Protein engineering of toluene-o-xylene monooxygenase from Pseudomonas stutzeri OX1 for oxidizing nitrobenzene to 3-nitrocatechol, 4-nitrocatechol, and nitrohydroquinone

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.

Concerns with computational protein engineering programmes IPRO and OptMAVEn and metabolic pathway engineering programme optStoic

It has become customary in engineering to require a modelling component in research endeavours. In addition, as the code for these models becomes more byzantine in complexity, it is difficult for reviewers and readers to discern their value and understand the underlying code. This opinion piece summarizes the negative experience of the author with the IPRO and OptMAVEn computational protein engineering models as well as problems with the optStoic metabolic pathway model. In our hands, these models often fail to predict reliable ways to engineer proteins and metabolic pathways.

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

Structural analysis of toluene-o-xylene monooxygenase (ToMO) hydroxylase revealed the presence of three hydrophobic cavities, a channel, and a pore leading from the protein surface to the active site. Here, saturation mutagenesis was used to investigate the catalytic roles of alpha-subunit (TouA) second cavity residue L95 and TouA channel residues Q204, D211, and F269. By testing the substrates toluene, phenol, nitrobenzene, and/or naphthalene, these positions were found to influence the catalytic activity of ToMO. Several regiospecific variants were identified from TouA positions Q204, F269, and L95. For example, TouA variant Q204H had the regiospecificity of nitrobenzene changed significantly from 30 to 61 % p-nitrophenol. Interestingly, a combination of mutations at Q204H and A106V altered the regiospecificity of nitrobenzene back to 27 % p-nitrophenol. TouA variants F269Y, F269P, Q204E, and L95D improved the meta-hydroxylating capability of nitrobenzene by producing 87, 85, 82, and 77 % m-nitrophenol, respectively. For naphthalene oxidation, TouA variants F269V, Q204A, Q204S/S222N, and F269T had the regiospecificity changed from 16 to 9, 10, 23, and 25 % 2-naphthol, respectively. Here, two additional TouA residues, S222 and A106, were also identified that may have important roles in catalysis. Most of the isolated variants from D211 remained active, whereas having a hydrophobic residue at this position appeared to diminish the catalytic activity toward naphthalene. The mutational effects on the ToMO regiospecificity described here suggest that it is possible to further fine tune and engineer the reactivity of multicomponent diiron monooxygenases toward different substrates at positions that are relatively distant from the active site.

The Toluene o-Xylene Monooxygenase Enzymatic Activity for the Biosynthesis of Aromatic Antioxidants

Monocyclic phenols and catechols are important antioxidant compounds for the food and pharmaceutic industries; their production through biotransformation of low-added value starting compounds is of major biotechnological interest. The toluene o-xylene monooxygenase (ToMO) from Pseudomonas sp. OX1 is a bacterial multicomponent monooxygenase (BMM) that is able to hydroxylate a wide array of aromatic compounds and has already proven to be a versatile biochemical tool to produce mono- and dihydroxylated derivatives of aromatic compounds. The molecular determinants of its regioselectivity and substrate specificity have been thoroughly investigated, and a computational strategy has been developed which allows designing mutants able to hydroxylate non-natural substrates of this enzyme to obtain high-added value compounds of commercial interest. In this work, we have investigated the use of recombinant ToMO, expressed in cells of Escherichia coli strain JM109, for the biotransformation of non-natural substrates of this enzyme such as 2-phenoxyethanol, phthalan and 2-indanol to produce six hydroxylated derivatives. The hydroxylated products obtained were identified, isolated and their antioxidant potential was assessed both in vitro, using the DPPH assay, and on the rat cardiomyoblast cell line H9c2. Incubation of H9c2 cells with the hydroxylated compounds obtained from ToMO-catalyzed biotransformation induced a differential protective effect towards a mild oxidative stress induced by the presence of sodium arsenite. The results obtained confirm once again the versatility of the ToMO system for oxyfunctionalization reactions of biotechnological importance. Moreover, the hydroxylated derivatives obtained possess an interesting antioxidant potential that encourages the use of the enzyme for further functionalization reactions and their possible use as scaffolds to design novel bioactive molecules.

