Characterization of the 2,6-Dimethylphenol Monooxygenase MpdAB and Evaluation of Its Potential in Vitamin E Precursor Synthesis.

2,6-Dimethylphenol (2,6-DMP) is a widely used chemical intermediate whose residue has been frequently detected in the environment, posing a threat to some aquatic organisms. Microbial degradation is an effective method to eliminate 2,6-DMP in nature. However, the genetic and biochemical mechanisms of 2,6-DMP metabolism remain unknown. Mycobacterium neoaurum B5-4 is a 2,6-DMP-degrading bacterium isolated in our previous study. Here, a 2,6-DMP degradation-deficient mutant of strain B5-4 was screened. Comparative genomic, transcriptomic, gene disruption, and genetic complementation data indicated that mpdA and mpdB are responsible for the initial step of 2,6-DMP degradation in M. neoaurum B5-4. MpdAB was predicted to be a two-component flavin-dependent monooxygenase system, which shows 32% and 36% identities with HsaAB from Mycobacterium tuberculosis CDC1551. The transcription of mpdA and mpdB was substantially increased upon exposure to 2,6-DMP. Nuclear magnetic resonance analysis showed that purified 6×His-MpdA and 6×His-MpdB hydroxylated 2,6-DMP and 2,3,6-trimethylphenol (2,3,6-TMP) at the para-position using NADH and flavin adenine dinucleotide (FAD) as cofactors. The apparent Km values of MpdAB for 2,6-DMP and 2,3,6-TMP were 0.12 ± 0.01 and 0.17 ± 0.01 mM, respectively, and the corresponding kcat/Km values were 4.02 and 2.84 s-1 mM-1, respectively. Since para-hydroxylated 2,3,6-TMP is a major precursor for vitamin E synthesis, the potential of MpdAB in vitamin E synthesis was preliminarily evaluated using whole-cell catalysis. Low expression levels of MpdA and 2,3,6-TMP cytotoxicity limited the efficiency of whole-cell catalysis. Together, this study reveals the genetic and biochemical basis for the initial step of 2,6-DMP biodegradation and provides candidate enzymes for vitamin E synthesis. IMPORTANCE Although the microbial degradation of the six isomers of dimethylphenol has been extensively studied, the genetic and biochemical mechanisms of 2,6-DMP degradation remain unclear. This study identified the genes responsible for the initial step in the 2,6-DMP catabolic pathway in M. neoaurum B5-4. Moreover, MpdAB also catalyzed the transformation of 2,3,6-TMP to 2,3,5-trimethylhydroquinone (2,3,5-TMHQ), a crucial step in vitamin E synthesis. Overall, this study provides candidate enzymes for both the bioremediation of 2,6-DMP contamination and the development of a green method to synthesize vitamin E.