than others? Alternatively, does obesity so overwhelm metabolism that the role of fat composition becomes much less significant? Whatever the answer, the issue cannot be overlooked.

The fatty acids consumed by Americans changed dramatically in the twentieth century. Seed oils such as soy, corn, and canola oils were rare before the industrial revolution of the 1800s because they require mechanical crushing or solvent extraction for efficient production. Fruit oils such as olive and palm oils, along with rendered animal fat (lard and tallow), were more widely used. The high-quality taste and low cost of seed oils drove a rise in seed oil production throughout the twentieth century. Today, soybean oil accounts for a staggering 20% of calories consumed by Americans, in the form of mayonnaise, deep frying fat, salad dressings, margarine, nondairy coffee creamers, snack foods, and sandwich spreads. It may be found in any food with an ingredient list that refers generically to “vegetable oil.”

The application of conventional and modern molecular methods (genetically modified) to engineer fatty acid composition of various food oils is resulting in the introduction of oils with modified fatty acid content into the food supply. A notable example is the genetic modification of soybeans to produce oils high in oleic acid. High oleic soy oil was developed to replace the use of trans fatty acid–rich hydrogenated fats for deep frying and other purposes for which an oil with high oxidative stability is needed. The high oleic soybean oil was generated by downregulating expression of the fatty acid desaturase gene that encodes the enzyme that converts monounsaturated oleic acid to the polyunsaturated linoleic acid. The oil from these soybeans contains about 80% oleic acid, compared to 25% for conventional soybean oil. At the same time, it contains less than 9% linoleic acid compared to 54% in conventional soybean oil, and less α-linolenic acid, 3% compared to 7% in conventional soybean oil. Commercial production of these high oleic soybeans was approved in North American countries in 2009–2010. Further modification of the fatty acid composition (e.g., increasing the α-linolenic acid content) is under development. Similarly, high oleic acid peanuts, with only 3% linoleic acid, are already on the consumer market in Australia as peanut butter and peanut-containing snacks. These current and upcoming changes to the fatty acid composition of the food supply will result in a major change, once again, in the fatty acid composition of fats in our diets and, with this change, we will likely see physiological consequences related to fatty acid composition rather than amount of fat.

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.

A solution is to number double bonds from the other end of the molecule, taking advantage of the fact that mammals cannot insert double bonds into the methyl end portion of the PUFA chain. Two conventions that are routinely used, which are effectively identical, are the IUPAC (International Union of Pure and Applied Chemistry) “n minus” convention and the omega convention. Examples of these notations are shown in Figure 6-3 for α-linolenic acid (18:3n−3) and docosahexaenoic acid (22:6n−3).

The IUPAC notation retains a close connection to the systematic organic chemistry notation. The “n” represents the number of carbons in the whole molecule, 18 in the case of α-linolenic acid and 22 in the case of docosahexaenoic acid. The number of double bonds follows the number of carbons, with the two separated by a colon (e.g., C18:3, or simply 18:3 for α-linolenic acid with three double bonds and 22:6 for docosahexaenoic acid with six double bonds). The location of the double bond closest to the methyl end of the fatty acid is indicated by the “n minus” nomenclature; for α-linolenic acid (18:3n−3), the double bond closest to the methyl end is carbon 18−3 or C15 (i.e., between C15 and C16). For docosahexaenoic acid (22:6n−3), the double bond closest to the methyl end is carbon 22−3 or C19 (i.e., between C19 and C20).

An alternative system called the omega notation, proposed by Holman, recognizes that the systematic organic chemistry numbering designates the carbon next to the carboxyl as “α,” and labels the last carbon in the acyl chain “ω.” The first double bond counting from the methyl end of α-linolenic or of docosahexaenoic acid appears at the third carbon and hence is designated ω3 (i.e., “omega three”). Although the IUPAC and omega numbering systems technically refer to different carbon atoms (e.g., C15 in 18:3n−3 versus C16 in 18:3ω3; or C19 in 22:6n−3 versus C20 in 22:6ω3), the n− and omega nomenclature effectively designate the equivalent fatty acids when the numeral following the n− or omega is the same. For example, 18:3n−3 and 18:3ω3 designate the same fatty acid (α-linolenic acid) and 20:4n−6 and 20:4ω6 both designate arachidonic acid.

