Structure and Properties of Lipids

OUTLINE

Lipids and Their Functions The Chemical Classes of Lipids Hydrocarbons Substituted Hydrocarbons Waxes, Esters, and Ethers Acylglycerols and Fats Glycerophospholipids Sphingolipids Steroids The General Properties of Lipids Packing of Lipids in the Solid State The Liquid Crystalline State of Lipids The Liquid State—Melts, Solutions, and

Suspensions Determinants of Lipid Melting Surface Behavior of Lipids at the Water Interface

Lipid Classification Based on Physical Interaction with Water Properties of Dietary Oils and Fats and Their Products: Diacylglycerols, Monoacylglycerols, and Fatty Acids Melting and Crystallization of Acylglycerols, Fatty Acids, and Acid-Soaps

Effects of Positions of Fatty Acids in Mixed-Chain Triacylglycerols upon Their Absorption from the Intestine Polymorphism of Fats and Oils The Surface Orientation of Different Triacylglycerols at the Water Interface

COMMON ABBREVIATIONS

CMC (critical micellar concentration) DHA (docosahexaenoic acid) EPA (eicosapentaenoic acid) PC (phosphatidylcholine) PE (phosphatidylethanolamine) PG (phosphatidylglycerol) PI (phosphatidylinositol) PS (phosphatidylserine)

LIPIDS AND THEIR FUNCTIONS

Lipids are one of the four major classes of biologically essential organic molecules found in all living organisms (the other classes are proteins, carbohydrates, and nucleic acids). A broad range of compounds are included in this class, based on their solubility characteristics. Lipids contain a substantial contiguous portion of aliphatic or aromatic hydrocarbon and have intermediate gram molecular weights that range between 100 and 5000. Included are hydrocarbons, steroids, soaps, detergents, and more complex molecules such as waxes, triacylglycerols (fats and oils), phospholipids, sphingolipids, fat-soluble vitamins, and lipopolysaccharides.

The major lipids are classified chemically in Table 3-1. They function as barriers, receptors, antigens, sensors, electrical insulators, biological detergents, membrane anchors for proteins, and, last but not least, a major energy source. Phospholipids play a critical role in maintaining the integrity of all living things, plant and animal, because they form the barrier separating the living cell from the extracellular environment. This barrier is called the cell or plasma membrane. It consists of a continuous bilayer of phospholipids into which other lipids, such as cholesterol and glycosphingolipids, are inserted. Into this lipid bilayer, protein channels, transporters, receptors, structural pillars such as integrins, and other functional elements are inserted to give the plasma membrane its unique characteristics. The fatty acid composition of the phospholipids and the cholesterol content regulate the fluidity and perhaps the thickness of the membrane. More cholesterol and saturated or irans-unsaturated fatty acyl groups in phospholipids and glycosphingolipids tend to stiffen the membrane. Glycosphingolipids are present only on the external surface of the plasma membrane. They act as receptors for toxins (e.g., ganglioside GM, is the receptor for cholera toxin) and probably as antigens to mark the cell as being of a certain type.

Phospholipids also form membranes, or barriers, between compartments of the cell (e.g., the membranes of the nucleus, the mitochondria, the endoplasmic reticulum (ER), the Golgi apparatus, secretory vesicles, and

TABLE 3-1

A Chemical Classification of Lipids*

  1. Hydrocarbons (normal, branched, saturated, unsaturated, cyclic, aromatic)
  2. Substituted hydrocarbons
  3. Alcohols
  4. Aldehydes
  5. Fatty acids, soaps, acid-soaps

D. Amines

  1. Waxes and other simple esters of fatty acids
  2. Fats and most oils (esters of fatty acids with glycerol)
  3. Triacylglycerols
  4. Diacylglycerols
  5. Monoacylglycerols

V Glycerophospholipids (diacyl; O-aikyl, acyl; di-O-alkyl)

  1. Phosphatidic acid
  2. Choline glycerophospholipids
  3. Ethanolamine glycerophospholipids
  4. Serine glycerophospholipids
  5. Inositol glycerophospholipids E Phosphatidylglycerols
  6. Lysoglycerophospholipids
  7. Glycoglycerolipids (including sulfates)
  8. Sphingolipids
  9. Sphingosine
  10. Ceramide
  11. Sphingomyelin
  12. Glycosphingolipids [ceramide monohexosides (cerebrosides), ceramide monohexoside sulfates, ceramide polyhexosides]
  13. Sialoglycosphingolipids (gangliosides)
  14. Steroids (sterols, bile acids, digitonin and other cardiac glycosides, sex and adrenal hormones)
  15. Other lipids [vitamins (A, D, E, K), eicosanoids, acyl CoA, acylcarnitine, glycosyl phosphatidylinositols, lipopolysaccharides, ubiquinone, dolichols]

*The chemical classification of lipids given in this table is necessarily incomplete and somewhat arbitrary. It progresses from hydrocarbons to more complex chemical structures. Simple esters and glycerol esters yield, on hydrolysis, the 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.

