title: Molecular identification of monomeric aspartate racemase from Bifidobacterium bifidum.
abstract: Bifidobacterium bifidum is a useful probiotic agent exhibiting health-promoting properties and contains d-aspartate as an essential component of the cross-linker moiety in the peptidoglycan. To help understand D-aspartate biosynthesis in B. bifidum NBRC 14252, aspartate racemase, which catalyzes the racemization of D- and L-aspartate, was purified to homogeneity and characterized. The enzyme was a monomer with a molecular mass of 27 kDa. This is the first report showing the presence of a monomeric aspartate racemase. Its enzymologic properties, such as its lack of cofactor requirement and susceptibility to thiol-modifying reagents in catalysis, were similar to those of the dimeric aspartate racemase from Streptococcus thermophilus. The monomeric enzyme, however, showed a novel characteristic, namely, that its thermal stability significantly increased in the presence of aspartate, especially the D-enantiomer. The gene encoding the monomeric aspartate racemase was cloned and overexpressed in Escherichia coli cells. The nucleotide sequence of the aspartate racemase gene encoded a peptide containing 241 amino acids with a calculated molecular mass of 26 784 Da. The recombinant enzyme was purified to homogeneity and its properties were almost the same as those of the B. bifidum enzyme.
authors.affiliation: Department of Bioresources Science, Kochi University, Nankoku, Kochi, Japan.
authors.forename: Tatsuyuki
authors.initials: T
authors.lastname: Yamashita
authors.forename: Makoto
authors.initials: M
authors.lastname: Ashiuchi
authors.forename: Kouhei
authors.initials: K
authors.lastname: Ohnishi
authors.forename: Shin'ichiro
authors.initials: S
authors.lastname: Kato
authors.forename: Shinji
authors.initials: S
authors.lastname: Nagata
authors.forename: Haruo
authors.initials: H
authors.lastname: Misono
mesh_terms: D000591:Amino Acid Isomerases / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D000595:Amino Acid Sequence; D001483:Base Sequence; D001644:Bifidobacterium / Q000201:enzymology*; D003001:Cloning, Molecular; D017931:DNA Primers; D004591:Electrophoresis, Polyacrylamide Gel; D004795:Enzyme Stability; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008969:Molecular Sequence Data; D008970:Molecular Weight; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D017931:DNA Primers; D011994:Recombinant Proteins; D000591:Amino Acid Isomerases; C019749:aspartate racemase