594
Chirality in Biology
O
CH
3
R
1
R
1
HO
O
OCH
3
COOR
2
O
Fig. 10
Spiro polyketide
metabolites. The chiral, spiro
carbon is indicated by the
symbol
.
mine and (
+
)-demethyldihydrogalantha-
mine (Fig. 9c, d).
In some polyketide metabolites of fungi,
the chiral center is a spiro carbon atom
(Fig. 10).
When
R
1
=
Cl,
R
2
=
H,
the
compound is named
erdin
and occurs
as a racemate in
Aspergillus terreus
;th
i
s
organism also
produces geodin, R
1
=
Cl, R
2
=
CH
3
,a
st
h
e(
+
) enantiomer.
Bisdechlorogeodin, R
1
=
H, R
2
=
CH
3
,
occurs as the (
+
) enantiomer in
Penicillium
frequentans
and as the (
) enantiomer in
Oospora sulfurea-ochracea
.
3
Physiological Responses to Enantiomers
Of
the
human
senses,
probably
only
hearing lacks a connection with chiral-
ity. Enantiomeric differences are com-
mon for taste and smell, and sight and
touch can detect mirror-image relation-
ships.
3.1
Taste
Classically, taste qualities in mammals
included sweet, bitter, salty, and sour. The
quality ‘‘umami’’ has now been added,
describing the meatlike, savory taste of L-
glutamate. Enantiomeric taste differences
for amino acids were ±rst noted in 1886
when D-asparagine was found to be sweet,
while L-asparagine was tasteless. For the
following, the D-enantiomers are sweet,
the L-forms being bitter or flat: histidine
leucine, phenylalanine, tryptophan, and
tyrosine. Alanine is an exception since the
L-enantiomer is sweet. Arginine, aspartate,
isoleucine, lysine, proline, serine, and
threonine are essentially tasteless, for both
enantiomers. Achiral glycine is sweet as
thenameimplies
.
The remarkable umami taste is re-
sponsible
for
much
of
the
flavor
of
soy sauce and monosodium L-glutamate,
MSG, which is widely used in the food
industries. In amounts greater than about
3.0 g, L-glutamate may show other phys-
iological effects since some individuals
experience the ‘‘Chinese restaurant syn-
drome’’ – a general feeling of tightness,
facial pressure, burning sensations, and
headache. In contrast, D-glutamate with
at most a flat taste is relatively inert
physiologically.
Another remarkable taste quality is that
of the methyl ester of
N
-L-
α
-aspartyl-L-
phenylalanine (trade name, Aspartame),
a
material
some
160
times
as
sweet
as sucrose. Interestingly, rodents do not
react to Aspartame. It is widely used but
should be avoided by individuals with
phenylketonuria. The DD enantiomer, as
well as the DL and LD diastereoisomers
have bitter tastes.
Speci±c taste receptors have been iden-
ti±ed in humans and animals. Recep-
tors designated as TRB (taste receptor,
family B) are G-protein-coupled receptors
(GPCR); human receptors were identi±ed
by searching for GPCR-encoding genes in
human genome regions implicated in taste
perception.
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