Cloning and characterization of a novel aspartate/glutamate racemase from the acorn worm Saccoglossus kowalevskii

Previously, we demonstrated that the animal aspartate racemase (AspR) gene has evolved from the serine racemase (SerR) gene by acquisition of three consecutive serine residues (Ser155-Ser156-Ser157) involved in the strong AspR activity, and this event has occurred independently and frequently during animal evolution. In the present study, we cloned and characterized two mammalian SerR homologous genes from the hemichordate acorn worm (Saccoglossus kowalevskii). The enzymes have been identified as an AspR and an aspartate/glutamate racemase (Asp/GluR) on the basis of their kinetic parameters. The S. kowalevskii Asp/GluR shows comparable substrate affinity and high catalytic efficiency (k_cat/K_m) for both aspartate and glutamate and is the first reported enzyme from animals that can synthesize D-glutamate. Amino acid sequence alignment analysis and site-directed mutagenesis studies have revealed that the amino acid residue at position 156, which is serine in AspR and alanine in Asp/GluR, is associated with binding and recognition of glutamate and aspartate. Phylogenetic analysis suggests that the S. kowalevskii AspR gene has evolved from the SerR gene after the divergence of hemichordata and vertebrate lineages by acquisition of the three serine residues at position 155 to 157 as in the case of other animal AspR genes. Furthermore, the S. kowalevskii Asp/GluR gene is the result of AspR gene duplication and several amino acid substitutions including that of the 156th serine residue with alanine. The fact that SerR has acquired substrate specificity towards aspartate or glutamate raises the possibility that synthesis of other D-amino acids is carried out by enzymes evolved from SerR.