D-amino acids and lactic acid bacteria

Abstract

Proteins are composed of L-amino acids except for glycine, which bears no asymmetric carbon atom. Accordingly, researchers hav0065 studied the function and metabolism of L-amino acids in living organisms but have paid less attention to the presence and roles of their D-enantiomers. However, with the recent developments in analytical techniques, the presence of various D-amino acids in the cells of various organisms and the importance of their roles have been revealed. For example, D-serine (D-Ser) and D-aspartate (D-Asp) act as neurotransmitters and hormone-like substances, respectively, in humans, whereas some kinds of D-amino acids act as a biofilm disassembly factor in bacteria. Interestingly, lactic acid bacteria produce various kinds of D-amino acids during fermentation, and many D-amino acids taste sweet, compared with the corresponding L-enantiomers. The influence of D-amino acids on human health and beauty has been reported in recent years. These facts suggest that the D-amino acids produced by lactic acid bacteria are important in terms of the taste and function of lactic-acid-fermented foods. Against this background, unique D-amino-acid-metabolizing enzymes have been searched for and observed in lactic acid bacteria. This review summarizes and introduces the importance of various D-amino acids in this regard.