The role of substrate binding pocket residues phenylalanine 176 and phenylalanine 196 on Pseudomonas sp. OX1 toluene o-xylene monooxygenase activity and regiospecificity

Saturation mutagenesis was used to generate eleven substitutions of toluene-o-xylene monooxygenase (ToMO) at alpha subunit (TouA) positions F176 and F196 among which nine were novel: F176H, F176N, F176S, F176T, F196A, F196L, F196T, F196Y, F196H, F196I, and F196V. By testing the substrates phenol, toluene, and naphthalene, these positions were found to influence ToMO oxidation activity and regiospecificity. Specifically, TouA variant F176H was identified that had 4.7-, 4.3-, and 1.8-fold faster hydroxylation activity towards phenol, toluene, and naphthalene, respectively, compared to native ToMO. The F176H variant also produced the novel product hydroquinone (61%) from phenol, made twofold more 2-naphthol from naphthalene (34% vs. 16% by the wild-type ToMO), and had the regiospecificity of toluene changed from 51% to 73% p-cresol. The TouA F176N variant had the most para-hydroxylation capability, forming p-cresol (92%) from toluene and hydroquinone (82%) from phenol as the major product, whereas native ToMO formed 30% o-cresol, 19% m-cresol, and 51% of p-cresol from toluene and 100% catechol from phenol. For naphthalene oxidation, TouA variant F176S exhibited the largest shift in the product distribution by producing threefold more 2-naphthol. Among the other F196 variants, F196L produced catechol from phenol two times faster than the wild-type enzyme. The TouA F196I variant produced twofold less o-cresol and 19% more p-cresol from toluene, and the TouA F196A variant produced 62% more 2-naphthol from naphthalene compared to wild-type ToMO. Both of these positions have never been studied through the saturation mutagenesis and some of the best substitutions uncovered here have never been predicted and characterized for aromatics hydroxylation.

Toward protein engineering for phytoremediation: possibilities and challenges

The combination of rational protein engineering and directed evolution techniques allow for the redesign of enzymes with tailored properties for use in environmental remediation. This review summarizes current molecular methods for either altering or improving protein function and highlights examples of how these methods can address bioremediation problems. Although much of the protein engineering applied to environmental clean-up employs microbial systems, there is great potential for and significant challenges to translating these approaches to plant systems for phytoremediation purposes. Protein engineering technologies combined with genomic information and metabolic engineering strategies hold promise for the design of plants and microbes to remediate organic and inorganic pollutants.

A process optimization for bio-catalytic production of substituted catechols (3-nitrocatechol and 3-methylcatechol

BACKGROUND

Substituted catechols are important precursors for large-scale synthesis of pharmaceuticals and other industrial products. Most of the reported chemical synthesis methods are expensive and insufficient at industrial level. However, biological processes for production of substituted catechols could be highly selective and suitable for industrial purposes.

RESULTS

We have optimized a process for bio-catalytic production of 3-substituted catechols viz. 3-nitrocatechol (3-NC) and 3-methylcatechol (3-MC) at pilot scale. Amongst the screened strains, two strains viz. Pseudomonas putida strain (F1) and recombinant Escherichia coli expression clone (pDTG602) harboring first two genes of toluene degradation pathway were found to accumulate 3-NC and 3-MC respectively. Various parameters such as amount of nutrients, pH, temperature, substrate concentration, aeration, inoculums size, culture volume, toxicity of substrate and product, down stream extraction, single step and two-step biotransformation were optimized at laboratory scale to obtain high yields of 3-substituted catechols. Subsequently, pilot scale studies were performed in 2.5 liter bioreactor. The rate of product accumulation at pilot scale significantly increased up to approximately 90-95% with time and high yields of 3-NC (10 mM) and 3-MC (12 mM) were obtained.

