386
chapter is
Lipids I: Fatty Acids and Eicosanoids
a -L in olen ic a cid , C 1 8 :3 (9 ,1 2 ,1 5 ) (from th e d iet)
(o cta d e c a tr ie n o ic acid )
A ctivation
a-L in o len o y l-C o A , 0 1 8 :3 (9 ,1 2 ,1 5 )
D esa tu ra tio n
O c ta d e c a te tr a e n o y l-C o A , 0 1 8 : 4 (6 ,9 ,1 2 ,1 5 )
C h a in -elo n g a tio n s y s te m
E ic o sa tetra e n o y l-C o A , 0 2 0 : 4 (8 ,1 1 ,1 4 ,1 7 )
D esa tu ra tio n
E ic o s a p e n ta e n o y i-C o A , 0 2 0 : 5 (5 ,8 ,1 1 ,1 4 ,1 7 )
c,-
C h a in -elo n g a tio n s y s te m
carbon chain length, number of double bonds, and double-
bond positions (in parentheses). Thus, palmitoleic acid
is designated 06:1(9) and linoleic acid is 08:2(9,12).
The location of the double bond is sometimes indicated
by A; for example, A
9
signifies that the double bond is
between carbon 9 and carbon 10. In both methods, the
carboxyl carbon is carbon 1. The double-bond position
can also be related to the co-end of the fatty acid molecule
(i.e., the methyl carbon farthest from the carboxyl end);
oleic acid is an
co-9
acid; linoleic acid has double bonds at
co
- 6
and co-9 carbons. The structures and names of some
naturally occurring unsaturated fatty acids are given in
Table 18-3.
The presence of a double bond in the hydrocarbon chain
gives rise to geometrical isomerism, which is due to re-
stricted rotation around carbon-carbon double bonds and
is exemplified by fumaric and maleic acids.
D o c o s a p e n ta e n o y l-C o A , C 2 2 :5 (7 ,1 0 ,1 3 ,1 6 ,1 9 )
D esa tu ra tio n
D o c o s a h e x a e n o y l-C o A , 0 2 2 : 6 (4 ,7 ,1 0 ,1 3 ,1 6 ,1 9 )
F I G U R E 1 8 -1 5
Synthesis of docosahexaenoic acid from a-Iinolenic acid. D esaturation and
chain elongation are sim ilar to the use described in Figure 18-13.
H O O C
H
H
C O O H
\ /
c
V
I
I
I
c
c
/ \
/ \
H O O C
H
H O O C
H
M a le ic a c id
F u m a ric a c id
(c /s -fo rm )
[trans-
fo rm )
18.5
Metabolism of Unsaturated
Fatty Acids
Structure and Nomenclature
of Unsaturated Fatty Acids
Unsaturated fatty acids contain one or more double bonds.
A common method for designating fatty acids gives the
Almost all naturally occurring, unsaturated, long-chain
fatty acids exist as the cis isomers, which are less
stable than the trans isomers. The cis configuration in-
troduces a bend (of about 30°) in the molecule, whereas
the trans isomer resembles the extended form of the satu-
rated chain (Figure 18-16). Arachidonic acid with four cis
double bonds is a U-shaped molecule. Some cis isomers
are biologically active as essential fatty acids. The trans
isomers cannot substitute for them but are metabolized
like the saturated fatty acids.
Saturated chain
f/ans-Monounsaturated chain
(uncommon in naturally
I
H
occurring fatty acids)
I
H
I
c/s-Monounsaturated chain
:=c.
(common in naturally
occurring fatty acids)
F I G U R E 1 8 -1 6
G eom etry o f saturated, trans m onounsaturated, and cis m onounsaturated
chains.
Functions of Unsaturated Fatty Acids
The cis unsaturated fatty acids provide fluidity of triacyl-
glycerol reserves and phospholipid membranes and many
serve as precursors of eicosanoids (prostaglandins, prosta-
cyclins, thromboxanes, and leukotrienes). The importance
of membrane fluidity and its relationship to the membrane
constituent phospholipids are discussed in Chapter 10.
Eicosanoids have numerous functions (see below).
18.6
Nonessential Fatty Acids
Palmitoleic and oleic acids, the two most abundant
monounsaturated fatty acids of animal lipids, can be
