Hn. Christensen et al., SPECIAL TRANSPORT AND NEUROLOGICAL SIGNIFICANCE OF 2 AMINO-ACIDS IN ACONFIGURATION CONVENTIONALLY DESIGNATED AS D, Journal of Experimental Biology, 196, 1994, pp. 297-305
We point out an ability of certain amino acids to be recognized at a b
iological receptor site as though their amino group bore, instead of a
n at relationship to a carboxylate group, a beta, gamma or delta relat
ionship to the same or a second carboxylate group. For aspartate, the
unbalanced position of its amino group between a pair of carboxylates
allows its occasional biorecognition as a beta- rather than as an alph
a-amino acid, whereas for proline and its homologs, their cyclic arran
gement may allow the imino group, without its being replicated, to be
sensed analogously as falling at either of two distances from the sing
le carboxylate group. The greater separation might allow proline to be
seen as biologically analogous to gamma-aminobutyric acid. This more
remote positioning of the imino group would allow the D-form of both a
mino acids to present its amino group in the orientation characteristi
c of the natural L-form. The dual modes of recognition should accordin
gly be signalled by what appears to be low stereospecificity, actually
due to a distinction in the enantiorecognition of the two isomers. Co
mpeting recognition for transport between their respective D- and L-fo
rms, although it does not prove that phenomenon, has been shown for pr
oline and, significantly, even more strongly for its lower homolog, 2-
azetidine carboxylate. Such indications have so far revealed themselve
s rather inconspicuously for the central nervous system binding of pro
line, reviewed here as a possible feature of a role suspected for prol
ine in neurotransmission.