15.1.1 Structure and Nomenclature of the Fatty Acids
565
a>
COOH
Fig.15.1a-d. The structure of unusual fatty acids in animals. a Iso-heptadecanoic acid; b anteiso-heptadecanoic
acid; c 2,4,6,8-tetramethyl-hexadecanoic acid from the
uropygial gland of the goose [108]; d cyclopropane fatty
acid (17-methyl-cis-9,1O-methylene octadecanoic acid)
from the parasitic flagellate Herpetomonas megaseliae [97]
results in anteiso-fatty acids with an odd number
of C atoms (Fig. 15.6). Methylmalonyl-CoA,
instead of malonyl-CoA, as the elongation substrate gives rise to internal methyl branches;
methyl transfer from methionine to fatty acids, as
known from the bacteria, apparently does not
occur in animals. Ethyl-, propyl-, and butylsubstituted fatty acids, as found in lipids of the
uropygial gland, are derived from the corresponding substituted elongation substrates [108].
Double bonds can be introduced into the fatty
acid chain by specific desaturases (Fig. 15.7).
These always have the cis configuration; trans isomers are only produced in bacteria and plants but
may find their way into animal lipids via the diet
or as products of gut symbionts. The house
cricket Gryllus domesticus is reported to contain
6 % elaidic acid, the trans isomer of oleic acid
[144]. Each cis double bond results in a kink in
the hydrocarbon chain.
The fatty acids have systematic names and the
most common also have trivial names. Many tables
and texts use a digital code in which the first digit
gives the number of C atoms and the second digit
gives the number of double bonds (Table 15.2).
The localization of the double bonds is also indicated for unsaturated fatty acids, e.g. 18:3(6,9,12)
for y-linolenic acid, where (6,9,12) represents the
numbers of C atoms relative to I-carboxyl from
which double bonds extend. Because the elongation of the fatty acids occurs at the carboxyl end,
the elongation of 18:3(6,9,12) gives rise to
20:3(8,11,14). This complicated change in the numbering is avoided if the position of the double
bonds is expressed relative to the methyl end.
Because the double bonds in the majority of unsaturated animal fatty acids are almost always separated by a CH2 group, notation of the double bond
closest to the methyl end is sufficient. Thus, ylinolenic acid and its elongation product are
described by 18:3(n-6) and 20:3(n-6); these are
sometimes written as 18:30)6 and 20:30)6. This
nomenclature system is especially useful in that
mutual conversion of the (n-3) , (n-6) and (n-9)
families is not possible in animal metabolism.
Typical animal fatty acids are unbranched,
even-numbered with 14-22 C atoms, and saturated or unsaturated with one to six double bonds.
Most frequent are the CI6 and CIS fatty acids, with
palmitic acid 16:0 being the predominant CI6 form
and oleic acid 18: l(n-9) and polyunsaturated fatty
acids (PUFA) constituting the predominant CIS
species. Also commonly found is vaccenic acid
18:1(n-7), which arises by elongation of palmitoleic acid 16:1(n-7) and in some invertebrates is
equivalent to, or exceeds, oleic acid in concentration [101]. Species-specific differences in the substrate specificity of the ~9-desaturase probably
determine whether stearic acid 18: 0 is desaturated
to oleic acid 18: l(n-9) rather than palmitic acid
16:0 to palmitoleic acid 16:1(n-7). Noteworthy in
this context is the large proportion of the unusual
fatty acid 20:1(n-11) in the triacylglycerols of the
cockchafer Melolontha vulgaris [251].
The proportion of nnsaturated fatty acids,
which determines the melting point of body fat, is
dependent upon the diet but also varies greatly
with the species (Table 15.3). The storage fat of
the ruminants is high in saturated fatty acids,
especially stearic acid (18:0), because the unsaturated fatty acids in the diet are completely
saturated by the rumen symbionts. Compared
with the mammals, the teleosts have a significantly higher proportion of polyunsaturated fatty
acids, in particular 18:2(n-6), 20:4(n-6), 20:5(n3), 22:5(n-3) and 22:6(n-3) [14]. The body fats of
the amphibians, reptiles and birds are intermediate in this respect. Large amounts of polyunsaturated fatty acids are also found in many invertebrate animals, e.g. the molluscs, echinoderms,
ascidians and free-living nematodes (Table 15.3)
[101, 103]. There are many interesting connections between habitat and fatty acid spectrum,
probably based upon differences in the composition of the diet. For example, marine mussels
always have more (n-3) than (n-6) fatty acids,
565
a>
COOH
Fig.15.1a-d. The structure of unusual fatty acids in animals. a Iso-heptadecanoic acid; b anteiso-heptadecanoic
acid; c 2,4,6,8-tetramethyl-hexadecanoic acid from the
uropygial gland of the goose [108]; d cyclopropane fatty
acid (17-methyl-cis-9,1O-methylene octadecanoic acid)
from the parasitic flagellate Herpetomonas megaseliae [97]
results in anteiso-fatty acids with an odd number
of C atoms (Fig. 15.6). Methylmalonyl-CoA,
instead of malonyl-CoA, as the elongation substrate gives rise to internal methyl branches;
methyl transfer from methionine to fatty acids, as
known from the bacteria, apparently does not
occur in animals. Ethyl-, propyl-, and butylsubstituted fatty acids, as found in lipids of the
uropygial gland, are derived from the corresponding substituted elongation substrates [108].
