Cavity residue leucine 95 and channel residues glutamine 204, aspartic acid 211, and phenylalanine 269 of toluene o-xylene monooxygenase influence catalysis

A multicomponent propane monooxygenase catalyzes the initial degradation of methyl tert-butyl ether in Mycobacterium vaccae JOB5

Methyl tert-butyl ether (MTBE) has been recognized as a groundwater contaminant due to its widespread distribution and potential threat to human health. The limited understanding of the enzymes catalyzing MTBE degradation restricts their application in MTBE bioremediation. In this study, an MTBE-degrading soluble di-iron monooxygenase that clusters phylogenetically with a known propane monooxygenase (PRM) encoded by the prmABCD gene cluster was identified and functionally characterized, revealing their role in MTBE metabolism by Mycobacterium vaccae JOB5. Transcriptome analysis demonstrated that the expression of prmABCD was upregulated when JOB5 was induced by MTBE. Escherichia coli Rosetta heterologously expressing prmABCD from JOB5 could transform MTBE, indicating that the PRM of JOB5 is capable of the initial degradation of MTBE. The loss of the gene encoding the oxygenase α-subunit or β-subunit, the coupling protein, or the reductase disrupted MTBE transformation by the recombinant E. coli Rosetta. In addition, the catalytic capacity of PRM is likely affected by residue G95 in the active site pocket and residues I84, P165, A269, and V270 in the substrate tunnel structure. Mutation of amino acids in the active site and substrate tunnel resulted in inefficiency or inactivation of MTBE degradation, and the activity in 1,4-dioxane (1,4-D) degradation was diminished less than that in MTBE degradation.IMPORTANCEMulticomponent monooxygenases catalyzing the initial hydroxylation of MTBE are important in MTBE biodegradation. Previous studies of MTBE degradation enzymes have focused on P450s, alkane monooxygenase and MTBE monooxygenase, but the vital role of soluble di-iron monooxygenases has rarely been reported. In this study, we deciphered the essential catalytic role of a PRM and revealed the key residues of the PRM in MTBE metabolism. Our findings provide new insight into the MTBE-degrading gene cluster and enzymes in bacteria. This characterization of the PRM associated with MTBE degradation expands our understanding of MTBE-degrading gene diversity and provides a novel candidate enzyme for the bioremediation of MTBE-contaminated sites.

Regiospecific Oxidation of Chlorobenzene to 4-Chlororesorcinol, Chlorohydroquinone, 3-Chlorocatechol and 4-Chlorocatechol by Engineered Toluene o-Xylene Monooxygenases

Toluene o-xylene monooxygenase (ToMO) was found to oxidize chlorobenzene to form 2-chlorophenol (2-CP, 4%), 3-CP (12%), and 4-CP (84%) with a total product formation rate of 1.2 ± 0.17 nmol/min/mg protein. It was also discovered that ToMO forms 4-chlorocatechol (4-CC) from 3-CP and 4-CP with initial rates of 0.54 ± 0.10 and 0.40 ± 0.04 nmol/min/mg protein, respectively, and chlorohydroquinone (CHQ, 13%), 4-chlororesorcinol (4-CR, 3%), and 3-CC (84%) from 2-CP with an initial product formation rate of 1.1 ± 0.32 nmol/min/mg protein. To increase the oxidation rate and alter the oxidation regiospecificity of chloroaromatics, as well as to study the roles of active site residues L192 and A107 of the alpha hydroxylase fragment of ToMO (TouA), we used the saturation mutagenesis approach of protein engineering. Thirteen TouA variants were isolated, among which some of the best substitutions uncovered here have never been studied before. Specifically, TouA variant L192V was identified which had 1.8-, 1.4-, 2.4-, and 4.8-fold faster hydroxylation activity toward chlorobenzene, 2-CP, 3-CP, and 4-CP, respectively, compared to the native ToMO. The L192V variant also had the regiospecificity of chlorobenzene changed from 4% to 13% 2-CP and produced the novel product 3-CC (4%) from 3-CP. Most of the isolated variants were identified to change the regiospecificity of oxidation. For example, compared to the native ToMO, variants A107T, A107N, and A107M produced 6.3-, 7.0-, and 7.3-fold more 4-CR from 2-CP, respectively, and variants A107G and A107G/L192V produced 3-CC (33 and 39%, respectively) from 3-CP whereas native ToMO did not. IMPORTANCE Chlorobenzene is a commonly used toxic solvent and listed as a priority environmental pollutant by the US Environmental Protection Agency. Here, we report that Escherichia coli TG1 cells expressing toluene o-xylene monooxygenase (ToMO) can successfully oxidize chlorobenzene to form dihydroxy chloroaromatics, which are valuable industrial compounds. ToMO performs this at room temperature in water using only molecular oxygen and a cofactor supplied by the cells. Using protein engineering techniques, we also isolated ToMO variants with enhanced oxidation activity as well as fine-tuned regiospecificities which make direct microbial oxygenations even more attractive. The significance of this work lies in the ability to degrade environmental pollutants while at the same time producing valuable chemicals using environmentally benign biological methods rather than expensive, complex chemical processes.

Mutagenesis and expression of methane monooxygenase to alter regioselectivity with aromatic substrates

Soluble methane monooxygenase (sMMO) from methane-oxidising bacteria can oxygenate more than 100 hydrocarbons and is one of the most catalytically versatile biological oxidation catalysts. Expression of recombinant sMMO has to date not been achieved in Escherichia coli and so an alternative expression system must be used to manipulate it genetically. Here we report substantial improvements to the previously described system for mutagenesis of sMMO and expression of recombinant enzymes in a methanotroph (Methylosinus trichosporium OB3b) expression system. This system has been utilised to make a number of new mutants and to engineer sMMO to increase its catalytic precision with a specific substrate whilst increasing activity by up to 6-fold. These results are the first 'proof-of-principle' experiments illustrating the feasibility of developing sMMO-derived catalysts for diverse applications.