From Small, D. M. and Zoeller, R. A. (1991) Lipids. In: Encyclopedia of Human Biology Vol. 4, pp. 725-748. Academic Press, Inc., Orlando, Florida.

peroxisomes). Phospholipids by themselves form very strong bilayers, which are effective barriers and do not let most molecules, including water and ions, cross readily Each membrane must have specific proteins bound to or inserted through the membranes to allow certain molecules to pass or be transported from one side to the other. These protein assemblies are often called pores, channels, or transporters. They transport very large molecules such as RNA through the nuclear pores and medium-sized molecules such as proteins through the ER. Small ions [e.g., hydrogen (H+), sodium (Na+), potassium (K+), and calcium (Ca2+)] and water are also carried by transporters. There are many such transporters, and they often have receptors for a specific molecule associated with them. These receptors bind the molecule and pass it to the transporter, which moves it across the cell membrane. It is widely believed that the function of these receptors and channels is affected by the type of membrane lipids in which they reside. However, firm proof of this hypothesis in vivo is lacking (Edidin, 1997).

Fats, oils, and waxes are stored in cytoplasmic droplets and represent a major source of cellular energy. An average human being in metabolic balance may easily ingest and absorb 100 g of fat in a day The 900 kcal derived from burning this fat would represent 35% to 45% of the total energy consumed. Components of fats (i.e., fatty acids, fatty alcohols, and glycerol) are used as building blocks for membranes during growth, maintenance, and repair. Many hormones are lipids. The steroid hormones (Cortisol, estrogens, proges-terones, and androgens), derived from cholesterol, and prostaglandins and leukotrienes, derived from polyunsaturated fatty acids, are examples. Low concentrations of these molecules affect important physiological changes. Cholesterol has recently been found to be essential in embryogenesis, and an absolute deficiency of this sterol leads to severe and often fatal defects (Porter et al., 1996; Salen et al., 1996). Other lipids act as regulators of intracellular processes (e.g., diacylglycerol, sphingosine, ceramides, and platelet-activating factor). Some lipids are secreted and act as pheromones that attract or repel other organisms. In higher animals, lipids are transported to and from cells in the form of small (10 to 10,000 nm diameter) aggregates called lipoproteins. Lipids in the brain, spinal cord, and nerves are ordered in a way that permits the transmission of electrical impulses without short-circuiting between nerves or tracts. Lipids also play a role in many diseases that afflict humans (e.g., atherosclerosis, obesity gallstone disease, Reye's syndrome, the famil ial lipidoses such as Tay-Sachs disease, Nie-mann-Pick disease, and Gaucher's disease, and the familial lipoproteinemias).

Most lipids, including most fatty acid building blocks of complex glycero- or sphin-golipids, can be made by cells. However, in humans, certain fatty acids cannot be synthesized de novo and must be ingested. These are called the essential fatty acids, and if they are not ingested, deficiencies will occur. Two key types of essential fatty acids are known, one having a cis double bond at the 3rd carbon from the methyl end and the other a cis double bond at the 6th carbon counting from the terminal methyl group. These are called the omega 3 («3) and omega 6 (co6) series of fatty acids (Nestlé Nutrition Workshop, 1992). These essential fatty acids are discussed in detail in Chapter 15. The co3 series is present in some plant oils as a-linolenic acid (Table 3-2). This 18 carbon fatty acid with 3 cis double bonds can be elongated and further unsaturated to form the 20-carbon eicosapen-taenoic acid (EPA) and then converted to the 22-carbon acid docosahexaenoic acid (DHA). EPA and DHA are found in high amounts in some fish oils. These fatty acids become acyl groups on some phospholipids. DHA is very enriched in parts of the brain and retina and appears essential for proper neural function and vision. The «6 series is widely found in plant oils as linoleic and -y-linolenic acids (Table 3-2). They are elongated and further unsaturated to arachidonic acid (C20:4a)6), which is essential for the formation of prostaglandins and leukotrienes.

