Fasan, R.

Curr. Opin. Green Sustain. Chem. 2021, 31, 100494, 10.1016/j.cogsc.2021.100494

The direct functionalization of C–H bonds constitutes a powerful strategy to construct and diversify organic molecules. However, controlling the chemo- and site-selectivity of this transformation, particularly in complex molecular settings, represents a significant challenge. Metalloenzymes are ideal platforms for achieving catalyst-controlled selective C–H bond functionalization as their reactivities can be tuned by protein engineering and/or redesign of their cofactor environment. In this review, we highlight recent progress in the development of engineered and artificial metalloenzymes for C–H functionalization, with a focus on biocatalytic strategies for selective C–H oxyfunctionalization and halogenation as well as C–H amination and C–H carbene insertion via abiological nitrene and carbene transfer chemistries. Engineered heme and nonheme iron dependent enzymes have emerged as promising scaffolds for executing these transformations with high chemo-, regio-, and stereocontrol as well as tunable selectivity. These emerging systems and methodologies have expanded the toolbox of sustainable strategies for organic synthesis and created new opportunities for the generation of chiral building blocks, the late-stage C–H functionalization of complex molecules, and the total synthesis of natural products.

Fasan, R.; Khare, S.D.

Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 12472-12477, 10.1073/pnas.1708907114

Thermostabilization represents a critical and often obligatory step toward enhancing the robustness of enzymes for organic synthesis and other applications. While directed evolution methods have provided valuable tools for this purpose, these protocols are laborious and time-consuming and typically require the accumulation of several mutations, potentially at the expense of catalytic function. Here, we report a minimally invasive strategy for enzyme stabilization that relies on the installation of genetically encoded, nonreducible covalent staples in a target protein scaffold using computational design. This methodology enables the rapid development of myoglobin-based cyclopropanation biocatalysts featuring dramatically enhanced thermostability (ΔTm = +18.0 °C and ΔT50 = +16.0 °C) as well as increased stability against chemical denaturation [ΔCm (GndHCl) = 0.53 M], without altering their catalytic efficiency and stereoselectivity properties. In addition, the stabilized variants offer superior performance and selectivity compared with the parent enzyme in the presence of a high concentration of organic cosolvents, enabling the more efficient cyclopropanation of a water-insoluble substrate. This work introduces and validates an approach for protein stabilization which should be applicable to a variety of other proteins and enzymes.

Arnold, F.H.; Fasan, R.

Curr. Opin. Biotechnol. 2017, 47, 102-111, 10.1016/j.copbio.2017.06.005

The surge in reports of heme-dependent proteins as catalysts for abiotic, synthetically valuable carbene and nitrene transfer reactions dramatically illustrates the evolvability of the protein world and our nascent ability to exploit that for new enzyme chemistry. We highlight the latest additions to the hemoprotein-catalyzed reaction repertoire (including carbene Si–H and C–H insertions, Doyle–Kirmse reactions, aldehyde olefinations, azide-to-aldehyde conversions, and intermolecular nitrene C–H insertion) and show how different hemoprotein scaffolds offer varied reactivity and selectivity. Preparative-scale syntheses of pharmaceutically relevant compounds accomplished with these new catalysts are beginning to demonstrate their biotechnological relevance. Insights into the determinants of enzyme lifetime and product yield are providing generalizable cues for engineering heme-dependent proteins to further broaden the scope and utility of these non-natural activities.

Fasan, R.

Bioorg. Med. Chem. 2014, 22, 5697-5704, 10.1016/j.bmc.2014.05.015

The direct conversion of aliphatic CH bonds into CN bonds provides an attractive approach to the introduction of nitrogen-containing functionalities in organic molecules. Following the recent discovery that cytochrome P450 enzymes can catalyze the cyclization of arylsulfonyl azide compounds via an intramolecular C(sp3)H amination reaction, we have explored here the CH amination reactivity of other hemoproteins. Various heme-containing proteins, and in particular myoglobin and horseradish peroxidase, were found to be capable of catalyzing this transformation. Based on this finding, a series of engineered and artificial myoglobin variants containing active site mutations and non-native Mn- and Co-protoporphyrin IX cofactors, respectively, were prepared to investigate the effect of these structural changes on the catalytic activity and selectivity of these catalysts. Our studies showed that metallo-substituted myoglobins constitute viable CH amination catalysts, revealing a distinctive reactivity trend as compared to synthetic metalloporphyrin counterparts. On the other hand, amino acid substitutions at the level of the heme pocket were found to be beneficial toward improving the stereo- and enantioselectivity of these Mb-catalyzed reactions. Mechanistic studies involving kinetic isotope effect experiments indicate that CH bond cleavage is implicated in the rate-limiting step of myoglobin-catalyzed amination of arylsulfonyl azides. Altogether, these studies indicate that myoglobin constitutes a promising scaffold for the design and development of CH amination catalysts.