Rational design of halohydrin dehalogenase for efficient chiral epichlorohydrin production with high activity and enantioselectivity in aqueous-organic two-phase system

Engineering of Conserved Sequence Motif 1 Residues in Halohydrin Dehalogenase HheC Simultaneously Enhances Activity, Stability, and Enantioselectivity

Halohydrin dehalogenases (HHDHs) are powerful enzymes for the asymmetric diversification of oxyfunctionalized synthons. They feature two characteristic sequence motifs that distinguish them from homologous short-chain dehydrogenases and reductases. Sequence motif 1, carrying a conserved threonine, glycine, and a central aromatic residue, lines the nucleophile binding pocket of HHDHs. It could therefore impact nucleophile binding and presumably also the activity of the enzymes. However, experimental evidence supporting this theory is largely missing. Herein, we systematically studied the mutability of the three conserved motif 1 residues as well as their resulting impact on enzyme activity, stability, and selectivity in two model HHDHs: HheC from Agrobacterium radiobacter AD1 and HheG from Ilumatobacter coccineus. In both HheC and HheG, the conserved threonine and glycine tolerated mutations to only structurally similar amino acids. In contrast, the central aromatic (i.e., phenylalanine or tyrosine) residue of motif 1 demonstrated much higher variability in HheC. Remarkably, some of these variants featured drastically altered activity, stability, and selectivity characteristics. For instance, variant HheC F12Y displayed up to 5-fold increased specific activity in various epoxide ring opening and dehalogenation reactions as well as enhanced enantioselectivity (e.g., an E-value of 74 in the azidolysis of epichlorohydrin compared to E = 22 for HheC wild type) while also exhibiting a 10 K higher apparent melting temperature. QM and MD simulations support the experimentally observed activity increase of HheC F12Y and reveal alterations in the hydrogen bonding network within the active site. As such, our results demonstrate that multiple enzyme properties of HHDHs can be altered through the targeted mutagenesis of conserved motif 1 residues. In addition, this work illustrates that motif 1 plays vital roles beyond nucleophile binding by impacting the solubility and stability properties. These insights advance our understanding of HHDH active sites and will facilitate their future engineering.

A Dynamic Loop in Halohydrin Dehalogenase HheG Regulates Activity and Enantioselectivity in Epoxide Ring Opening

Halohydrin dehalogenase HheG and its homologues are remarkable enzymes for the efficient ring opening of sterically demanding internal epoxides using a variety of nucleophiles. The enantioselectivity of the respective wild-type enzymes, however, is usually insufficient for application and frequently requires improvement by protein engineering. We herein demonstrate that the highly flexible N-terminal loop of HheG, comprising residues 39 to 47, has a tremendous impact on the activity as well as enantioselectivity of this enzyme in the ring opening of structurally diverse epoxide substrates. Thus, highly active and enantioselective HheG variants could be accessed through targeted engineering of this loop. In this regard, variant M45F displayed almost 10-fold higher specific activity than wild type in the azidolysis of cyclohexene oxide, yielding the corresponding product (1S,2S)-2-azidocyclohexan-1-ol in 96%eeP (in comparison to 49%eeP for HheG wild type). Moreover, this variant was also improved regarding activity and enantioselectivity in the ring opening of cyclohexene oxide with other nucleophiles, demonstrating even inverted enantioselectivity with cyanide and cyanate. In contrast, a complete loop deletion yielded an inactive enzyme. Concomitant computational analyses of HheG M45F in comparison to wild type enzyme revealed that mutation M45F promotes the productive binding of cyclohexene oxide and azide in the active site by establishing noncovalent C-H ··π interactions between epoxide and F45. These interactions further position one of the two carbon atoms of the epoxide ring closer to the azide, resulting in higher enantioselectivity. Additionally, stable and enantioselective cross-linked enzyme crystals of HheG M45F were successfully generated after combination with mutation D114C. Overall, our study highlights that a highly flexible loop in HheG governs the enzyme's activity and selectivity in epoxide ring opening and should thus be considered in future protein engineering campaigns of HheG.

Preparation of (S)-epichlorohydrin using a novel halohydrin dehalogenase by selective conformation adjustment

Chiral epichlorohydrin (ECH) is an attractive intermediate for chiral pharmaceuticals and chemicals preparation. The asymmetric synthesis of chiral ECH using 1,3-dicholoro-2-propanol (1,3-DCP) catalyzed by a haloalcohol dehalogenase (HHDH) was considered as a feasible approach. However, the reverse ring opening reaction caused low optical purity of chiral ECH, thus severely restricts the industrial application of HHDHs. In the present study, a novel selective conformation adjustment strategy was developed with an engineered HheCPS to regulate the kinetic parameters of the forward and reverse reactions, based on site saturation mutation and molecular simulation analysis. The HheCPS mutant E85P was constructed with a markable change in the conformation of (S)-ECH in the substrate pocket and a slight impact on the interaction between 1,3-DCP and the enzyme, which resulted in the kinetic deceleration of the reverse reactions. Compared with HheCPS, the catalytic efficiency (kcat(S)-ECH/Km(S)-ECH) of the reversed reaction dropped to 0.23-fold (from 0.13 to 0.03 mM-1 s-1), while the catalytic efficiency (kcat(1,3-DCP)/Km(1,3-DCP)) of the forward reaction only reduced from 0.83 to 0.71 mM-1 s-1. With 40 mM 1,3-DCP as substrate, HheCPS E85P catalyzed the synthesis of (S)-ECH with the yield up to 55.35% and the e.e. increased from 92.54 to >99%. Our work provided an effective approach for understanding the stereoselective catalytic mechanism as well as the green manufacturing of chiral epoxides.

Enzymatic synthesis of 2-phenethyl acetate in water catalyzed by an immobilized acyltransferase from Mycobacterium smegmatis

Although water is an ideal green solvent for organic synthesis, it is difficult for most biocatalysts to carry out transesterification reactions in water because of the reversible hydrolysis reaction. 3D structural characteristics and the microenvironment of an enzyme has an important effect on its selectivity for the transesterification reaction over the hydrolysis reaction. A novel 2-phenethyl acetate synthesis technology was developed using acyltransferase (EC 3.1.1.2) from Mycobacterium smegmatis (MsAcT) in water. Firstly, MsAcT was entrapped in a tetramethoxysilane gel network and the immobilization process of MsAcT increased its selectivity for the transesterification reaction over the hydrolysis reaction by 6.33-fold. Then, the synthesis technology of 2-phenethyl acetate using the immobilized MsAcT in water was optimized as follows: vinyl acetate was used as acyl donor, the molar ratio of vinyl acetate to 2-phenylethyl alcohol was 2 : 1, and the water content was 80% (w/w). The reaction was carried out at 40 °C for 30 min and conversion rate reached 99.17%. The immobilized MsAcT could be recycled for 10 batches. The synthesis method of 2-phenethyl acetate using MsAcT as a biocatalyst in water is a prospective green process technology.

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.