Essential fatty acid deficiency can occur in premature infants, children, and adults deprived of adequate w3 and/or «6 fatty acids as can occur in individuals with large resections of the intestine, fat malabsorption syndromes, or inadequate dietary intake. Adults need some fat (perhaps 30 to 50 g/day) and must have an adequate supply of essential fatty acids (a few grams a day), but average fat intake is much higher than this, perhaps 100 to 110 g/day in the United States and other westernized countries. Excess intake of cholesterol and fat, especially saturated fat, contributes greatly to obesity atherosclerosis, diabetes, gallstone disease, and perhaps cancer.

TABLE 3-2

Names, Formulas, and Selected Properties of Some Common Fatty Acids

TABLE 3-2

Names, Formulas, and Selected Properties of Some Common Fatty Acids

Fatty Acid

Chemical Name

A Formula*

<o Formula-]"

MW

(imoi/L

(25°Q

Saturated

Laurie (D)

Dodecanoic acid

12.0

200.31

44.2

11.5

Myristic (M)

Tetradecanoic acid

14.0

228.36

54.4

0.79

Palmitic (P)

Hexadecanoic acid

16.0

256.42

62.9

0.12

Stearic (S)

Octadecanoic acid

18.0

284.47

69.6

(18 X 10-3)^

Arachidic

Eicosanoic acid

20.0

312.52

75.4

(27 X 10"4)i

Behenic

Docosanoic acid

22.0

340.57

80.0

Lignoceric

Tetracosanoic acid

24.0

368.62

84.2

Monounsaturated

Myristoleic

ds-9-Tetradecenoic acid

14.1 9c

14: 1&>5

226.34

(2.00)$

Palmitoleic

c/s-9-Hexadecenoic acid

16.1 9c

16: lo>7

254.40

0.5

(0.30)|

Oleic (0)

c/s-9-Octadecenoic acid

18.1 9c

18: lo>9

282.45

13.4a; 16.3|J

(45 X 10"3)f

Elaidic (E)

irans-9-Octadecenoic acid

18.1 9t

18:1&>9 (trans)

282.45

46.5

c/s-Vaccenic

tis-1 l-Octadecenoic acid

18.1 lie

18: lco7

282.45

14.5

Petroselinic

ds-6-Octadecenoic acid

18.6 6c

18: lo)12

282.45

30

Erucic (Er)

cis-13-Docosenoic acid

22.1 13c

22: lo>9

348.55

34.7

(7 X 10-3)f

Polyunsaturated

Linoleic (L)

all c/s-9,12-0ctadecadienoic acid

18.2 9c 12c

18:2w6

280.44

-5

7-Linolenic

all c/s-6,9,12-Octadecatrienoic acid

18.3 6c9cl2c

18:3io6

278.44

Linolenic (Ln)

all as-9,12,15-Octadecatrienoic acid

18.3 9cl2cl5c

18:3a)3

278.44

-10 (-11.3)

ETA§

all ci's-5,8,ll-Eicosatrienoic acid

20.3 5c8cllc

20: 3o)9

Arachidonic (A)

all ci's-5,8,II,i4-Eicosatetraenoic acid

20.4 5c8cUcl4c

20:4(o6

304.5

-49.5

EPA||

all ci's-5,8,ll,14,17-Eicosapentaenoic acid

20.5 5c8cllcl4cl7c

20: 5&)3

302.5

DHA||

all ci's-4,7,10,13,16,19-Docosahexaenoic acid

22.6 4c7cl0cl3cl6cl9c

22:6(o3

328.5

'Formulas are shown as the number of carbon atoms followed by the number of double bonds. The position of each double bond is indicated by the lower of the numbers of the two doubly bonded carbon atoms, counting from the carboxyl carbon and specified as to cis (c) or trans (/) configuration. Thus, oleic (c/s-^octadecenoic) acid is 18.1 9c.

tThe number of the carbon atoms starts from the methyl end, and the location of the first (or only) double bond is indicated by a single number as a suffix preceded by "o>". Oleic acid is therefore designated as cl8: lw9, which informs the reader that the most distal double bond is 9 carbons from the methyl terminus. It is assumed that unless otherwise specified, all multiple double bonds in polyunsaturated fatty acids have the 1,4 relation to each other.

