144
chapter 9
Simple Carbohydrates
FIGURE 9-19
Structures of glycosides.
contain a steroid as the aglycone component (Figure 9-19).
Adenosine, a major constituent of nucleotides and nucleic
acids, is an N-glycoside.
If the glycosidic linkage occurs between two monosac-
charides, the compound is a
d isa cch a rid e.
Since the sec-
ond monosaccharide contains several hydroxyl groups,
various linkages are possible, thus giving rise to a num-
ber of isomers in which the anomeric carbon of at least
one of the monosaccharides forms part of the glycosidic
linkage.
D isa cch a rid es
Structures of some disaccharides are illustrated in
Figure 9-20.
M a lto se
is composed of two glucose residues joined by
an a-glycosidic linkage between Ci of one residue and C
4
ofthe other residue [designated a (l -> 4)]. In maltose, the
second sugar residue has an unsubstituted anomeric car-
bon atom and therefore can function as a reducing agent as
well as exhibit mutarotation. In
treh alose,
two glucose
residues are joined by an a-linkage through both anomeric
carbon atoms; therefore, the disaccharide is not a reducing
sugar, nor does it exhibit mutarotation.
L actose,
synthe-
sized only by secretory cells of the mammary gland during
lactation, is a disaccharide consisting of galactose and glu-
cose. The glycosidic bond is a ^-linkage between Q of
galactose and
C 4
of glucose (Figure 9-20). Lactose is a
reducing sugar and exhibits mutarotation by virtue of the
anomeric Q of the glucose residue.
L a ctu lo se
is a disac-
charide consisting of galactose and fructose linked through
a ^-linkage between Ci of galactose and
C 4
of fructose.
It is used in the treatment of some forms of chronic liver
disease (such as
h ep a tic en cep h a lo p a th y)
in which the
ammonia content in the blood is elevated (
h yp era m m o n e-
m ia
). Normally, ammonia produced in the gastrointestinal
