Mechanistic Insights into the Ene-Reductase-Catalyzed Promiscuous Reduction of Oximes to Amines

Redox Out of the Box: Catalytic Versatility Across NAD(P)H-Dependent Oxidoreductases

The asymmetric reduction of double bonds using NAD(P)H-dependent oxidoreductases has proven to be an efficient tool for the synthesis of important chiral molecules in research and on industrial scale. These enzymes are commercially available in screening kits for the reduction of C=O (ketones), C=C (activated alkenes), or C=N bonds (imines). Recent reports, however, indicate that the ability to accommodate multiple reductase activities on distinct C=X bonds occurs in different enzyme classes, either natively or after mutagenesis. This challenges the common perception of highly selective oxidoreductases for one type of electrophilic substrate. Consideration of this underexplored potential in enzyme screenings and protein engineering campaigns may contribute to the identification of complementary biocatalytic processes for the synthesis of chiral compounds. This review will contribute to a global understanding of the promiscuous behavior of NAD(P)H-dependent oxidoreductases on C=X bond reduction and inspire future discoveries with respect to unconventional biocatalytic routes in asymmetric synthesis.

Deciphering the Unconventional Reduction of C=N Bonds by Old Yellow Enzymes Using QM/MM

The reduction of C=X (X = N, O) bonds is a cornerstone in both synthetic organic chemistry and biocatalysis. Conventional reduction mechanisms usually involve a hydride ion targeting the less electronegative carbon atom. In a departure from this paradigm, our investigation into Old Yellow Enzymes (OYEs) reveals a mechanism involving transfer of hydride to the formally more electronegative nitrogen atom within a C=N bond. Beyond their known ability to reduce electronically activated C=C double bonds, e.g., in α, β-unsaturated ketones, these enzymes have recently been shown to reduce α-oximo-β-ketoesters to the corresponding amines. It has been proposed that this transformation involves two successive reduction steps and proceeds via imine intermediates formed by the reductive dehydration of the oxime moieties. We employ advanced quantum mechanics/molecular mechanics (QM/MM) simulations, enriched by a two-tiered approach incorporating QM/MM (UB3LYP-6-31G*/OPLS2005) geometry optimization, QM/MM (B3LYP-6-31G*/amberff19sb) steered molecular dynamics simulations, and detailed natural-bond-orbital analyses to decipher the unconventional hydride transfer to nitrogen in both reduction steps and to delineate the role of active site residues as well as of substituents present in the substrates. Our computational results confirm the proposed mechanism and agree well with experimental mutagenesis and enzyme kinetics data. According to our model, the catalysis of OYE involves hydride transfer from the flavin cofactor to the nitrogen atom in oximoketoesters as well as iminoketoesters followed by protonation at the adjacent oxygen or carbon atoms by conserved tyrosine residues and active site water molecules. Two histidine residues play a key role in the polarization and activation of the C=N bond, and conformational changes of the substrate observed along the reaction coordinate underline the crucial importance of dynamic electron delocalization for efficient catalysis.

Preparation of polyhydroxyalkanoate-based magnetic microspheres for carbonyl reductase purification and immobilization

A polyhydroxyalkanoate (PHA) magnetic microsphere was designed for one-step purification and immobilization of a novel carbonyl reductase (RLSR5) from recombinant Escherichia coli lysate. The hydrophobic core of this microsphere was composed of a highly biocompatible polymer, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), in which magnetic Fe3O4 particles were embedded during solvent evaporation. The hydrophilic shell of the fusion protein formed by PHA particle-binding protein (PhaP) and RLSR5 (PR) was expressed in recombinant E. coli. The magnetic core of Fe3O4@PHBHHx directly purified the hydrophilic shell from the E. coli lysate, and the two self-assembled to form Fe3O4@PHBHHx-PR through hydrophobic and hydrophilic interactions, eliminating the separation of the fusion protein. The microstructure, magnetic properties, morphology, size, and dispersion of Fe3O4@PHBHHx-PR were investigated by XRD, VSM, SEM, TEM, elemental mapping and DLS. It was found that Fe3O4@PHBHHx-PR correctly assembled, with a well dispersed spherical structure at the nanoscale and superparamagnetism properties. The amount of RLSR5 immobilized on PHA microspheres reached 121.9 mg/g. The Fe3O4@PHBHHx-PR was employed to synthesize (R)-tolvaptan with 99 % enantiomeric excess and 97 % bioconversion efficiency, and the catalyst maintained 78.6 % activity after 10 recovery cycles. These PHA magnetic microspheres are versatile carriers for enzyme immobilization and demonstrate improved stability and reusability of the free enzyme.