468
13 The Structure and Metabolism of Carbohydrates
primates, whereas the sugar contents of whale
and seal milk are extremely low. The sea lion
Zalophus californianus lacks a-lactalbumin and
therefore no lactose synthesis is possible. Low
milk-sugar concentrations are also found in bears,
fish, the sea otter, beaver, coypu and the lesser
white-toothed shrew Neomys fodiens [185,209].
In addition to lactose, more than 100 other oligosaccharides that are derived from lactose and
often contain sialic acids have been isolated from
human milk [79, 101, 150] (Fig. 13.4); similar oligosaccharides are also found in the milk of the
mouse and goat [48]. The carbohydrates of marsupial milk are very different in composition to
those in the milk of the Placentalia. Lactose
makes up only 3-5 % of the total carbohydrate in
the milk of the giant kangaroo Macropus giganteus. In this case, the carbohydrate content of the
milk increases in the first 6 months post partum to
13 % and then rapidly decreases to 2-3 %. Initially, a mixture of tri- and disaccharides predominates and later this changes to monosaccharides,
with galactose being more abundant than glucose
and glucosamine. Thus, the young in the pouch
receive mainly oligosaccharides, whereas the
mobile young animals receive monosaccharides
[59, 61, 183]. The carbohydrate spectrum of
monotreme milk is different again. The echidna
Tachyglossus aculeatus has mainly sialyl- and
fucosyllactose. In the duck-billed platypus Ornithorhyncus anatinus about one-half of the total
carbohydrate is difucosyllactose, and the rest is a
mixture of penta- and nonasaccharides with the
composition G1clGaI1_3G1cNacl_2FuCl_3; lactose is
completely absent [184].
Numerous di- and oligosaccharides have been
detected in bee honey and in the so-called honeydew of the aphids (Fig. 13.5). The formation of
honeydew stems from the curious mechanism of
nutrient uptake by the phloem sap-sucking aphids.
These tap into the phloem of the host
plant; because of the internal pressure of the
phloem, sap is forced through the proboscis and
the gut of the insect and emerges as honeydew
from the anus. Only a small fraction of the sap carbohydrate is absorbed by the aphid and, therefore,
the honeydew is rich in sugars; these are exploited
together with nectar for the production of honey
by the honey bee. Honey is composed mainly of
glucose and fructose arising from hydrolysis of the
sucrose in plant sap. Several of the oligosaccharides detected in honeydew and honey arise directly
from the sap without alteration, e.g. sucrose, raffinose and melezitose; others, e.g. maltose and maltose oligosaccharide, isomaltose and glucosucrose,
arise by transglycosylation through the action of
animal carbohydrases [40, 288].
13.3 Metabolism of Low Molecular
Weight Carbohydrates
Usually the requirement of an animal for various
sugars and sugar derivatives does not correspond
to the availability of these sugars in the diet.
Energy-yielding carbohydrate metabolism always
proceeds via glucose and fructose phosphates; the
biosynthesis of the body's own carbohydrate compounds and derivatives requires a range of sugars
that are present in strict proportions. In contrast,
the nutrition of herbivorous animals in particular
results in the uptake of a complex, changing spectrum of sugars. Thus, many different processes
must take place in each animal to bring about the
interconversion of sugars and sugar derivatives.
Lactose
Fucosyllactose
Lacto-N-tetraose
Lacto-N-neotetraose
Lacto-N-fucopentaose
Lacto-N-difucopentaose
Gall3<1-4)Glc
Fuca(1-2)Gal~(1-4)Glc
Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
Gal~(1-4)GlcNAc~(1-3)Gal~(1-4)Glc
Fuca(1-2)Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
Fuca(1-2)Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
t a1-4
LS-Tetrasaccharide a
LS-Tetrasaccharide b
LS-Tetrasaccharide c
DisialyllactoN-tetraose
Fuc
NeuNAca(2-3)Gal~(1-3)GlcNAC~(1-3)Gal~(1-4)Glc
NeuNAca(2-6)Gal~(1-4)GlcNAc~(1-3)Gal~(1-4)Glc
Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
ta2-6
NeuNAc
NeuNAca(2-3)Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
ta2-6
NeuNAc
Fig.13.4. Some of the oligosaccharides found in human milk [218]
13 The Structure and Metabolism of Carbohydrates
primates, whereas the sugar contents of whale
and seal milk are extremely low. The sea lion
Zalophus californianus lacks a-lactalbumin and
therefore no lactose synthesis is possible. Low
milk-sugar concentrations are also found in bears,
fish, the sea otter, beaver, coypu and the lesser
white-toothed shrew Neomys fodiens [185,209].