  • Extrapolated from solubility of shorter-chain acids.
  • ETA, eicosatrienoic acid, an acid that accumulates in essential fatty acid deficiency.
  • Found in marine animals in high concentration. EPA, eicosapentaenoic acid; DHA, docosahexaenoic acid.
  • Formulas are shown as the number of carbon atoms followed by the number of double bonds. The position of each double bond is indicated by the lower of the numbers of the two doubly bonded carbon atoms, counting from the carboxyl carbon and specified as to cis (c) or trans (/) configuration. Thus, oleic (c/s-^octadecenoic) acid is 18.1 9c.

tThe number of the carbon atoms starts from the methyl end, and the location of the first (or only) double bond is indicated by a single number as a suffix preceded by "o>". Oleic acid is therefore designated as cl8: lw9, which informs the reader that the most distal double bond is 9 carbons from the methyl terminus. It is assumed that unless otherwise specified, all multiple double bonds in polyunsaturated fatty acids have the 1,4 relation to each other.

  • Extrapolated from solubility of shorter-chain acids.
  • ETA, eicosatrienoic acid, an acid that accumulates in essential fatty acid deficiency.
  • Found in marine animals in high concentration. EPA, eicosapentaenoic acid; DHA, docosahexaenoic acid.

From Small, D. M. and Zoeller, R. A. (1991) Lipids. In: Encyclopedia of Human Biology, Vol. 4, pp. 725-748. Academic Press, Inc., Orlando, Florida.

THE CHEMICAL CLASSES OF LIPIDS

As summarized in Table 3-1, lipids include many types of compounds: hydrocarbons, steroids, soaps, detergents, and more complex molecules such as waxes, triacylglycerols (fats and oils), phospholipids, sphingolipids, fat-soluble vitamins, and lipopolysaccharides.

Hydrocarbons

Hydrocarbons contain only hydrogen and carbon. They may be saturated or unsaturated, branched or unbranched, cyclic or aliphatic, or they may exhibit a combination of these characteristics. The polyisoprenoids are a major source of hydrocarbons of biological origin (Ness and McKean, 1977). For example, a group of 40-carbon polyenoic compounds (phytoene, phytofluenes, lycopene, etc.), composed of isoprene units:

is synthesized from mevalonic acid in plant cells to produce the carotenoids (Fig. 3-1). These complex hydrocarbons are ingested by animals, and some proportion is taken up into the gut cells. One of these, (3-carotene, is oxidatively cleaved in the center to produce vitamin A, a 20-carbon polyisoprenoid alcohol, as discussed in Chapter 26. In animals, a similar pathway from mevalonic acid leads to the formation of ubiquinone (coenzyme Q), farnesyl (15 carbons) and geronylgeronyl (20 carbons) moieties, and the 30-carbon polyisoprenoid squalene (Fig. 3-1), which undergoes cyclization and oxidation to form steroids.

Dolichols are also derived through the isoprenoid synthetic pathway and consist of 15 to 19 isoprene units (75 to 90 carbon atoms), with phosphate esterified to the terminal alcohol group. They are, found in the ER and Golgi membranes and function as anchors for oligosaccharide chains being synthesized in the lumen of these organelles. Their organization within these membranes is not known. These 75- to 95-A-long molecules are either buried in the center of the bilayer, or they fold back and forth within the membrane three or four times, or their hydrophobic parts must be shielded by hydrophobic domains of proteins on either the cytosolic or luminal surface. The hydroxyl group of dolichol, which is usually esterified to phosphate, appears to be on the cytosolic side (outside) of the ER or Golgi apparatus, and to this a large number of sugars (up to 14) are attached by specific transferases. The oligosaccharide chain is then translocated en bloc to protein in the lumen during the posttranslational gly-cosylation process. Little is known about the physical properties of the dolichol phosphates and the dolichol glycophosphates.

The fat-soluble vitamins A, E, and K are also derived through modifications in the isoprenoid pathway. Vitamin D, which is derived from a sterol, contains isoprene units. These lipophilic molecules are found largely associated with lipid membranes or bound to specific carrier proteins.

Substituted Hydrocarbons

Alcohols (R-CH2OH), aldehydes (R-CHO), acids (R-COOH), and amines (R-CH2NH2) are examples of substituted hydrocarbons.

Figure 3-1. Line models of |3-carotene and squalene, showing the similarity of their structures.

Figure 3-1. Line models of |3-carotene and squalene, showing the similarity of their structures.

