Efficient protein expression in a robust Escherichia coli strain and its application for kinetic resolution of racemic glycidyl o-methylphenyl ether in high concentration

NosZ gene cloning, reduction performance and structure of Pseudomonas citronellolis WXP-4 nitrous oxide reductase

Nitrous oxide reductase (N₂OR) is the only known enzyme that can reduce the powerful greenhouse gas nitrous oxide (N₂O) to harmless nitrogen at the final step of bacterial denitrification. To alleviate the N₂O emission, emerging approaches aim at microbiome biotechnology. In this study, the genome sequence of facultative anaerobic bacteria Pseudomonas citronellolis WXP-4, which efficiently degrades N₂O, was obtained by de novo sequencing for the first time, and then, four key reductase structure coding genes related to complete denitrification were identified. The single structural encoding gene nosZ with a length of 1914 bp from strain WXP-4 was cloned in Escherichia coli BL21(DE3), and the N₂OR protein (76 kDa) was relatively highly efficiently expressed under the optimal inducing conditions of 1.0 mM IPTG, 5 h, and 30 °C. Denitrification experiment results confirmed that recombinant E. coli had strong denitrification ability and reduced 10 mg L⁻¹ of N₂O to N₂ within 15 h under the optimal conditions of pH 7.0 and 40 °C, its corresponding N₂O reduction rate was almost 2.3 times that of Alcaligenes denitrificans strain TB, but only 80% of that of wild strain WXP-4, meaning that nos gene cluster auxiliary gene deletion decreased the activity of N₂OR. The 3D structure of N₂OR predicted on the basis of sequence homology found that electron transfer center CuA had only five amino acid ligands, and the S2 of the catalytically active center CuZ only bound one Cu_I atom. The unique 3D structure was different from previous reports and may be closely related to the strong N₂O reduction ability of strain WXP-4 and recombinant E. coli. The findings show a potential application of recombinant E. coli in alleviating the greenhouse effect and provide a new perspective for researching the relationship between structure and function of N₂OR.

Nitrous oxide reductase (N₂OR) is the only known enzyme that can reduce the powerful greenhouse gas nitrous oxide (N₂O) to harmless nitrogen at the final step of bacterial denitrification. The recombinant E. coli and wild strain WXP-4 demonstrate strong N₂O reduction ability.

Vibrio natriegens as a pET-Compatible Expression Host Complementary to Escherichia coli

Efficient and novel recombinant protein expression systems can further reduce the production cost of enzymes. Vibrio natriegens is the fastest growing free-living bacterium with a doubling time of less than 10 min, which makes it highly attractive as a protein expression host. Here, 196 pET plasmids with different genes of interest (GOIs) were electroporated into the V. natriegens strain VnDX, which carries an integrated T7 RNA polymerase expression cassette. As a result, 65 and 75% of the tested GOIs obtained soluble expression in V. natriegens and Escherichia coli, respectively, 20 GOIs of which showed better expression in the former. Furthermore, we have adapted a consensus "what to try first" protocol for V. natriegens based on Terrific Broth medium. Six sampled GOIs encoding biocatalysts enzymes thus achieved 50-128% higher catalytic efficiency under the optimized expression conditions. Our study demonstrated V. natriegens as a pET-compatible expression host with a spectrum of highly expressed GOIs distinct from E. coli and an easy-to-use consensus protocol, solving the problem that some GOIs cannot be expressed well in E. coli.

Improving the enantioselectivity of halohydrin dehalogenase for the synthesis of (R)-benzyl glycidyl ether via biocatalytic azidolysis

Halohydrin dehalogenases (HHDHs) are valuable biocatalysts for the synthesis of enantiopure benzyl glycidyl ether (BGE) and its derivatives, which are important synthetic intermediates for anti-cancer and anti-obesity drugs. However, all the reported HHDHs exhibit low enantioselectivity. In this study, we screened site-saturation mutagenesis libraries of AbHHDH at positions R89, A136, V137, P178, N179, F180, I181, Y186 and F187 for mutants with enhanced enantioselectivity toward BGE. The four improved variant R89V, R89Y, R89K and V137I were identified, and the double mutant R89Y/V137I showed 2.9-fold higher enantioselectivity than the wild type. The regions of HHDH containing the identified mutations were analyzed by homology modeling to explain the changes of enantioselectivity. Kinetic resolution of 20 to 100 mM BGE using whole cells of Escherichia coli expressing the mutant R89Y/V137I resulted in (R)-BGE yields of 42 to 32.5%, with ee >99%. This study improves our understanding of the enantioselectivity of HHDHs and contributes improved biocatalysts for the kinetic resolution of BGE.