Double bonds can be introduced into the fatty
acid chain by specific desaturases (Fig. 15.7).
These always have the cis configuration; trans isomers are only produced in bacteria and plants but
may find their way into animal lipids via the diet
or as products of gut symbionts. The house
cricket Gryllus domesticus is reported to contain
6 % elaidic acid, the trans isomer of oleic acid
[144]. Each cis double bond results in a kink in
the hydrocarbon chain.
The fatty acids have systematic names and the
most common also have trivial names. Many tables
and texts use a digital code in which the first digit
gives the number of C atoms and the second digit
gives the number of double bonds (Table 15.2).
The localization of the double bonds is also indicated for unsaturated fatty acids, e.g. 18:3(6,9,12)
for y-linolenic acid, where (6,9,12) represents the
numbers of C atoms relative to I-carboxyl from
which double bonds extend. Because the elongation of the fatty acids occurs at the carboxyl end,
the elongation of 18:3(6,9,12) gives rise to
20:3(8,11,14). This complicated change in the numbering is avoided if the position of the double
bonds is expressed relative to the methyl end.
Because the double bonds in the majority of unsaturated animal fatty acids are almost always separated by a CH2 group, notation of the double bond
closest to the methyl end is sufficient. Thus, ylinolenic acid and its elongation product are
described by 18:3(n-6) and 20:3(n-6); these are
sometimes written as 18:30)6 and 20:30)6. This
nomenclature system is especially useful in that
mutual conversion of the (n-3) , (n-6) and (n-9)
families is not possible in animal metabolism.
Typical animal fatty acids are unbranched,
even-numbered with 14-22 C atoms, and saturated or unsaturated with one to six double bonds.
Most frequent are the CI6 and CIS fatty acids, with
palmitic acid 16:0 being the predominant CI6 form
and oleic acid 18: l(n-9) and polyunsaturated fatty
acids (PUFA) constituting the predominant CIS
species. Also commonly found is vaccenic acid
18:1(n-7), which arises by elongation of palmitoleic acid 16:1(n-7) and in some invertebrates is
equivalent to, or exceeds, oleic acid in concentration [101]. Species-specific differences in the substrate specificity of the ~9-desaturase probably
determine whether stearic acid 18: 0 is desaturated
to oleic acid 18: l(n-9) rather than palmitic acid
16:0 to palmitoleic acid 16:1(n-7). Noteworthy in
this context is the large proportion of the unusual
fatty acid 20:1(n-11) in the triacylglycerols of the
cockchafer Melolontha vulgaris [251].
The proportion of nnsaturated fatty acids,
which determines the melting point of body fat, is
dependent upon the diet but also varies greatly
with the species (Table 15.3). The storage fat of
the ruminants is high in saturated fatty acids,
especially stearic acid (18:0), because the unsaturated fatty acids in the diet are completely
saturated by the rumen symbionts. Compared
with the mammals, the teleosts have a significantly higher proportion of polyunsaturated fatty
acids, in particular 18:2(n-6), 20:4(n-6), 20:5(n3), 22:5(n-3) and 22:6(n-3) [14]. The body fats of
the amphibians, reptiles and birds are intermediate in this respect. Large amounts of polyunsaturated fatty acids are also found in many invertebrate animals, e.g. the molluscs, echinoderms,
ascidians and free-living nematodes (Table 15.3)
[101, 103]. There are many interesting connections between habitat and fatty acid spectrum,
probably based upon differences in the composition of the diet. For example, marine mussels
always have more (n-3) than (n-6) fatty acids,