Such molecules are usually found in low (usually micromolar) concentrations in cells, as they are rapidly metabolized. Fatty acids are the most abundant of the substituted hydrocarbons. The plasma concentration is about 0.5 to 1.0 mmol/L with about 99% bound to albumin. Fatty acids have the potential to reach high local concentrations during fat ca-tabolism. For instance, during lipolysis of chylomicron or very low density lipoprotein (VLDL) triacylglycerols by plasma lipoprotein lipase, free fatty acids are liberated. Sites of high lipolytic activity (e.g., the capillary beds in adipose tissue, muscle, and heart) may see very high concentrations of the fatty acids. In certain disease states (e.g., diabetic ketoacidosis, nephrotic syndrome, hypertriglyceridemia, and hyperthyroidism), the concentration of plasma fatty acids rises; this results in an elevation of the fatty acid to albumin ratio and facilitates greater partitioning of the fatty acids to plasma lipoproteins and to cell membranes, which may possibly cause local damage (Cistola et al., 1986).

The major saturated fatty acids in higher animals are palmitic (16 carbons) and stearic (18 carbons), followed by smaller amounts of 12-, 14-, and 20-carbon fatty acids (Table 3-2). The major monounsaturated fatty acids are oleic acid-(18:lco9), which has 18 carbon atoms and a cis double bond at carbon 9; vaccenic acid (18:lw7), which has the double bond at carbon 11 (numbering from the car-boxyl end); and palmitoleic acid, (16:lw7), which has a cis double bond at carbon 9. The major polyunsaturated fatty acids in plasma and tissues are Iinoleic (C18:2(o6), arachi-donic (C20:4w6), eicosapentaenoic (EPA; C20:5w3), and docosahexaenoic (DHA; C22:6w3). In polyunsaturated fatty acids, the double bonds are usually three carbons apart (-CH2-CH = CH-CH2-CH = CH-CHjr-). Arachi-donic acid is shown in Figure 3-2. Abbreviated nomenclature for fatty acids, numbering carbons from either the carboxyl (A system) or from the terminal methyl (<o system) end, is shown in Table 3-2.

Fatty acids are essential building blocks for membrane lipids. The essential fatty acids Iinoleic (C18:2a>6), arachidonic (C20:4co6), EPA (C20:5o>3), and DHA (C22:6w3) are necessary for proper growth and membrane function. Arachidonic acid, which is derived from Iinoleic acid by chain elongation and desatu-ration, is a major precursor of eicosanoids such as prostaglandins, prostacyclins, thromboxanes, and leukotrienes.

Recently it has been shown that some proteins are covalently linked to fatty acids. The reactions show high specificity for certain fatty acids. For instance, myristic acid (C14:0) is specifically attached to the amino group of the N-terminal glycine by an amide bond in a number of proteins, including the oncogenic viral Src protein, calcineurin, and recoverin (Johnson et al., 1994). Viral Src protein is oncogenic only when it is myristoylated; calcineurin and recoverin are Ca2+-sensing proteins in the brain and retina, respectively The myristoyl group acts as a switch when Ca2+ binds to the protein and allows the protein to anchor to a membrane to start a cascade of reactions (Ames et al., 1996). Palmitic acid (16:0) is covalently bound to many proteins at cysteinyl residues along the peptide chain through S-palmitoyl cysteine esters. These pal-mitates may anchor proteins such as caveolin to membranes. Many proteins are also linked to the polyisoprenoid alcohols farnesol (15 carbons) and geronylgeronol (20 carbons) as thioesters (Zhang and Casey 1996). These co-valent linkages are essential for proper function of such mammalian proteins as nuclear lamin, a structural protein of the nuclear membrane, and Ras, guanosine triphosphate (GTP)-binding proteins that play a crucial role in signaling pathways essential to cell differentiation and growth.

Waxes, Esters, and Ethers

Waxes are long-chained, rather nonpolar compounds found on the surfaces of plants and animals (Hamilton, 1995). Some waxes are

CH3(CH2)4(CH—CHCH2)4(CH2)2COOH

Figure 3-2. Arachidonic acid (all-c\s-5,8,1l,14-eico-satetraenoic acid).

straight-chained, branched, or unsaturated hydrocarbons, but many are esters of long-chain alcohols, R'-CH2OH, and long-chain fatty acids, R-COOH, (e.g., R-COOCH2-R'). These saturated long-chain waxes tend to be solids at ambient temperature.

Plankton and higher members of the aquatic food chain, including coral, mollusks, fish, sharks, and even whales, store large quantities of waxes. Waxes are also present in human skin lipids, beans, seeds, and leaves, serving to make the outer surface nonwetta-ble. Ethers (R-CH2OCH2-R') are also present in skin lipids, forming a barrier against water loss (Elias, 1991). Waxes of long-chain alcohols and acids are nonpolar molecules. They accumulate in intracellular droplets or on surfaces of leaves or skin and have almost no solubility in cellular membranes.

