564
15 Lipids
the different organs of an animal may have widely
different lipid compositions. The aliphatic lipid
components, i.e. the fatty acids and derived longchain alcohols and hydrocarbons, also exist in
various forms with differences in chain length,
the number and position of double bonds, branches and substituents. Because the components
of lipids can occur in almost any combination,
each class of lipids contains an almost astronomical number of molecular species. This large
variety of possible lipid structures allows each
organism to adapt its lipid spectrum to specific
conditions. However, as only a part of the necessary components can be produced by the animal
itself, the composition of lipids in the body is
strongly influenced by the diet.
One or more lipid classes is involved in each of
the three main functions of the lipids referred to
above. The fat stores that serve as an energy
reserve consist in most animals of triacylglycerols,
although in some marine species they may also
include large amounts of ether glycerides, wax
esters or the hydrocarbon squalene. The lipidcontaining secretions used to nourish young animals, e.g. mammalian milk, the crop milk of the
pigeons or the corresponding secretions of some
insect species, are, in principle, similar in composition, although they may differ quite specifically in detail. Wax esters and other hydrocarbons
are typical of the water-repellent lipid layers of
mammal, bird and arthropod body surfaces. In
addition to sterols, the construction of cellular
membranes involves, above all, glycerophospholipids, sphingophospolipids and glycolipids.
Thus, the subdivisions of this Chapter are
based upon these three main functions without
losing sight of the structural classification. However, we will consider only lipids that are directly
related metabolically to long-chain fatty acids,
i.e. the fatty acids themselves, the eicosonoates,
fatty alcohols and hydrocarbons derived during
metabolism, and the lipids with fatty acids as
components. The sterols and sterol esters will be
discussed in Chapter 16 and the non-polar substances that act as pheromones, the carotinoids
and the terpenes will be considered in Chapter 19.
15.1 Chemistry and Metabolism
of the Fatty Acids
15.1.1 Structure and Nomenclature
of the Fatty Acids
The fatty acids found in the body are in part taken
up in the diet, either unaltered or mostly changed
by metabolism, and in part synthesized de novo.
Thus, the structure of the fatty acids becomes
clear by considering their metabolism. The biosynthesis normally begins with acetyl-CoA as the
"primer" ; acyl-CoA is extended at each cycle by a
C2 unit and is usually released from the synthase
complex only on reaching a length of 14-18C
atoms. The fatty acids obtained in the diet or by
de novo synthesis can be further extended in an
elongation system by one or more C2 units. This
process explains why typical animal fatty acids
have chain lengths of C14 to Cz2 and an even number of C atoms (Table 15.2).
Fatty acids with an odd number of C atoms
arise from propionyl-CoA as primer. Methylbranched fatty acids (Table 15.1 and Fig. 15.1a-c)
are formed by fatty acid synthase using methylsubstituted primers of elongation substrates.
Isovaleryl-CoA (from leucine) as the primer gives
rise to iso- fatty acids with an odd number of C
atoms; iso-butyryl-CoA (from valine) produces
iso- fatty acids with an even number of C atoms·
and 2-methyl-butyryl-CoA (from isoleucine)
Table 15.2. Numerical symbols, systematic names and trivial names of some important fatty acids
Symbol
4:0
5:0
6:0
8:0
10:0
12:0
14:0
16:0
16: 1 (n-7)
18:0
18:1(n-7)
18: l(n-9)
18:2(n-6)
18:3(n-3)
18:3(n-6)
20:0
20:4(n-6)
22:0
24:0
24:1
Systematic name
Butanoic
Pentanoic
Hexanoic
Octanoic
Decanoic
Dodecanoic
Tetradecanoic
Hexadecanoic
9-Hexadecanoic
Octodecanoic
I-Octadecenoic
9-0ctadecenoic
9,12-0ctadecadienoic
9,12,15-0ctadecatrienoic
6,9,12-0ctadecatrienoic
Eicosanoic
5,8,1l,14-eicosatetraeneic
Docosanoic
Tetracosanoic
15-Tetracosenoic
Trivial name
Butyric
Valeric
Caproic
Caprylic
Capric
Lauric
Myristic
Palmitic
Palmitoleic
Stearic
Vaccenic
Oleic
Linoleic
a-Linolenic
y-Linolenic
Arachidic
Arachidonic
Behenic
Lignoceric
Nervonic
15 Lipids
the different organs of an animal may have widely
different lipid compositions. The aliphatic lipid
components, i.e. the fatty acids and derived longchain alcohols and hydrocarbons, also exist in
various forms with differences in chain length,
the number and position of double bonds, branches and substituents. Because the components
of lipids can occur in almost any combination,
each class of lipids contains an almost astronomical number of molecular species. This large
variety of possible lipid structures allows each
organism to adapt its lipid spectrum to specific
conditions. However, as only a part of the necessary components can be produced by the animal
itself, the composition of lipids in the body is
strongly influenced by the diet.
