Comparative biochemistry of bacterial N-acyl-d-amino acid amidohydrolase

N-acyl-d-amino acid amidohydrolases can be classified into three types based on substrate specificity. d-aminoacylase has been reported to occur in a very few bacteria such as Pseudomonas, Streptomyces, and Alcaligenes. N-acyl-d-aspartate amidohydrolase (d-AAase) has been reported in only Alcaligenes xylosoxydans subsp. xylosoxydans A-6 (Alcaligenes A-6) while N-acyl-d-glutamate amidohydrolase (d-AGase) has been isolated in two stains of Pseudomonas sp. 5f-1 and Alcaligenes A-6. The physiological roles of these enzymes in these microbes are not clear. They are individually characteristic in their substrate specificities, inducer profiles, inhibitors, isoelectric points, metal dependency, and some physicochemical properties. The primary structures of all the three types of N-acyl-d-amino acid amidohydrolases from Alcaligenes A-6 were determined from their nucleotide sequences. Comparison of their primary structures revealed high homology (46–56%) between the different enzymes. The three enzymes showed 26–27% sequence homology with l-aminoacylases from Bacillus stearothermophilus, porcine, and human. Chemical modification and site-directed mutagenesis identified the histidyl residues essential for catalysis. The Alcaligenes N-acyl-d-amino acid amidohydrolases share significant sequence similarities with some members of the urease-related amidohydrolase superfamily proposed by Holm and Sander [L. Holm, C. Sander, Proteins: Structure, Function and Genetics 28 (1997) 72].