Acylglycerols and Fats

Many complex lipids have a backbone of glycerol, a 3-carbon polyalcohol. When a substitution is made at one end of the glycerol molecule, the 2-carbon becomes optically active, as, for instance, in glycerophosphate. The standard nomenclature used is the sn terminology A single substitution of octadecanoic acid to form an ester bond, e.g., at the 1-position of glycerol, produces 1-octadecanoyl-sn-glycerol (Fig. 3-3). When two fatty acids are reacted with glycerol to form ester bonds, a diacylglycerol or diglyceride is formed (e.g., 1,3-diacyl-sn-glycerol or 1,2-diacyl-sn-glycerol). Diacylglycerols are biochemical intermediates in many lipolytic reactions, are critical building blocks used in the synthesis of more complex phospholipids and triacylglycerols, and are second messengers for some membrane-triggered reactions.

A triacylglycerol or triglyceride is formed when all three hydroxyls of glycerol form ester bonds with fatty acids. If all three fatty acids are the same, the triacylglycerol is called a simple triacylglycerol (e.g., triolein). If one of the fatty acids is different, it becomes a complex triacylglycerol. The properties of glycer-ides (e.g., melting point) depend greatly on the fatty acid chains involved. Triacylglycerols are the major storage lipids of plants and higher animals. Both plant oils (olive, corn, safflower) and animal fats (lard, suet, tallow) are nearly pure mixtures of complex triacylglycerols. Trace amounts of sterols, vitamins, free fatty acids, carotenoids, and other fat-soluble molecules can also be present in 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 triacylglycerols in intracellular oil droplets.

A single meal often contains dietary fat from a variety of sources (dairy, meat, vegetable). These mix with digestive secretions in the stomach and small intestine and undergo digestion and absorption (Carey et al., 1983; Patton, 1981). It is surprising that absorption from the lumen is generally quite efficient, and only about 4% of the ingested fat escapes into the feces (Carey et al., 1983). However, absorption of specific, highly saturated fats may be less efficient.

Vegetable oils, margarine, meat fats, butter, and other dietary fats are each composed of hundreds of different complex triacylglycerols, and most of these have not been analyzed completely The general structure of a triacyl-

1 -Octadecanoyl-sn-Glycerol (D-a-Monostearin)

Figure 3-3. Line model of a monoacylglycerol, showing the stereospecificity of the glycerol conformation. With the glycerol carbon in the 2 (middle) position in the plane of the page and carbons 1 and 3 behind the plane of the page, if the -OH on carbon 2 points up then the carbon on the right is designated carbon 7 and the one on the left, carbon 3. Glycerol carbon numbers are shown below the glycerol structure.

1 -Octadecanoyl-sn-Glycerol (D-a-Monostearin)

Figure 3-3. Line model of a monoacylglycerol, showing the stereospecificity of the glycerol conformation. With the glycerol carbon in the 2 (middle) position in the plane of the page and carbons 1 and 3 behind the plane of the page, if the -OH on carbon 2 points up then the carbon on the right is designated carbon 7 and the one on the left, carbon 3. Glycerol carbon numbers are shown below the glycerol structure.

glycerol is shown in Figure 3-4. The three R groups stand for different acyl groups. Using the International Union of Pure and Applied Chemistry/International Union of Biochemistry (IUPAC-IUB) nomenclature (Commission on Biochemical Nomenclature, 1968) with the carbon in the central (secondary) position in the plane of the page and the primary or end carbons behind the plane of the page, if the hydroxyl group on the midcarbon is drawn to the left, then the top carbon becomes sn-1, the midcarbon becomes sn-2, and the bottom carbon becomes sn-3. If the groups at the 1 and 3 positions are different (e.g., as shown for the different acyl groups in Figure 3-4), then the molecule has an asymmetrical carbon at the 2 position, and the optical isomers (enantiomers) 1R'-2R"-, 3R"'-s/7-glycerol and 1R'"-, 2R"-, 3R'-s/7-glyceroI are possible.

