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The fact that organotrophic organisms on Earth use L-amino acids and D-sugars as an energy source
is recognized as one of the universal features of life. The chirality of organic molecules with asymmetric location
of group-radicals was described a relatively long time ago. Louis Pasteur observed that abiotic (chemical)
processes produced mixtures with equal numbers (racemic) of the two forms but that living organisms possessed a
molecular asymmetry that included only one of the enantiomers (homochirality). He speculated that the origin of
the asymmetry of chiral biomolecules might hold the key to the nature of life.
All of the amino acids in proteins (except for Glycine which is symmetrical) exhibit the same absolute steric
configuration as L-glyceraldehyde. D-amino acids are never found in proteins, although they do exist in nature and
are often found in polypeptide antibiotics. Constitutional sugars of cells, opposite to the amino acids, are the D-enantiomers,
and the appearance of L-sugars in Nature is extremely rare. Notwithstanding this fact, the metabolism
of some bacteria does have the capability to use amino acids and sugars with alternative chirality. This property
may be caused by the function of specific enzymes belonging to the class of isomerases (racemases, epimerases,
isomerases, tautomerases).
In our laboratory, we have investigated several anaerobic bacterial strains, and have found that some of these
bacteria are capable of using D-amino acids and L-sugars. Strain BK1 is capable of growth on D-arginine, but its
growth characteristics on L-arginine are approximately twice as high. Another alkaliphilic strain SCAT (= ATCC
BAA-1084T = JCM 12857T = DSM 17722T = CIP 107910T) was found to be capable of growth on L-ribose and L-arabinose.
It is interesting that this strain was incapable of growth on D-arabinose, which suggests the involvement
of some alternative mechanism of enzyme activity. In this paper, we describe the preliminary results of these
microbiological studies and discuss some possible implications.
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