Characterization of 1,2-dibromoethane-degrading haloalkane dehalogenase from Bradyrhizobium japonicum USDA110

Kj-mhpC Enzyme in Klebsiella jilinsis 2N3 Is Involved in the Degradation of Chlorimuron-Ethyl via De-Esterification

Microbial degradation is a highly efficient and reliable approach for mitigating the contamination of sulfonylurea herbicides, such as chlorimuron-ethyl, in soil and water. In this study, we aimed to assess whether Kj-mhpC plays a pivotal role in the degradation of chlorimuron-ethyl. Kj-mhpC enzyme purified via prokaryotic expression exhibited the highest catalytic activity for chlorimuron-ethyl at 35 °C and pH 7. Bioinformatic analysis and three-dimensional homologous modeling of Kj-mhpC were conducted. Additionally, the presence of Mg+ and Cu2+ ions partially inhibited but Pb2+ ions completely inhibited the enzymatic activity of Kj-mhpC. LC/MS revealed that Kj-mhpC hydrolyzes the ester bond of chlorimuron-ethyl, resulting in the formation of 2-(4-chloro-6-methoxypyrimidine-2-amidoformamidesulfonyl) benzoic acid. Furthermore, the point mutation of serine at position 67 (Ser67) confirmed that it is the key amino acid at the active site for degrading chlorimuron-ethyl. This study enhanced the understanding of how chlorimuron-ethyl is degraded by microorganisms and provided a reference for bioremediation of the environment polluted with chlorimuron-ethyl.

Purification, characterization, and catalytic mechanism of N-Isopropylammelide isopropylaminohydrolase (AtzC) involved in the degradation of s-triazine herbicides

Deamination is ubiquitous in nature and has important biological significance. Leucobacter triazinivorans JW-1, recently isolated from sludge, can rapidly degrade s-triazine herbicides. The responsible enzymes, however, have not been purified and characterized. Herein, we purified an amidohydrolase, i.e., N-isopropylammelide isopropylaminohydrolase (AtzC) from JW-1 cells by ammonium sulfate precipitation and three chromatography steps. The purified AtzC catalyzed amidohydrolysis of N-isopropylammelide to cyanuric acid. The optimal catalytic conditions of the purified AtzC were 42 °C and pH 7.0, and the K_m and V_max of AtzC was 0.811 mM and 28.19 mmol/min·mg. AtzC could catalyze amidohydrolysis of an N-alkyl substituent from dihydroxy s-triazines to cyanuric acid. Molecular docking and structural alignments were used to infer AtzC catalytic mechanism. The structural architecture of AtzC resembled that of cytosine deaminase in class III amidohydrolase, with a single Zn²⁺ coordinated by His and Asp. Interestingly, the AtzC lacks an acidic residue putatively to activate water for hydrolysis as compared to the other amidohydrolases. His253 in AtzC probably functions as a single general acid-base catalyst. These findings further enhance our understanding how aminohydrolases catalyze the metabolism of s-triazine herbicides.

Purification, molecular characterization and metabolic mechanism of an aerobic tetrabromobisphenol A dehalogenase, a key enzyme of halorespiration in Ochrobactrum sp. T

Due to the detoxification of tetrabromobisphenol A (TBBPA) varies from different bacterial strains and depends on their specific enzymatic machinery, it is necessary to understand them for potential in situ bioremediation application. The special ability of our previously isolated Ochrobactrum sp. T to simultaneously debrominate and aerobic mineralize TBBPA urgent us to continuously study its degradation molecular mechanism. Herein, the purification and characterization of the dehalogenase which can debrominate TBBPA was investigated based on its corresponding encoding gene tbbpaA. Results showed that an enzyme with molecular mass of 117 kDa, K_m of 26.6 μM and V_max of 0.133 μM min^-1 mg^-1 was purified and designated as bromophenol dehalogenase. It was the only detected dehalogenase which exhibited TBBPA degradation ability (78%). Moreover, its activity was significantly enhanced by adding NADPH or methyl viologen to the reaction solution. The high similarity of substrate spectrum between the dehalogenase from the recombinant strain and the wild strain further indicated that it was the main dehalogenase responsible for the debromination in wild strain. Based on three identified metabolites, a metabolic pathway of TBBPA by purified enzyme under oxic condition was proposed. This study provides an excellent dehalogenase candidate for mechanistic study of aerobic dehalogenation of brominated aromatic compound.