Because all common PUFAs have cis double bonds arranged in methylene-interrupted positions, both notations are taken to imply cis geometry and methylene interruption of the double bonds. Thus a designation of n−6 (ω6) or n−3 (ω3), along with the number of carbons and number of double bonds, fully defines the structure of the PUFAs. For instance, “18:3n−6” or “18:3ω6” completely defines the structure of γ-linolenic acid as the fatty acid with the systematic name all-cis-6,9,12-octadecatrienoic acid.

Fatty acids with trans or conjugated double bonds do not ordinarily appear at other than trace levels in mammalian tissues (other than in the skin surface lipids), although they may be consumed as part of the diet. Example structures of trans and conjugated unsaturated fatty acids are shown in Figure 6-4. These fatty acids should not be designated with the n− or ω notation, unless further specification is provided. Unlike the situation with common fatty acids, there is no universal specialized notation for these more unusual fatty acids. Either systematic notation or some convenient adaptation of systematic numbering may be used. An example of acceptable notation would be “trans-18:1n−9” to specify the monounsaturated C18 fatty acid with a trans double bond between C9 and C10. For fatty acids with more than one double bond, a “Δ” notation system is frequently used. The superscript Δ precedes the designation of the sites of unsaturation counting from the carboxyl carbon. For instance, 18:3Δ6,9,12 designates γ-linolenic acid.

image Nutrition Insight

Vitamin F

The odd list of designations we now have for vitamins—A, D, E, and K for the fat-soluble vitamins, several Bs (with a few numbers missing) and C for the water-soluble vitamins, and some with no letters or numbers—is a remnant of the uneven but eventual progress in scientific understanding of the group of essential nutrients we call vitamins. What of other letters between E and K?

Vitamin F was proposed as the name of a factor associated with fat, and this proposal appeared in several papers in the 1920s. The discovery that fat-containing diets are essential for health is usually assigned to a 1929 paper of George and Mildred Burr (Burr and Burr, 1929), then working in the attic of the University of Minnesota medical school (Holman, 1992). They showed that rats fed fat-free diets developed scaly and sore skin, especially on the face and tail; had hair loss; grew at about two thirds of the normal rate; had a shortened life span; and were unable to reproduce. Animals that were emaciated and covered with very scaly skin were placed on 10 “drops” of lard per day. The animals immediately showed signs of recovery and within 10 weeks were fully “cured.” No recovery was seen when the non–fatty acid (nonsaponifiable) fraction of lard was used, thus indicating that the essential fat consisted of fatty acid(s). Subsequent studies established that the fat of greater unsaturation improved the ability of the fat to cure eczema (Brown et al., 1938). However, the inability of scientists to reproduce these effects in humans reduced their interest in vitamin F. We now know that periods greater than 6 months on fat-free diets are required to cause overt deficiency symptoms in humans. Today, the essentiality of the n−6 and n−3 fatty acids, linoleic (18:2n−6) and α-linolenic (18:3n−3) acid, is clearly established, and we would consider these to be components of the so-called vitamin F. There is still controversy about whether humans can synthesize sufficient long-chain PUFAs from 18:2n−6 and 18:3n−3 for optimum health. If, in fact, long-chain fatty acids are essential at some or all stages of the life cycle, PUFAs such as 20:4n−6, 20:5n−3, and 22:6n−3 might also be considered separate components of vitamin F. As progress continues toward defining the details of human requirements for PUFAs, as well as the role of specific PUFAs in human metabolism, the concept of a series of vitamin Fs may yet prove useful.

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.


Acylglycerols are esters of glycerol and fatty acids. Acylglycerols are synthesized by esterification of fatty acids to the hydroxyl groups of the three-carbon sugar, glycerol. Thus acylglycerols may have up to three fatty acid moieties esterified to the glycerol “backbone.” Dietary fats and oils are mainly mixtures of triacylglycerols. Before we can discuss the nomenclature of acylglycerols (and glycerophospholipids in the next section), it is important to understand the stereochemistry of glycerol.

