Characterization and multifunctionality of two amino acid aminotransferases from the hyperthermophile <i>Thermotoga maritima</i>

The hyperthermophile Thermotoga maritima possesses a unique peptidoglycan containing an unusual d-lysine. Previously, we identified enzymes involved in the production of d-lysine and d-glutamate, respectively, but the biosynthetic pathway of d-alanine remains unclear. Herein, we characterized two amino acid aminotransferases, aspartate aminotransferase (TM1255), and alanine aminotransferase (TM1698). TM1255 has specific aminotransferase activities toward l-aspartate and l-glutamate as amino donors, while TM1698 has broad substrate specificity with high activities toward l-alanine and l-aminobutyrate as amino donors. Intriguingly, these two enzymes possess racemase activities toward several amino acids and aspartate 4-decarboxylase activity. The catalytic efficiency of both enzymes was highest for aminotransferase activity, followed by aspartate 4-decarboxylase activity. Therefore, TM1255 and TM1698 are novel multifunctional enzymes that have three different activities.