CONCLUSION

The biocatalytic production of 3-substituted catechols viz. 3-NC and 3-MC depend on some crucial parameters to obtain maximum yields of the product at pilot scale. The process optimized for production of 3-substituted catechols by using the organisms P. putida (F1) and recombinant E. coli expression clone (pDTG602) may be useful for industrial application.

Rapid methods for high-throughput detection of sulfoxides

Enantiopure sulfoxides are prevalent in drugs and are useful chiral auxiliaries in organic synthesis. The biocatalytic enantioselective oxidation of prochiral sulfides is a direct and economical approach for the synthesis of optically pure sulfoxides. The selection of suitable biocatalysts requires rapid and reliable high-throughput screening methods. Here we present four different methods for detecting sulfoxides produced via whole-cell biocatalysis, three of which were exploited for high-throughput screening. Fluorescence detection based on the acid activation of omeprazole was utilized for high-throughput screening of mutant libraries of toluene monooxygenases, but no active variants have been discovered yet. The second method is based on the reduction of sulfoxides to sulfides, with the coupled release and measurement of iodine. The availability of solvent-resistant microtiter plates enabled us to modify the method to a high-throughput format. The third method, selective inhibition of horse liver alcohol dehydrogenase, was used to rapidly screen highly active and/or enantioselective variants at position V106 of toluene ortho-monooxygenase in a saturation mutagenesis library, using methyl-p-tolyl sulfide as the substrate. A success rate of 89% (i.e., 11% false positives) was obtained, and two new mutants were selected. The fourth method is based on the colorimetric detection of adrenochrome, a back-titration procedure which measures the concentration of the periodate-sensitive sulfide. Due to low sensitivity during whole-cell screening, this method was found to be useful only for determining the presence or absence of sulfoxide in the reaction. The methods described in the present work are simple and inexpensive and do not require special equipment.

Protein engineering of toluene monooxygenases for synthesis of chiral sulfoxides

Enantiopure sulfoxides are valuable asymmetric starting materials and are important chiral auxiliaries in organic synthesis. Toluene monooxygenases (TMOs) have been shown previously to catalyze regioselective hydroxylation of substituted benzenes and phenols. Here we show that TMOs are also capable of performing enantioselective oxidation reactions of aromatic sulfides. Mutagenesis of position V106 in the alpha-hydroxylase subunit of toluene ortho-monooxygenase (TOM) of Burkholderia cepacia G4 and the analogous position I100 in toluene 4-monooxygenase (T4MO) of Pseudomonas mendocina KR1 improved both rate and enantioselectivity. Variant TomA3 V106M of TOM oxidized methyl phenyl sulfide to the corresponding sulfoxide at a rate of 3.0 nmol/min/mg protein compared with 1.6 for the wild-type enzyme, and the enantiomeric excess (pro-S) increased from 51% for the wild type to 88% for this mutant. Similarly, T4MO variant TmoA I100G increased the wild-type oxidation rate by 1.7-fold, and the enantiomeric excess rose from 86% to 98% (pro-S). Both wild-type enzymes showed lower activity with methyl para-tolyl sulfide as a substrate, but the improvement in the activity and enantioselectivity of the mutants was more dramatic. For example, T4MO variant TmoA I100G oxidized methyl para-tolyl sulfide 11 times faster than the wild type did and changed the selectivity from 41% pro-R to 77% pro-S. A correlation between regioselectivity and enantioselectivity was shown for TMOs studied in this work. Using in silico homology modeling, it is shown that residue I100 in T4MO aids in steering the substrate into the active site at the end of the long entrance channel. It is further hypothesized that the main function of V106 in TOM is the proper positioning or docking of the substrate with respect to the diiron atoms. The results from this work suggest that when the substrate is not aligned correctly in the active site, the oxidation rate is decreased and enantioselectivity is impaired, resulting in products with both chiral configurations.

