Biocatalytic Asymmetric Cyclopropanations via Enzyme‐Bound Iminium Ion Intermediates

Expanding the Promiscuity of a Copper-Dependent Oxidase for Enantioselective Cross-Coupling of Indoles

Herein, we disclose the highly enantioselective oxidative cross-coupling of 3-hydroxyindole esters with various nucleophilic partners as catalyzed by copper efflux oxidase. The biocatalytic transformation delivers functionalized 2,2-disubstituted indolin-3-ones with excellent optical purity (90-99 % ee), which exhibited anticancer activity against MCF-7 cell lines, as shown by preliminary biological evaluation. Mechanistic studies and molecular docking results suggest the formation of a phenoxyl radical and enantiocontrol facilitated by a suited enzyme chiral pocket. This study is significant with regard to expanding the catalytic repertoire of natural multicopper oxidases as well as enlarging the synthetic toolbox for sustainable asymmetric oxidative coupling.

Enhancing the Peroxygenase Activity of a Cofactor-Independent Peroxyzyme by Directed Evolution Enabling Gram-Scale Epoxide Synthesis

Peroxygenases selectively incorporate oxygen into organic molecules making use of the environmentally friendly oxidant H₂ O₂ with water being the sole by-product. These biocatalysts can provide 'green' routes for the synthesis of enantioenriched epoxides, which are fundamental intermediates in the production of pharmaceuticals. The peroxyzyme 4-oxalocrotonate tautomerase (4-OT), catalysing the epoxidation of a variety of α,β-unsaturated aldehydes with H₂ O₂ , is outstanding because of its independence from any cost-intensive cofactor. However, its low-level peroxygenase activity and the decrease in the enantiomeric excess of the corresponding α,β-epoxy-aldehydes under preparative-scale conditions is limiting the potential of 4-OT. Herein we report the directed evolution of a tandem-fused 4-OT variant, which showed an ∼150-fold enhanced peroxygenase activity compared to 4-OT wild type, enabling the synthesis of α,β-epoxy-aldehydes in milligram- and gram-scale with high enantiopurity (up to 98 % ee) and excellent conversions. This engineered cofactor-independent peroxyzyme can provide new opportunities for the eco-friendly and practical synthesis of enantioenriched epoxides at large scale.

Unlocking New Reactivities in Enzymes by Iminium Catalysis

The application of biocatalysis in conquering challenging synthesis requires the constant input of new enzymes. Developing novel biocatalysts by absorbing catalysis modes from synthetic chemistry has yielded fruitful new-to-nature enzymes. Organocatalysis was originally bio-inspired and has become the third pillar of asymmetric catalysis. Transferring organocatalytic reactions back to enzyme platforms is a promising approach for biocatalyst creation. Herein, we summarize recent developments in the design of novel biocatalysts that adopt iminium catalysis, a fundamental branch in organocatalysis. By repurposing existing enzymes or constructing artificial enzymes, various biocatalysts for iminium catalysis have been created and optimized via protein engineering to promote valuable abiological transformations. Recent advances in iminium biocatalysis illustrate the power of combining chemomimetic biocatalyst design and directed evolution to generate useful new-to-nature enzymes.

Gene Fusion and Directed Evolution to Break Structural Symmetry and Boost Catalysis by an Oligomeric C-C Bond-Forming Enzyme

Gene duplication and fusion are among the primary natural processes that generate new proteins from simpler ancestors. Here we adopted this strategy to evolve a promiscuous homohexameric 4-oxalocrotonate tautomerase (4-OT) into an efficient biocatalyst for enantioselective Michael reactions. We first designed a tandem-fused 4-OT to allow independent sequence diversification of adjacent subunits by directed evolution. This fused 4-OT was then subjected to eleven rounds of directed evolution to give variant 4-OT(F11), which showed an up to 320-fold enhanced activity for the Michael addition of nitromethane to cinnamaldehydes. Crystallographic analysis revealed that 4-OT(F11) has an unusual asymmetric trimeric architecture in which one of the monomers is flipped 180° relative to the others. This gene duplication and fusion strategy to break structural symmetry is likely to become an indispensable asset of the enzyme engineering toolbox, finding wide use in engineering oligomeric proteins.