Chemical Nature of Fatty Acids and Acylglycerols

Page 363
Figure 9.1 Metabolic interrelationships of fatty acids in the human.
are supplied from the diet or by biosynthesis is described. The mechanism for storage as triacylglycerols and how fatty acids are mobilized and transported throughout the body to sites where they are needed are discussed. The central process of energy production from fatty acids is then examined, and finally the mechanisms by which the ketone bodies are synthesized and used are presented.
The Appendix includes the nomenclature and chemistry of lipids and in Chapter 26 there is a discussion of digestion and absorption of lipids.
9.2— Chemical Nature of Fatty Acids and Acylglycerols
Fatty Acids Are Alkyl Chains Terminating in a Carboxyl Group
Fatty acids consist of an alkyl chain with a terminal carboxyl group, the basic formula of completely saturated species being CH3–(CH2)n–COOH. The important fatty acids for humans have relatively simple structures, although in some organisms they may be quite complex, containing cyclopropane rings or extensive branching. Unsaturation occurs commonly in human fatty acids, with up to six double bonds per chain, these being almost always of the cis configuration. If there is more than one double bond per molecule, they are always separated by a methylene (–CH2–) group. The most common fatty acids
Page 364
in biological systems have an even number of carbon atoms, although some organisms do synthesize those with an odd number of carbon atoms. Humans can use the latter for energy but incorporate them into complex lipids to a minimal degree.
Figure 9.2 Isovaleric acid.
A few fatty acids with an a ­OH group are produced and used structurally by humans. However, more oxidized forms are normally produced only as metabolic intermediates during energy production or for specific physiological activity in the case of prostaglandins and thromboxanes. Some animals, including humans, also produce relatively simple branched­chain acids, branching being limited to methyl groups along the chain at one or more positions. These are apparently produced to contribute specific physical properties to some secretions and structures. For instance, large amounts of branched­chain fatty acids, particularly isovaleric acid (Figure 9.2), occur in lipids of echo­locating structures in marine mammals. Elucidation of the role of these compounds in sound focusing should be fascinating.
Most fatty acids in humans have C16, C18, or C20 atoms, but there are several with longer chains that occur principally in lipids of the nervous system. These include nervonic acid and a C22 acid with six double bonds (Figure 9.3).
Figure 9.3 Long­chain fatty acids.
Nomenclature of Fatty Acids
The most abundant fatty acids have common names that have been accepted for use in the official nomenclature. Examples are given in Table 9.1 with official systematic names. The approved abbreviations consist of the number of carbon atoms followed, after a colon, by the number of double bonds. Carbon atoms are numbered with the carboxyl carbon as number 1, and double bond locations are designated by the number of the carbon atom on the carboxyl side of it. These designations of double bonds are in parentheses after the rest of the symbol.
Most Fatty Acids in Humans Occur As Triacylglycerols
Fatty acids occur primarily as esters of glycerol, as shown in Figure 9.4, when they are stored for future use. Compounds with one (monoacylglycerols) or two (diacylglycerols) acids esterified are present only in relatively minor amounts and occur largely as metabolic intermediates in biosynthesis and degradation of glycerol­
containing lipids. Most fatty acids in humans exist as triacylglycerols, in which all three hydroxyl groups on glycerol are esterified with a fatty acid. These compounds have been called ''neutral fats or triglycerides." There are other types of "neutral fats" in the body, and the terms "triglyceride," "monoglyceride," and "diglyceride" are chemically incorrect and should not be used.
The distribution of different fatty acids in the three positions of the glycerol moiety of triacylglycerols in the body at any given time is influenced by many
TABLE 9.1 Fatty Acids of Importance to Humans
Numerical Symbol
16: 0
16: 1(9)
Structure
Trivial Name
Systematic Name
CH3–(CH2)14–COOH
Palmitic
Hexadecanoic
CH3–(CH2)5–CH=CH–(CH2)7–COOH
Palmitoleic
cis­9­Hexadecenoic
18: 0
CH3–(CH2)16–COOH
Stearic
Octadecanoic
18: 1(9)
CH3–(CH2)7–CH=CH–(CH2)7–COOH
Oleic
cis­9­Octadecenoic
18: 2(9,12)
CH3–(CH2)3–(CH2–CH=CH)2–(CH2)7–COOH
Linoleic
cis,cis­9,12­Octadecadienoic
18: 3(9,12,15)
CH3–(CH2–CH=CH)3–(CH2)7–COOH
Linolenic
cis,cis,cis­9,12,15­Octadecatrienoic
20: 4(5,8,11,14)
CH3–(CH2)3–(CH2–CH=CH)4–(CH2)3–COOH
Arachidonic
cis,cis,cis,cis­5,8,11,14­Eicosatetraenoic