590
Chirality in Biology
alternate:
L-Val-Gly-L-Ala-D-Leu-L-Ala-D-Val-L-Val-
D-Val-
(
L-Trp-D-Leu
)
3
-L-Trp
The N-terminal L-valine is modifed by
a Formyl group and the C-terminal
L-
tryptophan by ethanolamine. In the antibi-
otic, actinomycin D (
Streptomyces
strains)
there are two D-valine components and the
unusual amino acids, sarcosine, and
N
-
methyl-valine. Bacitracin A, produced by
B.
subtilis
and
B. licheniformis
and present in
some nonprescription antibiotic creams,
is a dodecapeptide with 4 D-amino acids
(aspartate, glutamate, ornithine, pheny-
lalanine). A very simple antibiotic with
as
ing
lech
ira
lcen
teris D-cycloserine (D-
4-amino-3-isoxazolidinone), C
3
H
6
N
2
O
2
.It
is produced by a
Streptomycete sp
., ulti-
mately From L-serine by way oF a ureido
derivative:
L-Ser
→→
O
-ureido-L-ser
O
-ureido-D-ser
D-cycloserine
.
2.5
Carbohydrates
Carbohydrates are also cited as exam-
ples oF homochirality. Again, the sheer
weight oF
D-glucose,
D-ribose,
and
D-
deoxyribose on planet Earth is enormous;
however, there are substantial amounts
oF some L-carbohydrates, For example, L-
arabinose. There have been several chem-
ical or biochemical syntheses oF ‘‘unnatu-
ral’’ carbohydrates, For example, L-glucose,
L-galactose, and
L-ribose. A biochemical
method involves the use oF
D-galactose
oxidase, EC 1.1.3.9. The type reaction
is the oxidation oF
D-galactose
at C-6,
thus producing D-
galacto
-hexodialdose and
H
2
O
2
, but some other alcohols are ox-
idized. To prepare
L-galactose, the
meso
compound, galactitol (±ig. 8b, R
1
=
OH,
R
2
=
H) is derived by reduction oF
D-
galactose
(±ig. 8a,
R
1
=
OH,
R
2
=
H).
Treatment oF galactitol with galactose oxi-
dase yields L-galactose (±ig. 8c, R
1
=
OH,
R
2
=
H). This provides a two-step racem-
ization, D-galactose
L-galactose. Oxida-
tion oF L(
+
)-glucitol (±ig. 8b, R
1
=
H,
R
2
=
OH) yields L-glucose (±ig. 8c, R
1
=
H, R
2
=
OH). L(
+
)-glucitol is prepared
by
reduction
oF
the
readily
available
D-gulonolactone
(±ig. 8a, R
1
=
H, R
2
=
OH). The oxidation eFfciencies were in-
creased by addition oF catalytic amounts oF
Ferricyanide.
L-Glucose has, apparently, not been iso-
lated as a natural product; however, some
microorganisms can oxidize it. Very Few
physiological actions have been attributed
to it or its derivatives. Thus, penta-
O
-
acetyl-
β
-L-glucopyranose caused a biphasic
increase in plasma insulin concentration
COX
C
OH
H
C
R
2
R
1
C
H
HO
C
OH
H
CH
2
OH
CH
2
OH
C
H
HO
C
OH
H
C
R
1
R
2
C
H
HO
COX
CH
2
OH
C
H
HO
C
OH
H
C
R
1
R
2
C
H
HO
CH
2
OH
CHO
C
H
HO
C
OH
H
C
R
1
R
2
C
H
HO
CH
2
OH
(a)
(b)
(c)
(d)
Fig. 8
Synthesis of L-galactose and L-glucose. In the second structure
from the left, (a) has been rotated through 180
. See text for details.
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