section
9.1
Classification
141
Aldonic acids cannot exist in hemiacetal ring forms but can
form cyclic structures by forming an ester linkage between
the carboxylic group and one of the hydroxyl groups of
the same molecule. This type of cyclic ester is known as
a
lactone
(e.g., D-glucono-1,5-lactone, Figure 9-12).
Uronic acids, in which the highest numbered carbon
atom bears the carboxyl group, can exist in cyclic hemi-
acetal ring structures because they have an accessible
carbonyl group. They can form glycosidic bonds (see
below) with other uronic acids or monosaccharides to
yield polysaccharides. At physiological pH, uronic acids
exist in the ionized form (e.g., D-glucuronate).
L -
Ascorbate
(vitamin C) in plants and in animals other than pri-
mates and guinea pigs is a biosynthetic derivative of
D-glucuronate (Figure 9-12).
Amino Sugars
Amino
sugars
are
obtained
by
replacing
a
hy-
droxyl group of a monosaccharide by an amino group
(Figure 9-13). The most common amino sugars are
the 2-aminoaldohexoses,
namely,
D-glucosamine and
H
N-Acetyl-a-D-galactosamine
(2-acetamido-2-deoxy-
a-o-galactosamine)
D-galactosamine. The amino groups usually occur as
the N-acetyl derivatives. Amino sugars are components
of structural polysaccharides and of glycosphingolipids
of membranes
(Chapter
10).
N-Acetylmuramic
acid
(Figure 9-14), a constituent of a bacterial cell wall polysac-
charide, has a lactyl side chain linked to C
3
of glu-
cosamine through an ether linkage. The polysaccharide
is a polymer of alternating N-acetylmuramic acid and N-
acetylglucosamine residues linked in a /3(1 —>■
4) glyco-
sidic bond.
Lysozyme
(also called
muramidase
) catalyzes
the hydrolysis of these bonds. The polysaccharide chains
are cross-linked through short peptide chains, and the
overall structure is known as a peptidoglycan (Chapter 11).
N-Acetylneuraminic acid (Figure 9-15), known as
sialic
o
II
C H j— c —
h o 2c —
c — O H
I
CH2
I
H— c — O H
I
— HN— C — H
I
--------------c — H
H— c — OH
I
H — c — OH
CH2O H
(a)
P -N-A cetylm uram ic acid
(3-O-lactyl-N-acetyl- p- D -glucosam ine)
F IG U R E 9-14
Structures of N-acetylated amino sugars.
FIGURE 9-15
Structure of N-acetyl-D-neuraminic acid in three different representations:
(a) Fischer projection; (b) Haworth projection, and (c) conformational
formula.
