Enantioselective hydrolysis of styrene oxide with the epoxide hydrolase of Sphingomonas sp. HXN-200

An In Vitro Hybrid Biocatalytic System Enabled by a Combination of Surface-Displayed, Purified, and Cell-Free Expressed Enzymes

Enzymatic cascades have become a green and sustainable approach for the synthesis of valuable chemicals and pharmaceuticals. Using sequential enzymes to construct a multienzyme complex is an effective way to enhance the overall performance of biosynthetic routes. Here we report the design of an efficient in vitro hybrid biocatalytic system by assembling three enzymes that can convert styrene to (S)-1-phenyl-1,2-ethanediol. Specifically, we prepared the three enzymes in different ways, which were cell surface-displayed, purified, and cell-free expressed. To assemble them, we fused two orthogonal peptide-protein pairs (i.e., SpyTag/SpyCatcher and SnoopTag/SnoopCatcher) to the three enzymes, allowing their spatial organization by covalent assembly. By doing this, we constructed a multienzyme complex, which could enhance the production of (S)-1-phenyl-1,2-ethanediol by 3 times compared to the free-floating enzyme system without assembly. After optimization of the reaction system, the final product yield reached 234.6 μM with a substrate conversion rate of 46.9% (based on 0.5 mM styrene). Taken together, our strategy integrates the merits of advanced biochemical engineering techniques, including cellular surface display, spatial enzyme organization, and cell-free expression, which offers a new solution for chemical biosynthesis by enzymatic cascade biotransformation. We, therefore, anticipate that our approach will hold great potential for designing and constructing highly efficient systems to synthesize chemicals of agricultural, industrial, and pharmaceutical significance.

Comparison and application of biofilter and suspended bioreactor in removing gaseous o-xylene

Two bioreactors, suspended-growth bioreactors (SPB) and biofilter (BF), were compared for the performances in removing gaseous o-xylene. Their efficiencies were investigated by varying the o-xylene loadings, gas flow rates, and gas-water ratios. High-throughput techniques were applied for the microbial populations assay. The conversion rate of carbon in o-xylene was calculated, and the relationship between biomass and removal efficiencies was also analyzed. Results indicated that both the SPB and BF could effectively treat gases containing o-xylene. The average removal efficiencies were 91.8% and 93.5%, respectively. The elimination capacity of the BF was much higher than that of the SPB when the intake load was below 150 g m^-3 h^-1. When the o-xylene loadings were over 150 g m^-3 h^-1, both the SPB and BF achieved similar o-xylene removal rates. The maximum elimination capacities were 28.36 g m^-3 h^-1 for the SPB and 30.67 g m^-3 h^-1 for BF. The SPB was more sensitive to the changes in the gas flow rate. Results of microbial assay indicated that bacteria e.g. Mycobacterium sp. and Rhodanobacter sp. might play important roles in removing o-xylene in the SPB, while the bacteria Pseudomonas sp., Sphingomonas sp., and Defluviicoccus sp., and the fungi Aspergillus sp. and Scedosporium sp., were the o-xylene degraders in the BF. The successful application of the integrated bioreactor in treating gases containing o-xylene exhausted from the electroplating plant indicated that the integration of SPB and BF could be an effective method for removing VOCs with Henry coefficient in the range of 0.01-1.

Production of (R)-styrene oxide by recombinant whole-cell biocatalyst in aqueous and biphasic system

The enantioselective resolution of racemic styrene oxide (rac-SO) to (R)-SO by whole cells of a recombinant Escherichia coli expressing epoxide hydrolase (EH) activity in aqueous and biphasic system were studied. Some parameters that may alter this bio-resolution, such as the concentration of recombinant cell, substrate and product were evaluated. The effect of the addition of different additives on the course of rac-SO biotransformation was also investigated. The results showed that the yield and the enantiomeric excess (ee) of (R)-SO were dependent on these variables. When the kinetic resolution was conducted with 350 mM of rac-SO, enantiopure (R)-SO with high (≥99%) ee was obtained with a yield of 38.2% yield at 12.2 h in the presence of 10% (v/v) Tween 80. An isooctane/aqueous system was developed to overcome the adverse factors in the aqueous phase, resulting in an improvement of yield from 38.2% to 42.9%. The results will provide a useful guidance for further application of this enzyme in the biocatalytic production of chiral synthons.

