Detection of D-glutamate Production from the Dual Function Enzyme, 4-amino-4-deoxychorismate Lyase/D-amino Acid Transaminase, in Mycobacterium smegmatis

D-amino acid transaminase (D-AAT) is able to synthesize both D-glutamate and D-alanine, according to the following reaction: D-alanine + alpha-ketoglutarate (sic) D-glutamate + pyruvate. These two D-amino acids are essential components of the peptidoglycan layer of bacteria. In our recently published work, MSMEG_5795 from Mycobacterium smegmatis was identified as having D-amino acid transaminase (D-AAT) activity, although it has primarily been annotated as 4-amino-4-deoxychorismate lyase (ADCL). To unequivocally demonstrate D-AAT activity from MSMEG_5795 protein two coupled enzyme assays were performed in series. First, D-alanine and alpha-ketoglutarate were converted to D-glutamate and pyruvate by MSMEG_5795 using the D-AAT assay. Next, the products of this reaction, following removal of all protein, were used as input into an assay for glutamate racemase in which D-glutamate is converted to L-glutamate by glutamate racemase (Gallo and Knowles, 1993; Poen et al., 2016). As the only source of D-glutamate in this assay would be from the reaction of D-alanine with MSMEG_5795, positive results from this assay would confirm the D-AAT activity of MSMEG_5795 and of any enzyme tested in this manner.