Biochemistry of Lipids: Fatty Acids, Omega Fats, Triglycerides, Phospholipids, Eicosanoids


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Major Roles of Biological Lipids

Biological molecules that are insoluble in aqueous solutions and soluble in organic solvents are classified as lipids. The lipids of physiological importance for humans have four major functions:

1. They serve as structural components of biological membranes.

2. They provide energy reserves, predominantly in the form of triacylglycerols.

3. Lipids and lipid derivatives serve as biologically active molecules exerting a wide range of functions.

4. Lipophilic bile acids aid in lipid solubilization.

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Fatty Acids

Fatty acids fill three major roles in the body:

1. as the components of more complex membrane lipids.

2. as the major components of stored fat in the form of triacylglycerols.

3. as the precursors for the synthesis of bioactive lipids.

Fatty acids are long-chain hydrocarbon molecules containing a carboxylic acid moiety at one end. The numbering of carbons in fatty acids begins with the carbon of the carboxylate group. At physiological pH, the carboxyl group is readily ionized, rendering a negative charge onto fatty acids in bodily fluids.

Fatty acids that contain no carbon-carbon double bonds are termed saturated fatty acids; those that contain double bonds are unsaturated fatty acids and fatty acids with multiple sites of unsaturation are termed polyunsaturated fatty acids (PUFAs). The numeric designations used for fatty acids come from the number of carbon atoms, followed by the number of sites of unsaturation (eg, palmitic acid is a 16-carbon fatty acid with no unsaturation and is designated by 16:0).

Structure of palmitic acid

Palmitic Acid

The melting point of fatty acids increases as the number of carbon atoms increases. In addition, the introduction of sites of unsaturation results in lower melting points when comparing a saturated and an unsaturated fatty acid of the same number of carbons. Saturated fatty acids of less than eight carbon atoms are liquid at physiological temperature, whereas those containing more than ten are solid. As a general rule, oils from vegetables contain many more unsaturated fatty acids and are therefore, liquids at room temperature. In contrast, animal oils contain more saturated fatty acids. The steric geometry of unsaturated fatty acids can also vary such that the acyl groups can be oriented on the same side or on opposite sides of the double bond. When the acyl groups are both on the same side of the double bond it is referred to as a cis bond, such as is the case for oleic acid (18:1). When the acyl groups are on opposite sides the bond is termed trans such as in elaidic acid, the trans isomer of oleic acid.

orientation of cis and trans double bonds

Orientation of cis (oleic acid) and trans (elaidic acid) double bonds

The majority of naturally occurring unsaturated fatty acids exist in the cis-conformation. Trans fatty acids occur in some foods and as byproducts of the process of hydrogenating saturated fatty acids to make them solids at room temperature, such as in partially hydrogenated vegetable oils. Diets high in trans fatty acids have been associated with an increased risk of cardiovascular disease and development of the metabolic syndrome.

The site of unsaturation in a fatty acid is indicated by the symbol Δ and the number of the first carbon of the double bond relative to the carboxylic acid group (–COOH) carbon which is designated carbon #1. For example, palmitoleic acid is a 16-carbon fatty acid with one site of unsaturation between carbons 9 and 10, and is designated by 16:1Δ9.

The majority of fatty acids found in the body are acquired in the diet. However, the lipid biosynthetic capacity of the body (fatty acid synthase and other fatty acid modifying enzymes) can supply the body with all the various fatty acid structures needed. Two key exceptions to this are the PUFAs known as linoleic acid and α-linolenic acid, containing unsaturation sites beyond carbons 9 and 10 (relative to the α-COOH group). These two fatty acids cannot be synthesized from precursors in the body, and are thus considered the essential fatty acids; essential in the sense that they must be provided in the diet. Since plants are capable of synthesizing linoleic and α-linolenic acid, humans can acquire these fats by consuming a variety of plants or else by eating the meat of animals that have consumed these plant fats. These two essential fatty acids are also referred to as omega fatty acids. The use of the greek omega, ω, refers to the end of the fatty acid opposite to that of the –COOH group. Linoleic acid is an omega-6 PUFA and α-linolenic is an omega-3 PUFA (see Table below). The role of PUFAs, such as linoleic and α-linolenic, in the synthesis of biologically important lipids is described briefly below and also in the Lipid Synthesis page, the Bioactive Lipids page, and the Lipid-Derived Modulators of Inflammation page.