In addition to lactose, more than 100 other oligosaccharides that are derived from lactose and
often contain sialic acids have been isolated from
human milk [79, 101, 150] (Fig. 13.4); similar oligosaccharides are also found in the milk of the
mouse and goat [48]. The carbohydrates of marsupial milk are very different in composition to
those in the milk of the Placentalia. Lactose
makes up only 3-5 % of the total carbohydrate in
the milk of the giant kangaroo Macropus giganteus. In this case, the carbohydrate content of the
milk increases in the first 6 months post partum to
13 % and then rapidly decreases to 2-3 %. Initially, a mixture of tri- and disaccharides predominates and later this changes to monosaccharides,
with galactose being more abundant than glucose
and glucosamine. Thus, the young in the pouch
receive mainly oligosaccharides, whereas the
mobile young animals receive monosaccharides
[59, 61, 183]. The carbohydrate spectrum of
monotreme milk is different again. The echidna
Tachyglossus aculeatus has mainly sialyl- and
fucosyllactose. In the duck-billed platypus Ornithorhyncus anatinus about one-half of the total
carbohydrate is difucosyllactose, and the rest is a
mixture of penta- and nonasaccharides with the
composition G1clGaI1_3G1cNacl_2FuCl_3; lactose is
completely absent [184].
Numerous di- and oligosaccharides have been
detected in bee honey and in the so-called honeydew of the aphids (Fig. 13.5). The formation of
honeydew stems from the curious mechanism of
nutrient uptake by the phloem sap-sucking aphids.
These tap into the phloem of the host
plant; because of the internal pressure of the
phloem, sap is forced through the proboscis and
the gut of the insect and emerges as honeydew
from the anus. Only a small fraction of the sap carbohydrate is absorbed by the aphid and, therefore,
the honeydew is rich in sugars; these are exploited
together with nectar for the production of honey
by the honey bee. Honey is composed mainly of
glucose and fructose arising from hydrolysis of the
sucrose in plant sap. Several of the oligosaccharides detected in honeydew and honey arise directly
from the sap without alteration, e.g. sucrose, raffinose and melezitose; others, e.g. maltose and maltose oligosaccharide, isomaltose and glucosucrose,
arise by transglycosylation through the action of
animal carbohydrases [40, 288].
13.3 Metabolism of Low Molecular
Weight Carbohydrates
Usually the requirement of an animal for various
sugars and sugar derivatives does not correspond
to the availability of these sugars in the diet.
Energy-yielding carbohydrate metabolism always
proceeds via glucose and fructose phosphates; the
biosynthesis of the body's own carbohydrate compounds and derivatives requires a range of sugars
that are present in strict proportions. In contrast,
the nutrition of herbivorous animals in particular
results in the uptake of a complex, changing spectrum of sugars. Thus, many different processes
must take place in each animal to bring about the
interconversion of sugars and sugar derivatives.
Lactose
Fucosyllactose
Lacto-N-tetraose
Lacto-N-neotetraose
Lacto-N-fucopentaose
Lacto-N-difucopentaose
Gall3<1-4)Glc
Fuca(1-2)Gal~(1-4)Glc
Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
Gal~(1-4)GlcNAc~(1-3)Gal~(1-4)Glc
Fuca(1-2)Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
Fuca(1-2)Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
t a1-4
LS-Tetrasaccharide a
LS-Tetrasaccharide b
LS-Tetrasaccharide c
DisialyllactoN-tetraose
Fuc
NeuNAca(2-3)Gal~(1-3)GlcNAC~(1-3)Gal~(1-4)Glc
NeuNAca(2-6)Gal~(1-4)GlcNAc~(1-3)Gal~(1-4)Glc
Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
ta2-6
NeuNAc
NeuNAca(2-3)Gal~(1-3)GlcNAc~(1-3)Gal~(1-4)Glc
ta2-6
NeuNAc
Fig.13.4. Some of the oligosaccharides found in human milk [218]