592
Chirality in Biology
next most abundant carbohydrate after cel-
lulose. The rare L-xylose occurs in some
antibiotics and lipopolysaccharides (e.g. of
Xanthomonas campestris
). With arabinose,
the
L-enantiomer
is common in many
plant products (e.g. gums, hemicelluloses,
and glycosides) and in bacterial polysac-
charides. The also-rare D-enantiomer oc-
curs in some plant glycosides and bacterial
polysaccharides. D-Lyxose is very rare but
is present in alkali-labile glycolipids of
Mycobacterium
smegmatis
. Naturally oc-
curring derivatives of
L-lyxose
are 2-
O
-
methyl-lyxose, a component of antibiotics
such as everninomycin B and D, and
3-
O
-methyl-lyxose, present in lipopolysac-
charides of
Pseudomonas maltophilia
.The
streptomycin component,
L-streptose, is
5-deoxy-3-formyl-L-lyxose, and a material
in
Coxiella burnettii
lipopolysaccharides is
3-
C
-hydroxymethyl-L-lyxose (dihydrodihy-
droxystreptose). Some strains of
E. coli
utilize the lyxose enantiomers. One mi-
croorganism,
Aerobacter aerogenes
strain
PRL-R3, utilizes all eight aldopentoses (i.e.
four each of the D
-and L-conFgurations)
and the four pentitols (D-and L-arabinitol,
ribitol, xylitol) as well.
Vitamin
C
(L-ascorbic
acid,
2-keto-
L-gulono-
γ
-lactone) occurs only as the
L-enantiomer.
It
is
biosynthesized
in
plants
by
way
of
L-galactose
as
fol-
lows:
D-glucose
6-phosphate
GDP-D-
mannose
GDP-L-galactose
L-galac-
tose 1-phosphate
L-galactose
L-galac-
tono-1,4-lactone
L-ascorbic
acid.
Sac-
charomyces cerevisiae
synthesizes L-ascorbic
acid when
incubated
with
L-galactose,
L-galactono-1,4-lactone, and
L-gulono-1,4-
lactone.
However,
incubation
with
D-
glucose,
D-galactose,
D-mannose,
or
D-
arabinose gave D-erythroascorbic acid, a
Fve-carbon analog of L-ascorbic acid.
Animals use a different pathway to make
vitamin C and in some, the capacity is
lacking (e.g. primates and guinea pigs).
In the rat, conversion of D-glucose to L-
ascorbic acid requires ‘‘inversion’’ of the
carbon chain sequence so that C
1
to C
6
of D-glucose appear as C
6
through C
1
in
ascorbate.
2.6
Other Natural Products
Many natural products occur both as enan-
tiomers and, sometimes, as racemates or
meso
structures; only a few examples can be
given. The case of tartaric acid is typical. In
the nineteenth century, a material named
paratartaric acid was sometimes formed in
wine production, and Pasteur separated
crystals of sodium ammonium paratar-
trate into two enantiomorphic forms by
‘‘crystal picking.’’ One of the free tartaric
acids obtained from the salts was dextro-
rotatory, the other levorotatory. In other
words, paratartaric acid was a racemic DL
mixture. The L(
+
)(or2
R
,3
R
) enantiomer
is found in some fungi and bacteria and is
particularly abundant in grape juice. The
D(
)(or2
S
,S3) enantiomer and the
meso
form have limited distribution.
Lactic acid formed in muscle glycol-
ysis is the L(
+
) enantiomer, (
S
)-(
+
)-2-
hydroxypropanoic acid. D(
)-lactic acid is
present in bacteria as the peptidoglycan
component,
N
-acetylmuramic acid, mu-
ramic acid being the 3-
O
-D-lactyl ether of
D-glucosamine. Racemic DL acid is found
in sour milk, molasses, and some fruit
juices. Bacterial lactate fermentations pro-
duce either the
D
-o
r
L-enantiomer
or
the racemate depending on the organ-
isms used.
Alkaloids, terpenes, and many other sec-
ondary metabolites frequently occur in
both enantiomeric forms or as racemates.
Examples of alkaloids that occur as (
+
),
(
), and racemic forms are the tobacco
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