title: Molecular cloning and enzymological characterization of pyridoxal 5'-phosphate independent aspartate racemase from hyperthermophilic archaeon Thermococcus litoralis DSM 5473.
abstract: We succeeded in expressing the aspartate racemase homolog gene from Thermococcus litoralis DSM 5473 in Escherichia coli Rosetta (DE3) and found that the gene encodes aspartate racemase. The aspartate racemase gene consisted of 687 bp and encoded 228 amino acid residues. The purified enzyme showed aspartate racemase activity with a specific activity of 1590 U/mg. The enzyme was a homodimer with a molecular mass of 56 kDa and did not require pyridoxal 5'-phosphate as a coenzyme. The enzyme showed aspartate racemase activity even at 95 °C, and the activation energy of the enzyme was calculated to be 51.8 kJ/mol. The enzyme was highly thermostable, and approximately 50 % of its initial activity remained even after incubation at 90 °C for 11 h. The enzyme showed a maximum activity at a pH of 7.5 and was stable between pH 6.0 and 7.0. The enzyme acted on L-cysteic acid and L-cysteine sulfinic acid in addition to D- and L-aspartic acids, and was strongly inhibited by iodoacetic acid. The site-directed mutagenesis of the enzyme showed that the essential cysteine residues were conserved as Cys83 and Cys194. D-Forms of aspartic acid, serine, alanine, and valine were contained in T. litoralis DSM 5473 cells.
authors.affiliation: Department of Life Science and Biotechnology, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-Cho, Suita, Osaka-Fu, 564-8680, Japan.
authors.forename: Tsubasa
authors.initials: T
authors.lastname: Washio
authors.affiliation: Kansai University High Technology Research Center, 3-3-35 Yamate-Cho, Suita, Osaka-Fu, 564-8680, Japan.
authors.forename: Shiro
authors.initials: S
authors.lastname: Kato
authors.affiliation: Department of Life Science and Biotechnology, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-Cho, Suita, Osaka-Fu, 564-8680, Japan. oikawa@kansai-u.ac.jp.|Kansai University High Technology Research Center, 3-3-35 Yamate-Cho, Suita, Osaka-Fu, 564-8680, Japan. oikawa@kansai-u.ac.jp.
authors.forename: Tadao
authors.initials: T
authors.lastname: Oikawa
mesh_terms: D000591:Amino Acid Isomerases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019943:Amino Acid Substitution; D019843:Archaeal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017124:Conserved Sequence; D004795:Enzyme Stability; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D011732:Pyridoxal Phosphate / Q000378:metabolism*; D013379:Substrate Specificity; D019712:Thermococcus / Q000201:enzymology* / Q000235:genetics
chemical_list: D019843:Archaeal Proteins; D011732:Pyridoxal Phosphate; D000591:Amino Acid Isomerases; C019749:aspartate racemase
keywords: Aspartate racemase; D-Aspartic acid; Hyperthermophilic archaeon; Molecular cloning; Thermococcus litoralis