section 9.1
Classification
143
HOH2C
2 -D e o x y -a - D -ribose
T h e p h o sp h o ry la te d d eriv ativ e
of d e o x y rib o se (Fig 9 -1 6 )
is a c o m p o n e n t of DNA.
a -L - R h a m n o s e
(6 -d eo x y - L -m an n o se)
a - L - F u c o s e
(6 -d eo x y - l -g a la c to s e )
T h e s e tw o d e o x y s u g a r s a re th e rare m o n o s a c c h a rid e s
th at a re in th e l -configuration. L -R h a m n o se is a
c o n stitu e n t o f lip o p o ly sa cc h a rid e s of th e o u te r m e m b ra n e
lay er of g ra m -n e g a tiv e b acteria. L -F u co se is a c o n stitu en t
o f g ly c o p ro te in s in cell m e m b ra n e s.
FIGURE 9-17
D eoxy sugars.
(6-deoxyl-L-galactose), a constituent of cell membrane
glycoproteins and glycolipids and one of the few monosac-
charides that exist in the L-configuration (Figure 9-17).
L-Fucose occurs either at terminal or preterminal posi-
tions of many cell surface oligosaccharide ligands. These
fucosylated oligosaccharides mediate cell-cell recognition
and adhesion-signaling pathways. Some of these processes
include early embryologic development, blood group
recognition (Chapter 10), and pathological processes such
as inflammation and neoplasia. The cell-cell adhesion
mediated by fucosylated oligosaccharide involves cell
surface calcium-dependent binding proteins known as
selectins.
Glycosides
Glycosides are formed when the anomeric (hemiac-
etal or hemiketal) hydroxyl group of a monosaccharide
undergoes condensation with the hydroxyl group of a
second molecule, with the elimination of water. For-
mation of glycosides is an example of
acetal
forma-
tion, which is a reaction between a hemiacetal group
and another hydroxyl group. The linkage resulting from
such a reaction is known as a
glycosidic bond.
Glyco-
sides are named for the sugar that provides the hemi-
acetal group. Thus, if glucose provides the hemiac-
etal group, the resultant molecule is a
glucoside;
if
galactose provides the hemiacetal group, the result is
a
galactoside.
An example of the formation of a gly-
coside is shown in Figure 9-18. When glucose reacts
with methanol at an elevated temperature in the pres-
ence of an acid catalyst, a mixture of
a-
and /
1
-methyl
glucopyranosides is obtained. Once the anomeric hy-
droxyl group is substituted, properties associated with the
anomeric carbon atom, namely, mutarotation, reduction,
and ring size (pyranose versus furanose), are permanently
lost.
The noncarbohydrate moiety of a glycoside is known
as the
aglycone.
Methanol, glycerol, sterols, and phe-
nols may serve as aglycones. Glycosides, which stimulate
cardiac muscle contractions and are used therapeutically,
CH2OH
-o
HO
OH
> H O H + C H ,O H
-KO
O H
D -G lucopyranose
C ataly st (H-*)
CH O H
' “I
O — C H ,
P-M ethyl- D-
g lu co sid e
O H
FIGURE 9-18
Form ation o f glycosides.