Aerobic and Anaerobic Biodegradation of 1,2-Dibromoethane by a Microbial Consortium under Simulated Groundwater Conditions

This study was conducted to explore the potential for 1,2-Dibromoethane (EDB) biodegradation by an acclimated microbial consortium under simulated dynamic groundwater conditions. The enriched EDB-degrading consortium consisted of anaerobic bacteria Desulfovibrio, facultative anaerobe Chromobacterium, and other potential EDB degraders. The results showed that the biodegradation efficiency of EDB was more than 61% at 15 °C, and the EDB biodegradation can be best described by the apparent pseudo-first-order kinetics. EDB biodegradation occurred at a relatively broad range of initial dissolved oxygen (DO) from 1.2 to 5.1 mg/L, indicating that the microbial consortium had a strong ability to adapt. The addition of 40 mg/L of rhamnolipid and 0.3 mM of sodium lactate increased the biodegradation. A two-phase biodegradation scheme was proposed for the EDB biodegradation in this study: an aerobic biodegradation to carbon dioxide and an anaerobic biodegradation via a two-electron transfer pathway of dihaloelimination. To our knowledge, this is the first study that reported EDB biodegradation by an acclimated consortium under both aerobic and anaerobic conditions, a dynamic DO condition often encountered during enhanced biodegradation of EDB in the field.

Purifying, cloning and characterizing a novel dehalogenase from Bacillus sp. GZT to enhance the biodegradation of 2,4,6-tribromophenol in water

2,4,6-Tribromophenol (TBP), an intermediate of brominated flame retardants, can easily release to environment and recalcitrant to degradation. Previously, Bacillus sp. GZT, a pure aerobic strain capable of simultaneously debrominating and mineralizing TBP, was successfully isolated by us. To further obtain a practical application and dig up its TBP degradation mechanism, a total of 46.7-fold purification of a novel dehalogenase with a final specific activity of 18.9 U mg^-1 and a molecular mass of 63.4 kDa was achieved. Under optimal conditions (35 °C and 200 rpm), up to 80% degradation efficiencies were achieved within 120 min. Adding H₂O₂, NADPH, Mn²⁺ and Mg²⁺ promoted enzyme reaction effectively; while EDTA, methyl viologen, Ni²⁺, Cu²⁺, Ca²⁺ and Fe²⁺ strongly inhibited reaction activities. The debromination of TBP was catalyzed by the enzyme at a Km of 78 μM and a Vmax of 0.65 min^-1 mg protein^-1, which indicated that this dehalogenase could specifically eliminate TBP with a high efficiency and stability. Based on MALDI-TOF/TOF analysis, the dehalogenase shared 98% identity with peptide ABC transporter substrate-binding protein. One open reading frame (ORF) encoding this peptide was found in Strain GZT genome, subjected to clone and expressed in Escherichia coli (E. coli) to characterize the encoding gene. Result showed that this recombinant strain could also remove as similar amount of TBP as Bacillus sp. GZT under the identical condition. Based on these results, we suggest that this newly-isolated TBP dehalogenase highlights a new approach for remediating TBP pollution.

One-step process for debromination and aerobic mineralization of tetrabromobisphenol-A by a novel Ochrobactrum sp. T isolated from an e-waste recycling site

A novel bacterium, Ochrobactrum sp. T, capable of simultaneous debromination and aerobic mineralization of tetrabromobisphenol-A (TBBPA), was isolated from a sludge sample collected from an electronic-waste recycling site. The bacterium exhibited maximal debrominase activity at pH 6.5, 35 °C, and 200 rpm in Luria-Bertani culture medium. Initial TBBPA concentration and pH had more significant effects on degradation efficiency than those of temperature and inoculum size. Degradation and debromination efficiencies of 91.8% and 86.7%, respectively, were achieved within 72 h under optimized conditions of 35 °C, pH 7.0, inoculum volume of 25 mL, and TBBPA concentration of 3 mg L⁻¹. In addition, a 35.6% decrease in total organic carbon was observed after the degradation of 5 mg L⁻¹ TBBPA for 120 h. Eight metabolic intermediates were identified during the biodegradation of TBBPA. This study is the first report to propose a one-step process for TBBPA debromination and mineralization by a single bacterial strain.