Prochiral Glycerol

Stereochemistry is an important property of glycerolipids. The glycerol molecule possesses a plane of symmetry such that the central carbon atom can be considered prochiral, a chemical term referring to a carbon atom with four substituents, three of which are different. A prochiral carbon is not chiral, but substitution of one of the equivalent substituent groups with a fourth, nonequivalent group renders the central carbon chiral. This is the case for the central carbon of glycerol: when non–chemically equivalent moieties are added to the −CH2OH groups, the central carbon becomes chiral. Chirality is very important to biochemical properties and thus must be described unambiguously. Here, the systematic notation of organic chemistry, with rules for designating chiral centers as R or S, leads to even more confusing designations than in the case of fatty acid double bond position. However, a single notation that closely parallels metabolism was introduced in the 1960s and is widely used. As shown in Figure 6-5, glycerol can be positioned so that the top and bottom −CH2OH are oriented with their −OH groups extending to the right and the middle −OH extending to the left. By convention, the positions are referred to as sn-1, sn-2, and sn-3 (top to bottom), with sn being an abbreviation for “stereospecific numbering.” In some contexts, the sn-1 and sn-3 positions are metabolically equivalent and are designated the α positions, with the center sn-2 position then being designated the β position. When non–chemically equivalent groups are added to the sn-1 and sn-3 positions, glycerol becomes chiral.

Monoacylglycerols, Diacylglycerols, and Triacylglycerols

Acylglycerols are formed by the esterification of one or more of the glycerol –OH groups with a fatty acid carboxyl group by means of an ester linkage. A single acyl substitution to form an ester bond forms a monoacylglycerol, which may be designated as a 1-, 2-, or 3-monoacyl-sn-glycerol, depending on location of the fatty acid on the glycerol moiety. For example, esterification of hexadecanoic acid (palmitic acid) to the 1-position of glycerol produces 1-hexadecanoyl-sn-glycerol (1-palmitoyl-sn-glycerol). When two fatty acids are reacted with glycerol to form ester bonds, a diacylglycerol is formed (e.g., 1,3-diacyl-sn-glycerol or 1,2-diacyl-sn-glycerol). Both monoacylglycerols and diacylglycerols occur in relatively low proportion in mammals but are important as biochemical intermediates in many lipolytic reactions and are critical building blocks in the synthesis of more complex phospholipids and triacylglycerols. Diacylglycerols also act as second messengers for some membrane-triggered reactions.

A triacylglycerol (TAG) is formed when all three hydroxyls of glycerol form ester bonds with fatty acids. TAGs are the most common form of lipids in food and in mammalian tissues. The older abbreviated term triglyceride and its abbreviation TG are also used as synonyms for TAG, primarily in the medical literature. The properties of acylglycerols (e.g., melting point) depend greatly on the fatty acid chains involved. The fatty acids in a TAG may be all the same, all different, or two of a kind with one different one. If all three fatty acids are the same, the TAG is called a simple TAG (e.g., triolein). If one of the fatty acids is different, it becomes a complex TAG. If the fatty acids at the sn-1 and sn-3 positions are different, the TAG is chiral. Chirality in TAG is of less physiological importance than in other glycerolipids because many enzymes catalyzing reactions involving TAGs do not distinguish between the sn-1 and sn-3 (i.e., α) positions. Saturated fatty acyl chains tend to be found in the sn-1 and sn-3 positions, whereas unsaturated fatty acyl chains tend to be found in the sn-2 position of acylglycerols. There are notable exceptions, however, as in lard (pork fat) and human milk, for which 16:0 is predominantly in the sn-2 position and unsaturated fatty acyl chains are in the sn-1 and sn-3 positions.

The three unique positions of the glycerol backbone permit a tremendous number of positional isomers—that is, TAG with the same three fatty acids arranged in many different ways on the three positions of glycerol. For instance, consider the number of isomeric TAGs with the three fatty acids: palmitic (P), oleic (O), and stearic (S) acids. They could be arranged in six different ways listed in sn-1,2,3 order: POS and SOP, PSO and OSP, OPS and SPO, where the pairs are stereoisomers. Moreover, a fat containing these three fatty acids may not contain all fatty acids on all distinct TAG molecules; it may also contain PPP, PPS, OSO, and other combinations. In general, the number of unique TAG isomers is F3, where F is the number of fatty acids present in a sample. Even simple fats have more than 10 different fatty acids, which could be present as 103, or 1,000, chemically distinct TAG molecules.

TAGs are the major storage lipids of plants and higher animals. Both plant oils (olive, corn, safflower) and animal fats (lard, suet, tallow) are predominantly mixtures of complex TAGs. A few percent of sterols, vitamins, free fatty acids, carotenoids, and other fat-soluble molecules are usually present in oils and fats. In animals, adipose tissue is the main source of fat, but skeletal muscle, heart, liver, skin, and bone marrow often contain appreciable amounts of TAGs in intracellular oil droplets.