Characterization of the arene-oxidizing intermediate in ToMOH as a diiron(III) species

We report the generation and characterization of a diiron(III) intermediate formed during reaction with dioxygen of the reduced hydroxylase component of toluene/o-xylene monooxygenase from Pseudomonas sp. OX1. The decay rate of this species is accelerated upon mixing with phenol, a substrate for this system. Under steady-state conditions, hydrogen peroxide was generated in the absence of substrate. The oxidized hydroxylase also decomposed hydrogen peroxide to liberate dioxygen in the absence of reducing equivalents. This activity suggests that dioxygen activation may be reversible. The linear free energy relationship determined from hydroxylation of para-substituted phenols under steady-state turnover has a negative slope. A value of rho < 0 is consistent with electrophilic attack by the oxidizing intermediate on the aromatic substrates. The results from these steady and pre-steady-state experiments provide compelling evidence that the diiron(III) intermediate is the active oxidant in the toluene/o-xylene monooxygenase system and is a peroxodiiron(III) transient, despite differences between its optical and Mössbauer spectroscopic parameters and those of other peroxodiiron(III) centers.

Application of gene-shuffling for the rapid generation of novel [FeFe]-hydrogenase libraries

A gene-shuffling technique was identified, optimized and used to generate diverse libraries of recombinant [FeFe]-hydrogenases. Six native [FeFe]-hydrogenase genes from species of Clostridia were first cloned and separately expressed in Escherichia coli concomitantly with the assembly proteins required for [FeFe]-hydrogenase maturation. All enzymes, with the exception of C. thermocellum HydA, exhibited significant activity when expressed. Single-stranded DNA fragments from genes encoding the two most active [FeFe]-hydrogenases were used to optimize a gene-shuffling protocol and generate recombinant enzyme libraries. Random sampling demonstrates that several shuffled products are active. This represents the first successful application of gene-shuffling using hydrogenases. Moreover, we demonstrate that a single set of [FeFe]-hydrogenase maturation proteins is sufficient for the heterologous assembly of the bioinorganic active site of several native and shuffled [FeFe]-hydrogenases.

Biology of Pseudomonas stutzeri

Pseudomonas stutzeri is a nonfluorescent denitrifying bacterium widely distributed in the environment, and it has also been isolated as an opportunistic pathogen from humans. Over the past 15 years, much progress has been made in elucidating the taxonomy of this diverse taxonomical group, demonstrating the clonality of its populations. The species has received much attention because of its particular metabolic properties: it has been proposed as a model organism for denitrification studies; many strains have natural transformation properties, making it relevant for study of the transfer of genes in the environment; several strains are able to fix dinitrogen; and others participate in the degradation of pollutants or interact with toxic metals. This review considers the history of the discovery, nomenclatural changes, and early studies, together with the relevant biological and ecological properties, of P. stutzeri.

Directed evolution of enzymes and biosynthetic pathways

Directed evolution is an important tool for overcoming the limitations of natural enzymes as biocatalysts. Recent advances have focused on applying directed evolution to a variety of enzymes, such as epoxide hydrolase, glyphosate N-acetyltransferase, xylanase and phosphotriesterase, in order to improve their activity, selectivity, stability and solubility. The focus has also shifted to manipulating biosynthetic pathways for the production of many naturally synthesized compounds, as well as the production of novel 'unnatural' compounds. A combined directed evolution and computational design approach is becoming increasingly important in exploring enzyme sequence-space and creating improved or novel enzymes. Fueled by recent breakthroughs in genomics and metagenomics, these developments should help expand the use of biocatalysts in industry.