One or more lipid classes is involved in each of
the three main functions of the lipids referred to
above. The fat stores that serve as an energy
reserve consist in most animals of triacylglycerols,
although in some marine species they may also
include large amounts of ether glycerides, wax
esters or the hydrocarbon squalene. The lipidcontaining secretions used to nourish young animals, e.g. mammalian milk, the crop milk of the
pigeons or the corresponding secretions of some
insect species, are, in principle, similar in composition, although they may differ quite specifically in detail. Wax esters and other hydrocarbons
are typical of the water-repellent lipid layers of
mammal, bird and arthropod body surfaces. In
addition to sterols, the construction of cellular
membranes involves, above all, glycerophospholipids, sphingophospolipids and glycolipids.
Thus, the subdivisions of this Chapter are
based upon these three main functions without
losing sight of the structural classification. However, we will consider only lipids that are directly
related metabolically to long-chain fatty acids,
i.e. the fatty acids themselves, the eicosonoates,
fatty alcohols and hydrocarbons derived during
metabolism, and the lipids with fatty acids as
components. The sterols and sterol esters will be
discussed in Chapter 16 and the non-polar substances that act as pheromones, the carotinoids
and the terpenes will be considered in Chapter 19.
15.1 Chemistry and Metabolism
of the Fatty Acids
15.1.1 Structure and Nomenclature
of the Fatty Acids
The fatty acids found in the body are in part taken
up in the diet, either unaltered or mostly changed
by metabolism, and in part synthesized de novo.
Thus, the structure of the fatty acids becomes
clear by considering their metabolism. The biosynthesis normally begins with acetyl-CoA as the
"primer" ; acyl-CoA is extended at each cycle by a
C2 unit and is usually released from the synthase
complex only on reaching a length of 14-18C
atoms. The fatty acids obtained in the diet or by
de novo synthesis can be further extended in an
elongation system by one or more C2 units. This
process explains why typical animal fatty acids
have chain lengths of C14 to Cz2 and an even number of C atoms (Table 15.2).
Fatty acids with an odd number of C atoms
arise from propionyl-CoA as primer. Methylbranched fatty acids (Table 15.1 and Fig. 15.1a-c)
are formed by fatty acid synthase using methylsubstituted primers of elongation substrates.
Isovaleryl-CoA (from leucine) as the primer gives
rise to iso- fatty acids with an odd number of C
atoms; iso-butyryl-CoA (from valine) produces
iso- fatty acids with an even number of C atoms·
and 2-methyl-butyryl-CoA (from isoleucine)
Table 15.2. Numerical symbols, systematic names and trivial names of some important fatty acids
Symbol
4:0
5:0
6:0
8:0
10:0
12:0
14:0
16:0
16: 1 (n-7)
18:0
18:1(n-7)
18: l(n-9)
18:2(n-6)
18:3(n-3)
18:3(n-6)
20:0
20:4(n-6)
22:0
24:0
24:1
Systematic name
Butanoic
Pentanoic
Hexanoic
Octanoic
Decanoic
Dodecanoic
Tetradecanoic
Hexadecanoic
9-Hexadecanoic
Octodecanoic
I-Octadecenoic
9-0ctadecenoic
9,12-0ctadecadienoic
9,12,15-0ctadecatrienoic
6,9,12-0ctadecatrienoic
Eicosanoic
5,8,1l,14-eicosatetraeneic
Docosanoic
Tetracosanoic
15-Tetracosenoic
Trivial name
Butyric
Valeric
Caproic
Caprylic
Capric
Lauric
Myristic
Palmitic
Palmitoleic
Stearic
Vaccenic
Oleic
Linoleic
a-Linolenic
y-Linolenic
Arachidic
Arachidonic
Behenic
Lignoceric
Nervonic