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).


Glycerophospholipids are commonly described as phospholipids or phosphoglycerides. These lipids are derived from the parent compound phosphatidic acid, which is also known as diacylglycerol 3-phosphate. Other members of the glycerophospholipids include lysophospholipids and diphosphatidylglycerols. All glycerophospholipids contain at least one fatty acid esterified to the glycerol backbone.

Diacylphospholipids, the Common Phospholipids

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.

The simplest phospholipid is phosphatidic acid (diacylglycerol 3-phosphate), in which phosphoric acid is esterified to

image Nutrition Insight

Trans Fatty Acids: The Good, the Bad, and the Ugly

Trans fats refers to the presence of trans double bonds in an unsaturated fatty acyl group in the fat. Trans double bonds arise from two sources: chemical catalytic hydrogenation of unsaturated oils and as normal products of rumen bacterial and bovine physiology in dairy products. Those arising from hydrogenation of oils have a generally bad reputation as unnatural promoters of heart disease. Food labeling rules implemented by the U.S. Food and Drug Administration in 2006 specified that the content of trans fats be shown on all food labels, and this resulted in many food producers removing trans fats from their prepared foods. In contrast to the trans fats in hydrogenated fats, the trans fats in dairy products are associated with beneficial physiological effects, at least in animal studies.

Chemical hydrogenation adjusts the melting point of oils, turning them into solid fats, by saturating many double bonds with hydrogen and isomerizing others from cis to trans. Saturated and trans fatty acids in oils have higher melting points than corresponding fatty acids with cis double bonds, and thus any oil can have its melting point finely adjusted by adding just the right amount of hydrogen. Hydrogenation lowers the cost of shortening for baked goods because any inexpensive, high-quality, deodorized/flavor-neutral oil available at a particular time can be hydrogenated to make shortening. Furthermore, hydrogenation destroys PUFAs, particularly α-linolenic acid, which is considered to be a major cause of rancidity that limits the life of frying oil and other high temperature–treated foods.

The trans fatty acids resulting from hydrogenation mainly constitute a series of monoenes with trans double bonds distributed at various positions along the hydrocarbon chain, centered at the site of the original cis double bond. Conjugated double bonds of various positions and geometries are also created. These distributions of trans monoene and diene fatty acids are thought to be atherogenic due to their increasing plasma cholesterol in a manner similar to some saturated fats (Hu and Willett, 2002), especially myristic and lauric acids.

In contrast to hydrogenated fats, dairy products have a very specific distribution of trans fatty acids. The most prominent dairy trans fat is a monoene, trans-11-18:1 (vaccenic acid), with smaller amounts of other monoenes with double bonds located at the C4 to C16 positions. Also present at lower concentrations are dienes known as conjugated linoleic acids (CLAs). The most prominent of these is the cis-9, trans-11-18:2 fatty acid, named rumenic acid (Kramer et al., 1998), constituting around 90% of all CLA in dairy products. This fatty acid is a product of the rumen production of vaccenic acid, which is acted upon by a Δ9-desaturase in the cow’s mammary gland (Kay et al., 2004). The conversion of vaccenic acid to rumenic acid also occurs in humans (Turpeinen et al., 2002). The trans-7, cis-9 isomer of 18:2 is made in a similar way but accounts for much less of the total. Rumenic acid has potent anticarcinogenic properties in rats, and a close structural isomer, trans-10, cis-12-18:2, has antiobesity effects in rats (Pariza, 2004). Neither effect has been confirmed in humans, but research is continuing on the possible health benefits of these trans double bond–containing isomers of monoene and diene fatty acids found in dairy products.

Most regulatory definitions of trans fat, such as those of the U.S. Food and Drug Administration and the Codex Alimentarius for international trade, define trans fat as the geometrical isomers of monounsaturated and polyunsaturated fatty acids having nonconjugated carbon–carbon double bonds in the trans configuration. These definitions thus specifically exclude the trans fats (CLA and vaccenic acid) that are found in dairy products and beef.

Stay updated, free articles. Join our Telegram channel

Feb 26, 2017 | Posted by in PHARMACY | Comments Off on Structure, Nomenclature, and Properties of Lipids

Full access? Get Clinical Tree

Get Clinical Tree app for offline access