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Physiologically Relevant Fatty Acids

Numerical Symbol Common Name and Struture Comments
14:0 Myristic acid
Structure of myristic acid
Often found attached to the N-term. of plasma membrane-associated cytoplasmic proteins
16:0 Palmitic acid
Structure of palmitic acid
End product of mammalian fatty acid synthesis
16:1Δ9 Palmitoleic acid
Structure of palmitoleic acid
 
18:0 Stearic acid
Structure of stearic acid
 
18:1Δ9 Oleic acid
Structure of oleic acid
An omega-9 monounsaturated fatty acid
18:2Δ9,12 Linoleic acid
Structure of linoleic acid
Essential fatty acid
An omega-6 polyunsaturated fatty acid
18:3Δ9,12,15 α-Linolenic acid (ALA)
Structure of linolenic acid
Essential fatty acid
An omega-3 polyunsaturated fatty acid
20:4Δ5,8,11,14 Arachidonic acid
Structure of arachidonic acid
An omega-6 polyunsaturated fatty acid
Precursor for eicosanoid synthesis
20:5Δ5,8,11,14,17 Eicosapentaenoic acid (EPA)
Structure of arachidonic acid
An omega-3 polyunsaturated fatty acid
enriched in fish oils
22:6Δ4,7,10,13,16,19 Docosahexaenoic acid (DHA)
Structure of arachidonic acid
An omega-3 polyunsaturated fatty acid
enriched in fish oils

Oleic acid (18:0) is the most abundant monounsaturated fatty acid (MUFA) in the the in the human body. Palmitoleic acid (16:1) is also an abundant MUFA in hunam cells. These two fatty acids represent the majority of MUFAs present in membrane phospholipids, triglycerides, and cholesterol esters. The health benefits of oleic acid are broad and profound. Numerous studies have shown that consumption of MUFAs is important to maintain low levels of LDL in the blood and is also likely to be associated with the potential for elevated HDL. Another physiologically significant effect of oleic acid is the result of its conversion to oleoylethanolamide (OEA) in the small intestine. OEA has demonstrated effects in the CNS related to the control of apetite and feeding behaviors. For more information on this latter function of oleic acid go the the Bioactive Lipids page.

Excellent vegetarian and vegan sources of oleic acid are olive oil in which up to 85% of the triglyceride in this oil contains oleic acid. Free oleic acid levels in food grade olive oil are mandated to be less than 2% as more than this makes the oil inedible. Extra virgin olive oils will have less than 0.8% free oleic acid. Other vegetable and nut oils also contain high levels of oleic acid in their triglycerides with canola oil (60%-65%) having the second highest amount compared to olive oil, pecan oil has 60%-75% oleic in triglyceride, peanut oil is 35%-70%, sunflower oil is 20%-80%, and grape seed oil is 15%-20%. Animal fats are also high in oleic acid with lard and tallow containing 44%-47% in the triglyceride component.

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Omega-3, and -6, Polyunsaturated Fatty Acids (PUFAs)

The term omega, as it relates to fatty acids, refers to the terminal carbon atom farthest from the functional carboxylic acid group (–COOH). The designation of a polyunsaturated fatty acid (PUFA) as an omega-3 fatty acid, for example, defines the position of the first site of unsaturation relative to the omega end of that fatty acid. Thus, an omega-3 fatty acid like α-linolenic acid (ALA), which harbors three carbon-carbon double bonds (i.e sites of unsaturation), has a site of unsaturation between the third and fourth carbons from the omega end (see Figure in Table above). There are three major types of omega-3 fatty acids that are ingested in foods and used by the body: ALA, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Once eaten, the body converts ALA to EPA and DHA, the two types of omega-3 fatty acids more readily used by the body and which serve as important precursors for lipid-derived modulators of cell signaling, gene expression and inflammatory processes.

It is important to denote that when discussing omega-3 fatty acids, their dietary origin is quite important. Omega-3 fats from plants, such as those in flax seed oil, are enriched in ALA. As indicated above, ALA must first be converted to EPA (requiring three independent reactions) and then to DHA (requiring an additional four reactions). Omega-3 fats from fish are enriched in EPA and DHA and thus do not need to undergo the complex conversion steps required of ALA. In addition, the conversion of ALA to EPA and DHA is inefficient in individuals consuming a typical Western diet rich in animal fats.

