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Many strains of lactic acid bacteria produce high concentrations of  d -amino acids. Among them,  Lactobacillus salivarius  UCC 118 produces  d -alanine at a relative concentration much greater than 50 % of the total  d ,  l -alanine (100 d / d ,  l -alanine). We characterized the  L. salivarius  alanine racemase (ALR) likely responsible for this  d -alanine production and found that the enzyme was activated by carboxylates, which is an unique characteristic among ALRs. In addition, alignment of the amino acid sequences of several ALRs revealed that A131 of  L. salivarius  ALR is likely involved in the activation. To confirm that finding, an  L. salivarius  ALR variant with an A131K (ALR A131K ) substitution was prepared, and its properties were compared with those of ALR. The activity of ALR A131K  was about three times greater than that of ALR. In addition, whereas  L. salivarius  ALR was strongly activated by low concentrations (e.g., 1 mM) of short chain carboxylates, and was inhibited at higher concentrations (e.g., 10 mM), ALR A131K  was clearly inhibited at all carboxylate concentrations tested (1–40 mM). Acetate also increased the stability of ALR such that maximum activity was observed at 35 °C and pH 8.0 without acetate, but at 50 °C in the presence of 1 mM acetate. On the other hand, maximum ALR A131K  activity was observed at 45 °C and around pH 9.0 with or without acetate. It thus appears that A131 mediates the activation and stabilization of  L. salivarius  ALR by short chain carboxylates.

Characterization of Lactobacillus salivarius alanine racemase: short-chain carboxylate-activation and the role of A131