The triacylglycerol composition of oils, fats, and lipoproteins is usually reported as the total overall fatty acid composition of the triacylglycerol mixture. This information is easily obtained (Bailey 1950; Consumer and Food Economics Institute, 1976-1990; Gun-stone et al., 1994; Kuksis, 1978; Sonntag, 1979) and is valuable because it tells us which major fatty acids are esterified to the glycerol; however, it does not tell us their position on glycerol or how many specific triacylglycerol species are present in a given sample (Small, 1991). If the sample is treated with pancreatic lipase to cleave the fatty acids in the primary 1 and 3 positions of each triacylglycerol to

Figure 3-4. The Commission on Biochemical Nomenclature (1968) has recommended nomenclature for substituted glycerides—e.g., the phosphoglycerides. With the carbon in the 2 (middle) position in the plane of the page and carbons 1 and 3 behind the plane of the page, if the -OH on carbon 2 of the glycerol is drawn to the left, then the top carbon is designated carbon 1 and the bottom carbon becomes 3. A triacylglycerol with myristic acid on position 1, oleic acid on position 2, and palmitic acid on position 3 would be described as sn-glyceroi-1-myristate-2-oleate-3-palmitate, or simply as sn -MOP.

produce two fatty acids and a 2-monoacyl-glycerol, and if these in turn are analyzed, then we learn globally which fatty acids are in the 2 position and in the 1 and 3 positions. This procedure does not distinguish between the 1 and 3 positions or reveal which primary fatty acids were linked to which monoacyl-glycerol. In theory, the number of triacylglyc-erols (N) that could be present in a sample having n different fatty acids is n3 (Farines et al., 1988).

For example, if a sample of fat had 10 different fatty acids, the possible number of individual triacylglycerols would be 103 = 1000. This includes positional isomers and enantiomers—that is, optical isomers in which a specific fatty acid is either at the s/?-l or sn-3 position. If one simply considers race-mic mixtures in which the 1 and 3 positions can be interchanged, then N = (n3 + n2)/2. If one considers only the fatty acid combinations on the three positions and ignores positional isomers, then N = (n3 + n2 + 2n)/6. Thus, if a specific sample contains only three different fatty acids (e.g., R', R", R"9, then the total number of isomers, including positional isomers and enantiomers, would be 27. If we exclude optical isomers, there would be 18, and if we exclude positional isomers, there would be 7. Considering that many dietary fats and oils often have 10 or more major fatty acids, the number of potential individual triacylglycerols becomes enormous. In fact, butterfat has both short- and long-chain fatty acids as well as many unsaturated ones (Breckinridge, 1978) and, therefore, probably consists of thousands of individual stereospe-cific triacylglycerols. For this reason even cow's milk fat has not been completely analyzed (Myher et al., 1988).

In a careful search of the literature (Small, 1991), the major triacylglycerols found in a variety of natural fats and oils were identified and are listed in Table 3-3. Most of these have not been fully analyzed, and the specific stereoisomers are generally not reported. In some examples positional isomers are indicated, and in a few rare cases the optical isomers are given (i.e., mustard seed oil; Myher et al., 1979). The melting point (MP) (or range of melting) of the fat is also given. Fats with a high content of saturated long-chain fats have high MPs. Butter, lard, and

TABLE 3-3

Composition and Melting of Some Natural Fats and Oils*

Fat or Oil Melting Point (°Qt Major Triacylglycerols

TABLE 3-3

Composition and Melting of Some Natural Fats and Oils*

Fat or Oil Melting Point (°Qt Major Triacylglycerols

Butterfat

37 to 38

PPBf PPCf POP*

Horse fat

000 POO LOO

Lard

46 to 49

SPOf OPLf OPOi

Tallow (beef)

40

POO POP POS

Cocoa butter

28 to 36

POS SOS POP

Coconut oil

24 to 27

DDD CDD CDM

Palm kernel oil

24 to 29

DDD MOD ODO

Almond oil

000 OLO OLL

Corn oil

-14

LLL LOL LLP

Cottonseed oil

5 to 11

PLL POL LLL

Egg triglycerides

POO PLO POS

Grapeseed oil

8

LLL OLL POL

Hazelnut oil

OOO OLO POO

Olive oil

-7

OOO OOP OLO

Palm oil

30 to 36

POP POO POL

Peanut oil

.. -8 to +12

OOL POL OLL

Rice bran oil

PLO OOL POO

Safflower oil

-15

LLL LLO LLP

Soybean oil

-14

LLL LLO LLP

Sunflower oil

-17

LLL OLL LOO

Walnut oil

LLL OLL PLL

Rapeseed oil (low Er)

5

OOO LOO OOLn

Linseed oil

-17

LnLnLn LnLnL LnLnO

Rapeseed oil (high Er)

ErOEr ErLEr ErLnEr

Mustard seed oil§

ErOErf ErLErf OOErf

•Abbreviations used for acyl chains in the triacylglycerols: B = C4:0 (butyric), C = C-10:0 (capric), D = C-12:0 (dodecanoic), M = C-l4:0 (myristic), P = C-16:0 (palmitic), S = C-18:0 (stearic), 0 = C-I8:l (cis) (oleic), E = 0-18:1 (trans) (elaidic), L = C-18:2 (linoleic), Ln = C-18:3 (linolenic), G = C-20:l (gogoleic), Er = C-22:l (erucic).

fThe melting points or ranges of melting of these fats and oils are taken from references reported in Small (1991). ^Specific triacylglycerols estimated from stereospecific fatty acid analyses, as in lS,2P,30-sn-glycerol.