Most of the omega-6 PUFAs consumed in the diet are from vegetable oils and consist of linoleic acid. Linoleic acid is converted to γ-linolenic acid (GLA) in the body. GLA should not be confused with ALA which, as pointed out above, is an essential omega-3 PUFA. GLA is converted to dihomo-γ-linolenic acid (DGLA) and then to arachidonic acid as shown in the Eicosanoid Synthesis page. Due to the limited activity of the Δ5-desaturase most of the DGLA formed from GLA is inserted into membrane phospholipids at the same C-2 position as for arachidonic acid. GLA can be ingested from several plant-based oils including evening primrose oil, borage oil, and black currant seed oil.


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Basic Structure of Triacylglycerides

Triacylglycerides are composed of a glycerol backbone to which 3 fatty acids are esterified.

Structure of a typical triglyceride

Basic composition of a triacylglyceride.


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Basic Structure of Phospholipids

The basic structure of phospolipids is very similar to that of the triacylglycerides except that C–3 (sn3)of the glycerol backbone is esterified to phosphoric acid. The building block of the phospholipids is phosphatidic acid which results when the X substitution in the basic structure shown in the Figure below is a hydrogen atom. Substitutions include ethanolamine (phosphatidylethanolamine), choline (phosphatidylcholine, also called lecithins), serine (phosphatidylserine), glycerol (phosphatidylglycerol), myo-inositol (phosphatidylinositol, these compounds can have a variety in the numbers of inositol alcohols that are phosphorylated generating polyphosphatidylinositols), and phosphatidylglycerol (diphosphatidylglycerol more commonly known as cardiolipins). See the Lipid Synthesis page for images of the various phospholipids.

Basic structure of a phopsholipid

Basic composition of a phospholipid.
X can be a number of different substituents.


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Basic Structure of Plasmalogens

Plasmalogens are complex membrane lipids that resemble phospholipids, principally phosphatidylcholine. The major difference is that the fatty acid at C–1 (sn1) of glycerol contains either an O-alkyl (–O–CH2–) or O-alkenyl ether (–O–CH=CH–) species. A basic O-alkenyl ether species is shown in the Figure below where –X can be substituents such as those found in phospholipids described above.

Structure of a plasmalogen

Basic composition of O-alkenyl plasmalogens

One of the most potent plasmalogens is platelet activating factor (PAF: 1-O-1'-enyl-2-acetyl-sn-glycero-3-phosphocholine) which is a choline plasmalogen in which the C–2 (sn2) position of glycerol is esterified with an acetyl group instead of a long chain fatty acid.

PAF functions as a mediator of hypersensitivity, acute inflammatory reactions and anaphylactic shock. PAF is synthesized in response to the formation of antigen-IgE complexes on the surfaces of basophils, neutrophils, eosinophils, macrophages and monocytes. The synthesis and release of PAF from cells leads to platelet aggregation and the release of serotonin from platelets. PAF also produces responses in liver, heart, smooth muscle, and uterine and lung tissues.

Structure of platelet activating factor (PAF)

Structure of PAF


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Basic Structure of Sphingolipids

Sphingolipids are composed of a backbone of sphingosine which is derived itself from glycerol. Sphingosine is N-acetylated by a variety of fatty acids generating a family of molecules referred to as ceramides. Sphingolipids predominate in the myelin sheath of nerve fibers. Sphingomyelin is an abundant sphingolipid generated by transfer of the phosphocholine moiety of phosphatidylcholine to a ceramide, thus sphingomyelin is a unique form of a phospholipid.

The other major class of sphingolipids (besides the sphingomyelins) are the glycosphingolipids generated by substitution of carbohydrates to the sn1 carbon of the sphingosine backbone of a ceramide. There are 4 major classes of glycosphingolipids:

Cerebrosides: contain a single moiety, principally galactose.

Sulfatides: sulfuric acid esters of galactocerebrosides.

Globosides: contain 2 or more sugars.

Gangliosides: similar to globosides except also contain sialic acid (N-acetylneuraminic acid, NANA).

Structure of sphingosine

Structure of a ceramide

"n" indicates any fatty acid may be N-acetylated at this position.

Top: Sphingosine
Bottom: Basic composition of a ceramide


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Michael W King, PhD | © 1996–2013 themedicalbiochemistrypage.org, LLC | info @ themedicalbiochemistrypage.org

Last modified: August 19, 2013