Alanine 101 and alanine 110 of the alpha subunit of Pseudomonas stutzeri OX1 toluene-o-xylene monooxygenase influence the regiospecific oxidation of aromatics

Saturation mutagenesis was used to generate 10 mutants of toluene-o-xylene monooxygenase (ToMO) at alpha subunit (TouA) positions A101 and A110: A101G, A101I, A101M, A101VE, A101V, A110G, A110C, A110S, A110P, and A110T; by testing the substrates toluene, o-cresol, m-cresol, p-cresol, phenol, naphthalene, o-methoxyphenol, m-methoxyphenol, p-methoxyphenol, o-xylene, and nitrobenzene, these positions were found to influence the regiospecific oxidation of aromatics. For example, compared to wild-type ToMO, TouA variant A101V produced threefold more 3-methoxycatechol from m-methoxyphenol as well as produced methylhydroquinone from o-cresol whereas wild-type ToMO did not. Similarly, variant A110C synthesized 1.8-fold more o-cresol from toluene and 1.8-fold more 3-methoxycatechol from m-methoxyphenol, and variant A110G synthesized more m-nitrophenol and twofold less p-nitrophenol from nitrobenzene. The A101V and A110C mutations did not affect the rate of reaction with the natural substrate toluene, so the variants had high activity. This is the first report that these or analogous residues influence the catalysis with this class of enzymes. Wild-type ToMO was found to oxidize o-methoxyphenol to methoxyhydroquinone (60%) and 4-methoxyresorcinol (40%), m-methoxyphenol to 4-methoxycatechol (96%) and 3-methoxycatechol (4%), and p-methoxyphenol to 4-methoxycatechol (100%).

Protein engineering of toluene-o-xylene monooxygenase from Pseudomonas stutzeri OX1 for enhanced chlorinated ethene degradation and o-xylene oxidation

Toluene-o-xylene monooxygenase (ToMO) from Pseudomonas stutzeri OX1 has been shown to degrade all chlorinated ethenes individually and as mixtures. Here, DNA shuffling of the alpha hydroxylase fragment of ToMO (TouA) and saturation mutagenesis of the TouA active site residues I100, Q141, T201, F205, and E214 were used to enhance the degradation of chlorinated aliphatics. The ToMO mutants were identified using a chloride ion screen and then were further examined by gas chromatography. Escherichia coli TG1/pBS(Kan)ToMO expressing TouA saturation mutagenesis variant I100Q was identified that has 2.8-fold better trichloroethylene (TCE) degradation activity (apparent Vmax of 1.77 nmol min-1 mg-1 protein-1 vs 0.63 nmol min-1 mg-1 protein-1). Another variant, E214G/D312N/M399V, has 2.5-fold better cis-1,2-dichloroethylene (cis-DCE) degradation activity (apparent Vmax of 8.4 nmol min-1 mg-1 protein-1 vs 3.3 nmol min-1 mg-1 protein-1). Additionally, the hydroxylation regiospecificity of o-xylene and naphthalene were altered significantly for ToMO variants A107T/E214A, T201G, and T201S. Variant T201S produced 2.0-fold more 2,3-dimethylphenol (2,3-DMP) from o-xylene than the wild-type ToMO, whereas variant A107T/E214A had 6.0-fold altered regiospecificity for 2,3-DMP formation. Variant A107T/E214A also produced 3.0-fold more 2-naphthol from naphthalene than the wild-type ToMO, whereas the regiospecificity of variant T201S was altered to synthesize 3.0-fold less 2-naphthol, so that it made almost exclusively 1-naphthol (96%). Variant T201G was more regiospecific than variants A107T/E214A and T201S and produced 100% 3,4-DMP from o-xylene and >99% 1-naphthol from naphthalene. Hence, ToMO activity was enhanced for the degradation of TCE and cis-DCE and for the regiospecific hydroxylation of o-xylene and naphthalene through DNA shuffling and saturation mutagenesis.

