Chirality in Biology
595
A receptor in mammals, designated
T1R1
+
3, is a heteromer of the taste-
speciFc T1R1 and T1R3 GPCRs. This
receptor responds to the classical 19 chiral
amino acids as L-butnot D-forms, and the
effect is potentiated by IMP; it is probably
a constituent of the umami response.
Sequence differences in T1R receptors
between human and mouse influence
the selectivity and speciFcity of the taste
response. A different receptor, T1R2
+
3is
activated by those D-amino acids that taste
sweet and attract mice; L-amino acids did
not cause this activation.
A complex system occurs in a prepara-
tion from catFsh (
Ictalurus punctatus
)taste
epithelium where L-alanine is a potent taste
stimulus; D-alanine has a lesser effect. ±or
a portion of the observed response, there
may be a common receptor/transduction
process, but in addition there appear
to be independent processes for each
enantiomer. The Caribbean spiny lobster
(
Panulirus argus
) contains populations of
chemosensory receptors that are differen-
tially sensitive to D
-and L-alanine. Of 77
neurons tested, 44% and 34% were clas-
siFed as L-alanine and D-alanine-sensitive
respectively.
Taste differences are generally rather
subtle, especially for carbohydrates. While
sucrose might be regarded as the epit-
ome of sweet substances, it has been
reported that ‘‘at the threshold level, su-
crose yields a tactual sensation, which may
be called bitter, medicinal, sour, and so
on, but as the concentration is increased,
the taste becomes bitter, then bitter-sweet,
and Fnally, purely sweet.’’ Apparently,
‘‘enantio-sucrose’’ (containing
L-glucose
and L-fructose) has never been prepared
or tasted. Carbohydrate solutions are gen-
erally complex equilibrium mixtures of
anomeric pyranose/furanose and straight-
chain forms. Some taste differences for
anomers and equilibrium solutions are
known. There are also temperature and
concentration effects (e.g. with sucrose)
and considerable variation from one indi-
vidual to another.
There is no strong evidence for general
enantiomeric differences among carbohy-
drates. Thus,
L-mannose was rated less
sweet than D-mannose by Fve individu-
als, but three others said the reverse and
still others found D-mannose to be bitter.
Identical ‘‘sweetness scores’’ have been
claimed for enantiomers of glucose, man-
nose, galactose, fructose, and xylose. In
fact, there have been proposals to use L-
hexoses as nonnutritive sweetners. While
syntheses for L-glucose are available (see
earlier) they are presumably too expensive
to compete with materials such as aspar-
tame. Penta-
O
-acetyl derivatives of various
monosaccharides are not sweet; the follow-
ing were actually rated as bitter: penta-
O
-
acetyl-D-glucose (both anomers), penta-
O
-
acetyl-
α
-D-mannopyranose, and penta-
O
-
acetyl-
β
-L-glucopyranose.
‘‘Pseudo’’-carbohydrates,
in
which
CH
2
replaces
O
in
the
ring
structure,
also
have
sweet
tastes,
but
again, there are ambiguities. ±ive people
found
the
enantiomers
of
pseudo-
β
-
fructopyranose to be as sweet as ordinary
D-fructose; three out of Fve judged the D-
enantiomer to be somewhat sweeter than
the L. The thiopyranoid analogs of
α
-D-
glucopyranose and
β
-D-fructopyranose are
signiFcantly sweeter than the usual forms.
3.2
Odor
The subject of enantiomeric odor dif-
ferences has been controversial. Unfor-
tunately, there is considerable variation
in human olfactory sensitivity, and the
nose is generally very sensitive to small
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