Asymmetric diboration of terminal alkenes with a rhodium catalyst and subsequent oxidation: enantioselective synthesis of optically active 1,2-diols

Pin it down: A highly enantioselective diboration of terminal alkenes with chiral 1 and bis(pinacolato)diboron (B2 pin2 ) was realized. Subsequent oxidation of the diboron adducts with sodium peroxoborate readily gave the corresponding optically active 1,2-diols in high yields and high enantioselectivities.

Isolation and characterization of styrene metabolism genes from styrene-assimilating soil bacteria Rhodococcus sp. ST-5 and ST-10

Styrene metabolism genes were isolated from styrene-assimilating bacteria Rhodococcus sp. ST-5 and ST-10. Strain ST-5 had a gene cluster containing four open reading frames which encoded styrene degradation enzymes. The genes showed high similarity to styABCD of Pseudomonas sp. Y2. On the other hand, strain ST-10 had only two genes which encoded styrene monooxygenase and flavin oxidoreductase (styAB). Escherichia coli transformants possessing the sty genes of strains ST-5 and ST-10 produced (S)-styrene oxide from styrene, indicating that these genes function as styrene degradation enzymes. Metabolite analysis by resting-cell reaction with gas chromatography-mass spectrometry revealed that strain ST-5 converts styrene to phenylacetaldehyde via styrene oxide by styrene oxide isomerase (styC) reaction. On the other hand, strain ST-10 lacked this enzyme, and thus accumulated styrene oxide as an intermediate. HPLC analysis showed that styrene oxide was spontaneously isomerized to phenylacetaldehyde by chemical reaction. The produced phenylacetaldehyde was converted to phenylacetic acid (PAA) in strain ST-10 as well as in strain ST-5. Furthermore, phenylacetic acid was converted to phenylacetyl-CoA by the catalysis of phenylacetate-CoA ligase in strains ST-5 and ST-10. This study proposes possible styrene metabolism pathways in Rhodococcus sp. strains ST-5 and ST-10.

Biochemical characterization and transcriptional analysis of the epoxide hydrolase from white-rot fungus Phanerochaete chrysosporium

The white-rot basidiomycetes Phanerochaete chrysosporium is a model fungus used to investigate the secondary metabolism and lignin degradation. Genomic sequencing reveals the presence of at least 18 genes encoding putative epoxide hydrolases (EHs). One cDNA encoding EH (designated as PchEHA) was cloned and expressed in Escherichia coli. Transcriptional analysis demonstrated that the transcripts of PchEHA could be detected under the ligninolytic and nonligninolytic conditions as well as amended with anthracene. The recombinant enzyme exhibits broad hydrolytic activity toward several racemic epoxides including styrene oxide, epichlorohydrin, and 1,2-epoxybutane, but with different specificity. Using racemic styrene oxide as the substrate, the optimal pH and temperature are pH 9.0 and 40 degrees C, respectively. The enzyme is not sensitive to EDTA, and is inhibited by H2O2, and several metal ions including Zn(2+), Cd(2+), and Hg(2+) at various extents. Several organic cosolvents including acetone, dimethylsulfoxide, formamide, glycerol and ethanol at 10% (v/v) cause slight or no inhibition of the hydrolytic reaction. More importantly, the recombinant enzyme displays distinct enantioselective preference to several chiral epoxides. The enzyme showed good enantioselectivity toward chiral styrene oxide with preferential hydrolysis of (R)-enantiomer. PchEHA is likely a novel soluble EH based on the sequence analysis and catalytic properties, and is a great potential biocatalyst for the preparation of enantiopure styrene oxide in racemic kinetic resolution.

Cloning, expression and enantioselective hydrolytic catalysis of a microsomal epoxide hydrolase from a marine fish, Mugil cephalus

The cDNA of a marine fish microsomal epoxide hydrolase (mEH) gene from Mugil cephalus was cloned by rapid amplification of cDNA ends (RACE) techniques. The homology model for the mEH of M. cephalus showed a characteristic structure of alpha/beta-hydrolase-fold main domain with a lid domain over the active site. The characteristic catalytic triad, consisting of Asp(238), His(444), and Glu(417), was highly conserved. The cloned mEH gene was expressed in Escherichia coli and the recombinant mEH exhibited (R)-preferred hydrolysis activity toward racemic styrene oxide. We obtained enantiopure (S)-styrene oxide with a high enantiopurity of more than 99% enantiomeric excess and yield of 15.4% by batch kinetic resolution of 20 mM racemic styrene oxide.