Alanine-valine structural change

Certain D-amino acids can be formed in bacteria by processes involving a racemase and D-amino acid transaminases. Amino acid activating systems have been found for D-alanine in Staph, aureus and a number of other bac-teria 34 is thus possible that free D-amino acids are sometimes incorporated as such into peptide antibiotics. On the other hand, the D-valine fragment of the penicillins arises from changes in an intermediate peptide that contains an L-valine residue. L-Valine in the culture fluid appears to be the precursor of the D-valine residue found in some of the actinomycins, but this relationship does not reveal the stage at which the inversion of configuration occurs. Differences in the permeability of cells or intracellular structures to l- and D-amino acids are liable to complicate the interpretation of experiments in this field. 

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