Identification of a novel <scp>d</scp>‐amino acid aminotransferase involved in <scp>d</scp>‐glutamate biosynthetic pathways in the hyperthermophile <i>Thermotoga maritima</i>

Abstract

The hyperthermophilic bacterium Thermotoga maritima has an atypical peptidoglycan that contains d ‐lysine alongside the usual d ‐alanine and d ‐glutamate. We previously identified a lysine racemase involved in d ‐lysine biosynthesis, and this enzyme also possesses alanine racemase activity. However, T . maritima has neither alanine racemase nor glutamate racemase enzymes; hence, the precise biosynthetic pathways of d ‐alanine and d ‐glutamate remain unclear in T . maritima . In the present study, we identified and characterized a novel d ‐amino acid aminotransferase (TM0831) in T . maritima . TM0831 exhibited aminotransferase activity towards 23 d ‐amino acids, but did not display activity towards l ‐amino acids. It displayed high specific activities towards d ‐homoserine and d ‐glutamine as amino donors. The most preferred acceptor was 2‐oxoglutarate, followed by glyoxylate. Additionally, TM0831 displayed racemase activity towards four amino acids including aspartate and glutamate. Catalytic efficiency ( k cat / K m ) for aminotransferase activity was higher than for racemase activity, and pH profiles were distinct between these two activities. To evaluate the functions of TM0831, we constructed a TTHA1643 (encoding glutamate racemase)‐deficient Thermus thermophilus strain ( ∆TTHA1643 ) and integrated the TM0831 gene into the genome of ∆TTHA1643 . The growth of this TM0831 ‐integrated strain was promoted compared with ∆TTHA1643 and was restored to almost the same level as that of the wild‐type strain. These results suggest that TM0831 is involved in d ‐glutamate production. TM0831 is a novel d ‐amino acid aminotransferase with racemase activity that is involved in the production of d ‐amino acids in T . maritima .