Chirality in Biology
591
when injected into anesthetized rats. In
mice, L-glucose increased production of
advanced glycation end products and there
was accelerated aging. Mice injected with
L-glucose also showed an improved mem-
ory effect reversed by peripherally acting
cholinergic drugs. The
L-glucose
effect
may involve facilitation of acetylcholine
synthesis and/or its release. Commer-
cially available
L-glucose
is used as a
nonmetabolizable analog for D-glucose in
transport and other metabolic studies. The
derivative,
N
-methyl-L-glucosamine, is a
component of streptomycin and is biosyn-
thesized from D-glucose.
D-Galactose predominates with L-galac-
tose derivatives in some polysaccharides.
The gelling material, agar-agar, contains
about 70% of agarose. In this heteropoly-
mer, units with D
-and L-conFgurations
alternate;
the
two
monomers
are
β
-
D-galactopyranose
and
3,6-anhydro-
α
-L-
galactopyranose. Liganded agarose deriva-
tives are used in afFnity chromatography.
L-Galactose has a role in ascorbic acid
biosynthesis in plants (see later). Both
enantiomers of 6-deoxygalactose, trivial
name fucose, occur widely – D-fucose in
plant glycosides and L-fucose in seaweed
polysaccharides and in the cell wall matrix
of higher plants. L-±ucosyl residues are
also present in animal glycoproteins and
glycolipids including human erythrocyte
surface antigens.
Among other hexoses and their deriva-
tives,
residues
of
L-iduronic
acid
are
present in chondroitin sulfate B (dermatan
sulfate), a glycosaminoglycan present in
ground
substances,
connective
tissue,
and
so
on;
a
second
component
is
N
-acetyl-D-galactosamine. Alginic acid – a
hydrophilic polysaccharide found in brown
seaweeds, especially the California giant
kelp (
Macrocystis pyrifera
)a
n
dh
o
r
s
e
t
a
i
l
kelp
(
Laminaria
digitata
) – has
a
com-
plex interrupted structure of stretches
of
α
-L-gulopyranosyluronic acid and
β
-D-
mannopyranosyluronic acid residues. The
sodium salt is used extensively as an emul-
siFer and thickener.
The rather rare 6-deoxytalose, 6-deoxy-
3-
O
-methylglucose, and fucosamine oc-
cur as both enantiomeric forms. In two
cases, enantiomers have different triv-
ial names; the
D
-a
n
d
L-forms
of 3,6-
dideoxygalactose are respectively abequose
(
Salmonella
lipopolysaccharides) and coli-
tose (
E. coli
lipopolysaccharides). The cor-
responding forms of 3,6-dideoxymannose
aretyve
lose(
l
ipopo
lysacchar
ideso
fsome
gram-negative bacteria) and ascarylose (a
glycolipid component in
Ascaris
eggs and
Yersinia pseudotuberculosis
).
The pentoses, D-ribose and 2-deoxy-D-
ribose, occur in large amounts in nucleic
acid structures. Although generally as-
sumed not to occur in nature, isolation
of
L-ribose
from water-soluble polysac-
charides of
Adonis vernalis
(pheasant’s
eye, ox’s eye) has been reported. Mu-
tants of
E. coli
and some other bacte-
ria metabolize
L-ribose by an NADPH-
linked reductase to ribitol; subsequently,
ribitol pathway enzymes convert ribitol
to
D-ribulose
(D-
erythro
-pentulose).
An
L-ribose
isomerase, converting
L-ribose
to
L-ribulose
(L-
erythro
-pentulose),
has
been isolated from
Acinetobacter sp
.The
gene for this enzyme was cloned into
E.
coli
and
sequenced;
the
recombi-
nant enzyme was puriFed to homogene-
ity. ±urther metabolism presumably in-
volved the pentose phosphate phosphoke-
tolase pathway: L-ribulose
→
L-ribulose 5-
phosphate
→
D-xylulose
5-phosphate
→
acetyl phosphate
+
glyceraldehyde 3-phos-
phate.
The next most abundant pentose is D-
xylose, present in the xylans; xylan is the