Regiospecific oxidation of naphthalene and fluorene by toluene monooxygenases and engineered toluene 4-monooxygenases of Pseudomonas mendocina KR1

The regiospecific oxidation of the polycyclic aromatic hydrocarbons naphthalene and fluorene was examined with Escherichia coli strains expressing wildtype toluene 4-monooxygenase (T4MO) from Pseudomonas mendocina KR1, toluene para-monooxygenase (TpMO) from Ralstonia pickettii PKO1, toluene ortho-monooxygenase (TOM) from Burkholderia cepacia G4, and toluene/ortho-xylene monooxygenase (ToMO) from P. stutzeri OX1. T4MO oxidized toluene (12.1+/-0.8 nmol/min/mg protein at 109 microM), naphthalene (7.7+/-1.5 nmol/min/mg protein at 5 mM), and fluorene (0.68+/-0.04 nmol/min/mg protein at 0.2 mM) faster than the other wildtype enzymes (2-22-fold) and produced a mixture of 1-naphthol (52%) and 2-naphthol (48%) from naphthalene, which was successively transformed to a mixture of 2,3-, 2,7-, 1,7-, and 2,6-dihydroxynaphthalenes (7%, 10%, 20%, and 63%, respectively). TOM and ToMO made 1,7-dihydroxynaphthalene from 1-naphthol, and ToMO made a mixture of 2,3-, 2,6-, 2,7-, and 1,7-dihydroxynaphthalene (26%, 22%, 1%, and 44%, respectively) from 2-naphthol. TOM had no activity on 2-naphthol, and T4MO had no activity on 1-naphthol. To take advantage of the high activity of wildtype T4MO but to increase its regiospecificity on naphthalene, seven engineered enzymes containing mutations in T4MO alpha hydroxylase TmoA were examined; the selectivity for 2-naphthol by T4MO I100A, I100S, and I100G was enhanced to 88-95%, and the selectivity for 1-naphthol was enhanced to 87% and 99% by T4MO I100L and G103S/A107G, respectively, while high oxidation rates were maintained except for G103S/A107G. Therefore, the regiospecificity for naphthalene oxidation was altered to practically pure 1-naphthol or 2-naphthol. All four wildtype monooxygenases were able to oxidize fluorene to different monohydroxylated products; T4MO oxidized fluorene successively to 3-hydroxyfluorene and 3,6-dihydroxyfluorene, which was confirmed by gas chromatography-mass spectrometry and 1H nuclear magnetic resonance analysis. TOM and its variant TomA3 V106A oxidize fluorene to a mixture of 1-, 2-, 3-, and 4-hydroxyfluorene. This is the first report of using enzymes to synthesize 1-, 3-, and 4-hydroxyfluorene, and 3,6-dihydroxyfluorene from fluorene as well as 2-naphthol and 2,6-dihydroxynaphthalene from naphthalene.

Alpha-subunit positions methionine 180 and glutamate 214 of Pseudomonas stutzeri OX1 toluene-o-xylene monooxygenase influence catalysis

Alpha-subunit position M180 of toluene-o-xylene monooxygenase influences the regiospecific oxidation of aromatics (e.g., from o-cresol, M180H forms 3-methylcatechol, methylhydroquinone, and 4-methylresorcinol, whereas the wild type forms only 3-methylcatechol). Position E214 influences the rate of reaction (e.g., E214G increases p-nitrophenol oxidation 15-fold) by controlling substrate entrance and product efflux as a gate residue.

Controlling the regiospecific oxidation of aromatics via active site engineering of toluene para-monooxygenase of Ralstonia pickettii PKO1

A primary goal of protein engineering is to control catalytic activity. Here we show that through mutagenesis of three active site residues, the catalytic activity of a multicomponent monooxygenase is altered so that it hydroxylates all three positions of toluene as well as both positions of naphthalene. Hence, for the first time, an enzyme has been engineered so that its regiospecific oxidation of a substrate can be controlled. Through the A107G mutation in the alpha-subunit of toluene para-monooxygenase, a variant was formed that hydroxylated toluene primarily at the ortho-position while converting naphthalene to 1-naphthol. Conversely, the A107T variant produced >98% p-cresol and p-nitrophenol from toluene and nitrobenzene, respectively, as well as produced 2-naphthol from naphthalene. The mutation I100S/G103S produced a toluene para-monooxygenase variant that formed 75% m-cresol from toluene and 100% m-nitrophenol from nitrobenzene; thus, for the first time a true meta-hydroxylating toluene monooxygenase was created.