Identification and characterization of a new gene from Variovorax paradoxus Iso1 encoding N-acyl-D-amino acid amidohydrolase responsible for D-amino acid production

An N-acyl-D-amino acid amidohydrolase (N -D-AAase) was identified in cell extracts of a strain, Iso1, isolated from an environment containing N-acetyl-D-methionine. The bacterium was classified as Variovorax paradoxus by phylogenetic analysis. The gene was cloned and sequenced. The gene consisted of a 1467-bp ORF encoding a polypeptide of 488 amino acids. The V. paradoxus N-D-AAase showed significant amino acid similarity to the N-acyl-D-amino acid amidohydrolases of the two eubacteria Alcaligenes xylosoxydans A-6 (44-56% identity), Alcaligenes facelis DA1 (54% identity) and the hyperthermophilic archaeon Pyrococcus abyssi (42% identity). After over-expression of the N-D-AAase protein in Escherichia coli , the enzyme was purified by multistep chromatography. The native molecular mass was 52.8 kDa, which agreed with the predicted molecular mass of 52 798 Da and the enzyme appeared to be a monomer protein by gel-filtration chromatography. A homogenous protein with a specific activity of 516 U.mg(-1) was finally obtained. After peptide sequencing by LC/MS/MS, the results were in agreement with the deduced amino acid sequence of the N -D-AAase. The pI of the enzyme was 5.12 and it had an optimal pH and temperature of 7.5 and 50 degreesC, respectively. After 30 min heat treatment at 45 degreesC, between pH 6 and pH 8, 80% activity remained. The N-D-AAase had higher hydrolysing activity against N-acetyl-D-amino acid derivates containing D-methionine, D-leucine and D-alanine and against N-chloroacetyl-D-phenylalanine. Importantly, the enzyme does not act on the N -acetyl-L-amino acid derivatives. The enzyme was inhibited by chelating agents and certain metal ions, but was activated by 1 mm of Co2+ and Mg2+. Thus, the N-D-AAase from V. paradoxus can be considered a chiral specific and metal-dependent enzyme.