Identification and functional validation of kynureninases from oral bacteria.

Background: The kynurenine (KYN) pathway produces key metabolites for immunoregulation and neuromodulation in humans, but its presence and activity in the oral microbiome are unclear. This study investigates the functionality of the key kynureninase (KynU), which catalyses kynurenine to anthranilic acid (AA), in oral bacteria.

Methods: Bioinformatic analysis identified putative kynU genes in oral bacterial genomes, and structural similarity of the predicted proteins was evaluated using Template Modeling (TM)-score and Root Mean Square Deviation (RMSD) analyses. Selected kynU sequences were cloned into the pBAD-His A expression vector. Enzymatic activity was accessed by quantifying AA concentrations using liquid chromatography-mass spectrometry (LC-MS).

Results: Among 71 species, seven oral bacteria were identified to possess the kynU. Structural analyses indicated KynU from four species may fold into functional enzymes. Three recombinant KynU from Burkholderia cepacia, Ralstonia pickettii, and Stenotrophomonas maltophilia produced detectable levels of AA (21.27 ± 12.0 µM, 19.59 ± 8.6 µM, and 46.43 ± 36.8 µM, respectively), confirming functional KYN-to-AA conversion.

Conclusions: This study demonstrates KynU activity in oral bacteria, revealing an unrecognised aspect of microbial metabolism with potential implications for host-microbe interactions. Further investigation is required to elucidate the biological significance of bacterial KYN metabolites and their role in oral diseases.