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-...
Background Due to various environmental problems, biodegradable polymers such as poly (3-hydroxybutyrate) (PHB) have gained much attention in recent years. Purple non-sulfur (PNS) bacteria have various attractive characteristics useful for environmentally harmless PHB production. However, production of PHB by PNS bacteria using genetic engineering has never been reported. This study is the first report of a genetically engineered PNS bacterial strain with a high PHB production. Results We constructed a poly (3-hydroxyalkanoate) depolymerase ( phaZ ) gene-disrupted Rhodobacter sphaeroides HJ strain. This R. sphaeroides HJΔ phaZ (pLP-1.2) strain showed about 2.9-fold higher volumetric PHB production than that of the parent HJ (pLP-1.2) strain after 5 days of culture. The HJΔ phaZ strain was further improved for PHB production by constructing strains overexpressing each of the eight genes including those newly found and annotated as PHB biosynthesis genes in the KEGG GENES Database. Among these constructed strains, all of gene products exhibited annotated enzyme activities in the recombinant strain cells, and HJΔ phaZ ( phaA3 ), HJΔ phaZ ( phaB2 ), and HJΔ phaZ ( phaC1 ) showed about 1.1-, 1.1-, and 1.2-fold higher volumetric PHB production than that of the parent HJΔ phaZ (pLP-1.2) strain. Furthermore, we constructed a strain that simultaneously overexpresses all three phaA3 , phaB2 , and phaC1 genes; this HJΔ phaZ ( phaA3 / phaB2 / phaC1 ) strain showed about 1.7- to 3.9-fold higher volumetric PHB production (without ammonium sulfate; 1.88 ± 0.08 g l −1 and with 100 mM ammonium sulfate; 0.99 ± 0.05 g l −1 ) than those of the parent HJ (pLP-1.2) strain grown under nitrogen limited and rich conditions, respectively. Conclusion In this study, we identified eight different genes involved in PHB biosynthesis in the genome of R. sphaeroides 2.4.1, and revealed that their overexpression increased PHB accumulation in an R. sphaeroides HJ strain. In addition, we demonstrated the effectiveness of a phaZ disruption for high PHB accumulation, especially under nitrogen rich conditions. Furthermore, we showed that PNS bacteria may have some unidentified genes involved in poly (3-hydroxyalkanoates) (PHA) biosynthesis. Our findings could lead to further improvement of environmentally harmless PHA production techniques ...
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