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 have 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.Proteins are basically made up of 20 kinds of α-amino acids in the form of monomers, and these amino acids have at least one asymmetric carbon, except for glycine. This yields 19 kinds of amino acid (glycine has been excepted) with l-and d-form varieties. As proteins are made up of l-amino acids, many researchers, in fields such as medical, food, and nutritional science, have paid close attention to l-amino acids, giving rise to considerable knowledge of l-amino acids, including proteins. Until the 1970s, many researchers thought that d-amino acids were rare in nature and did not play an important role in living organisms. After that, however, d-alanine (d-Ala), d-glutamate (d-Glu), and d-aspartate (d-Asp) were found in bacterial cell walls as constituents, and d-amino acids were observed to exist in nature. This finding led to the conclusion that d-amino acids were only rarely present in bacterial cell walls, but in the 1990s, the development of analytical techniques, such as chiral column chromatography, meant that various kinds of d-amino acids were identified in free forms in diverse organisms. These d-amino acids were initially considered to be derived from foods or enterobacteria, but it is now known that some parts of d-amino acids such as d-serine (d-Ser) and d-Asp can be synthesized in mammalian cells. d-Amino acids have similar physical and chemical characteristics with l-amino acids, but the physiological functions of d-amino acids are completely different with those of l-amino acids. For example, d-Ser works as neurotransmitter in the brain, but l-Ser has no such role. Furthermore, d-amino acids are uncommon in living organisms, compared with l-amino acids, and despite the fact that some d-...

d-glutamate and Gut Microbiota in Alzheimer’s Disease

“…Different fermentation processes may contribute different formation of d-amino acids. Lactic fermentation with lactic acid bacteria produces various kinds of D-amino acids [44]. For example, Lactobacillus plantarum glutamate racemase converts l-glutamate into d-glutamate [45].…”

Bioconversion of Fish Discards through the Production of Lactic Acid Bacteria and Metabolites: Sustainable Application of Fish Peptones in Nutritive Fermentation Media

“…Nevertheless, the capacity of peptones was almost always similar or higher than that found in commercial meat extract and bactopeptone from MRS, proving that the fish peptones studied here present a good balance in the amino acid content, including those essential for many lactic acid bacteria ( L. helveticus , L. plantarum , etc. ), such as Ile, Leu, Cys, Glu, or Val [ 51 , 52 , 53 ].…”

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“…42 For example, it promotes testosterone synthesis in the testicle, 43 and regulates synthesis of oxytocin, vasopressin, and prolactin in the posterior pituitary gland. [44][45][46] The role of D-Asp as a neurotransmitter has been demonstrated in recent years. 47 The generation of D-Asp might be catalyzed by Srr in mice 48 ; however, some mechanisms are unclear regarding its synthesis in mammals.…”

“…47 The generation of D-Asp might be catalyzed by Srr in mice 48 ; however, some mechanisms are unclear regarding its synthesis in mammals. 46 In addition to the synthesized D-Ser and D-Asp catalyzed by various enzymes in cells, some Damino acids from various foods and enteric bacteria are also ingested and metabolized in the human body. D'aniello et al investigated the ingestion of D-amino acids by feeding adult animals of different species, including man, rat, mouse, rabbit, chicks, with a given quantity of D-amino acids followed by measuring the levels of D-amino acids and their oxidative products in the blood and solid tissues after a time interval.…”

“…In the 1970s, D-Ala, D-glutamate, and D-Asp were found in bacterial cell walls as constituents, and D-amino acids were observed to exist widely in nature. The initial findings led to the conclusion that D-amino acids were only rarely present in bacterial cell walls, but with the development of chiral analytical techniques, various kinds of D-amino acids were identified in free forms in diverse organisms 46. In 2013, Mutaguchi et al reported that lactic fermentation is responsible for the D-amino acid production.…”