  • liie stereospecific composition of the eight most prevalent triacylglycerols of mustard seed oil, which comprise 40% of the total, is ErOEr = 8.2%, ErLEr = 6.8%, OOEr = 5.9%, ErLnEr = 5.3%, OLEr = 4.9%. OLnEr = 3.8%, GOEr = 3.3%, and GLEr = 2.7%.
  • Abbreviations used for acyl chains in the triacylglycerols: B = C4:0 (butyric), C = C-10:0 (capric), D = C-12:0 (dodecanoic), M = C-l4:0 (myristic), P = C-16:0 (palmitic), S = C-18:0 (stearic), 0 = C-I8:l (cis) (oleic), E = 0-18:1 (trans) (elaidic), L = C-18:2 (linoleic), Ln = C-18:3 (linolenic), G = C-20:l (gogoleic), Er = C-22:l (erucic).

fThe melting points or ranges of melting of these fats and oils are taken from references reported in Small (1991). ^Specific triacylglycerols estimated from stereospecific fatty acid analyses, as in lS,2P,30-sn-glycerol.

§liie stereospecific composition of the eight most prevalent triacylglycerols of mustard seed oil, which comprise 40% of the total, is ErOEr = 8.2%, ErLEr = 6.8%, OOEr = 5.9%, ErLnEr = 5.3%, OLEr = 4.9%. OLnEr = 3.8%, GOEr = 3.3%, and GLEr = 2.7%.

From Small, D. M. (1991) The effects of glyceride structure on absorption and metabolism. Annu Rev Nutr 11:413—434. Used with permission, from the Annual Review of Nutrition, Volume 11 © 1991 by Annual Reviews.

tallow, all animal fats, have MPs at or above body temperature (37°C). Excess intake of high-melting fats has been shown to increase plasma concentrations of beta lipoproteins (LDL, VLDL) (Grundy 1991). The concentrations of these lipoproteins are raised by ingestion of high amounts of saturated fat (>30 g intake per day). This "bad fat" increases the risk of cardiovascular disease, such as stroke and heart attack, and dietary intake of saturated fats by adults should be limited to 20 to 30 g/day (less than 10% of total calories).

Glycerophospholipids

Glycerophospholipids have a phosphate at the 3 position of glycerol and acyl or alkyl groups on at least one (usually both) of the other glycerol carbons. The phosphate esterified to the sn-3-glycerol may be free (phosphatidic acid) or esterified to other small molecules (Fig. 3-5) to form phosphatidylcholine (PC, lecithin), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphati.dylglycerol (PG), or phosphatidylinositol (PI). All these phospholipids have two acyl chains esterified to the glycerol but different head groups, some carrying a net negative charge (PS, PI, PG) and some zwitterionic (PC, PE) at pH 7. The net charge is due to ionization of the phosphate and of the amino alcohols. In some cases alkyl chains are linked to the glycerol moiety by ether bonds £-C-0-C-). One ether-linked phospholipid, platelet-activating factor (hexadecyl, 2-acetyl-sn-glycerol-3-phos-phocholine) causes marked vasoactivity Plas-malogens have a vinyl ether -C-0-C = C-and these may act as antioxidants.

1,2-Distearoyl-sn-Glycerol-3-Phosphatidic Acid (R=H)

1,2-Distearoyl-sn-Glycerol-3-Phosphatidic Acid (R=H)

Choline Moiety h3n h2c-

Ethanolamine Moiety c h2c-

II o

Serine Moiety hoc h?c-

Glycerol Moiety

C1 4C

Keep Your Weight In Check During The Holidays

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Responses

  • olavi
    What are the properties of lipids structure the fats and oils?
    8 years ago
  • FOLCARD
    What are the chemical structure and functions lipids?
    8 years ago
  • thorsten
    What are the contiguous isoprene groupings of lycopene?
    8 years ago
  • Jody Monroy
    How are glycerophospholipids different to triacylglycerols?
    6 years ago
  • frankie massey
    How is cholesterol different from triacylglycerols chemically?
    5 years ago

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