Novel amino acid aminotransferases mediate the final steps in biosynthetic pathways of branched‐chain and aromatic amino acids in the hyperthermophile <i>Thermotoga maritima</i>

The hyperthermophile Thermotoga maritima does not possess a typical branched-chain amino acid aminotransferase or aromatic amino acid aminotransferase, leaving the biosynthetic pathways of these amino acids unclear. In this study, we identified and characterized a novel branched-chain and aromatic amino acid aminotransferase (TM1131). We also characterized a histidinol-phosphate aminotransferase (TM1040) with reported aminotransferase activity toward aromatic amino acids. TM1131 exhibited broad substrate specificity and the highest activity toward branched-chain and aromatic amino acids as an amino donor and toward corresponding 2-oxoacids as an amino acceptor. TM1040 also showed broad substrate specificity, with the highest activity toward l-lysine and l-arginine as an amino donor, and toward 2-oxoacids corresponding to l-methionine, l-leucine, and l-phenylalanine. Additionally, we investigated the multifunctionality of these two enzymes to explore other potential amino acid metabolic activities. Intriguingly, TM1131 displayed aspartate 4-decarboxylase activity, albeit with lower catalytic efficiency than measured for aminotransferase activity. TM1131 is involved in the final step of the biosynthetic pathways of branched-chain and aromatic amino acids, to which TM1040 also likely contributes.