High-Yield Terms
Essential fatty acid: fatty acid required in the diet due to the inability of human cells to synthesize
Omega fatty acid: refers to the location of sites of unsaturation relative to the omega end (farthest from the carboxylic acid) of a fatty acid
Monounsaturated fatty acid (MUFA): a fatty acid with a single site of unsaturation, oleic acid, an omega-9 MUFA, is the most physiologically significant MUFA
Polyunsaturated fatty acid (PUFA): any fatty acid with multiple sites of unsaturation; omega-3 and omega-6 PUFA are the most significant clinically
Plasmalogen: any of a group of ether phospholipids
Sphingosine: an amino alcohol that serves as the backbone for the sphingolipid class of lipid, which includes the sphingomyelins and the glycosphingolipids
Ceramide: sphingosine containing a fatty N-acylation, serves as the backbone for the glycosphingolipids
Glycosphingolipid: any ceramide to which a carbohydrate or carbohydrates have been added, constitutes the cerebrosides, globosides, sulfatides, and gangliosides
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 8 carbon atoms are liquid at physiological temperature, whereas those containing more than 10 are solid.
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 serve as structural components of biological membranes; provide energy reserves, predominantly in the form of triglycerides, serve as biologically active molecules exerting a wide range of regulatory functions, and the lipophilic bile acids aid in lipid emulsification during digestion of fats. The biologically relevant lipids consist of the fatty acids, triglycerides, phospholipids, sphingolipids, ceramides, cholesterol, bile acids, eicosanoids, omega fatty acid derivatives, and bioactive lipid derivatives, which also include the inflammation-modulating lipid derivatives.
Fatty Acids
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 play 3 major roles in the body: (1) they serve as components of more complex membrane lipids; (2) they are the major components of stored energy in the form of triglycerides; and (3) they serve as the precursors for the synthesis of the numerous types of bioactive lipids.
Fatty acids that do not contain carbon–carbon double bonds are termed saturated fatty acids; those that contain double bonds are unsaturated fatty acids. 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 sites of unsaturation and is designated by 16:0; see Table 3-1).
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 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 (Figure 3-1).
FIGURE 3-1: Geometric isomerism of Δ9, 18:1 fatty acids (oleic and elaidic acids). Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper’s Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.
The majority of naturally occurring unsaturated fatty acids exist in the cis-conformation.
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 16:1D9.
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 2 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.
High-Yield Concept
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.
These 2 essential fatty acids are also referred to as omega fatty acids. The use of the Greek letter 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. The role of PUFAs, such as linoleic and α-linolenic, in the synthesis of biologically important lipids is discussed in Chapter 20 (Table 3-1).
Omega-3, and -6 PUFAs
The term omega, as it relates to fatty acids, refers to the terminal carbon atom farthest from the carboxylic acid group (–COOH). The designation of a 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 3 sites of unsaturation (carbon–carbon double bonds), has a site of unsaturation between the third and fourth carbons from the omega end. There are 3 major 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 2 omega-3 fatty acids, which serve as important precursors for lipid-derived modulators of cell signaling, gene expression, and inflammatory processes.
Most of the omega-6 PUFAs consumed in the diet are from vegetable oils and consist of linoleic acid. Linoleic acid is first converted to γ-linolenic acid (GLA), then to dihomo-γ-linolenic acid (DGLA), and finally to arachidonic acid (see Chapter 22). Due to the limited activity of human 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.
Triglycerides (Triacylglycerols)
Triglycerides represent the storage form of fatty acids. In this form, the large amount of energy produced from the oxidation of fatty acids is readily available to the cell. All tissues store triglycerides but adipose tissue is by far the largest reservoir of this form of energy storage. Triglycerides are generated by the esterification of fatty acids to glycerol (Figure 3-2).
FIGURE 3-2: Triacylglycerol. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper’s Illustrated Biochemistry, 29th ed. New York, NY: McGraw-Hill; 2012.
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