C-H functionalization reactions catalyzed by artificial metalloenzymes

CH functionalization, a promising frontier in modern organic chemistry, facilitates the direct conversion of inert CH bonds into many valuable functional groups. Despite its merits, traditional homogeneous catalysis, often faces challenges in efficiency, selectivity, and sustainability towards this transformation. In this context, artificial metalloenzymes (ArMs), resulting from the incorporation of a catalytically-competent metal cofactor within an evolvable protein scaffold, bridges the gap between the efficiency of enzymatic transformations and the versatility of transition metal catalysis. Accordingly, ArMs have emerged as attractive tools for various challenging catalytic transformations. Additionally, the coming of age of directed evolution has unlocked unprecedented avenues for optimizing enzymatic catalysis. Taking advantage of their genetically-encoded protein scaffold, ArMs have been evolved to catalyze various CH functionalization reactions. This review delves into the recent developments of ArM-catalyzed CH functionalization reactions, highlighting the benefits of engineering the second coordination sphere around a metal cofactor within a host protein.

Biocatalytic strategy for the construction of sp3-rich polycyclic compounds from directed evolution and computational modelling

Catalysis with engineered enzymes has provided more efficient routes for the production of active pharmaceutical agents. However, the potential of biocatalysis to assist in early-stage drug discovery campaigns remains largely untapped. In this study, we have developed a biocatalytic strategy for the construction of sp3-rich polycyclic compounds via the intramolecular cyclopropanation of benzothiophenes and related heterocycles. Two carbene transferases with complementary regioisomer selectivity were evolved to catalyse the stereoselective cyclization of benzothiophene substrates bearing diazo ester groups at the C2 or C3 position of the heterocycle. The detailed mechanisms of these reactions were elucidated by a combination of crystallographic and computational analyses. Leveraging these insights, the substrate scope of one of the biocatalysts could be expanded to include previously unreactive substrates, highlighting the value of integrating evolutionary and rational strategies to develop enzymes for new-to-nature transformations. The molecular scaffolds accessed here feature a combination of three-dimensional and stereochemical complexity with 'rule-of-three' properties, which should make them highly valuable for fragment-based drug discovery campaigns.

Copper-Catalyzed Sulfimidation in Aqueous Media: a Fast, Chemoselective and Biomolecule-Compatible Reaction

Performing transition metal-catalyzed reactions in cells and living systems has equipped scientists with a toolbox to study biological processes and release drugs on demand. Thus far, an impressive scope of reactions has been performed in these settings, but many are yet to be introduced. Nitrene transfer presents a rather unexplored new-to-nature reaction. The reaction products are frequently encountered motifs in pharmaceuticals, presenting opportunities for the controlled, intracellular synthesis of drugs. Hence, we explored the transition metal-catalyzed sulfimidation reaction in water for future in vivo application. Two Cu(I) complexes containing trispyrazolylborate ligands (Tpx ) were selected, and the catalytic system was evaluated with the aid of three fitness factors. The excellent nitrene transfer reactivity and high chemoselectivity of the catalysts, coupled with good biomolecule compatibility, successfully enabled the sulfimidation of thioethers in aqueous media. We envision that this copper-catalyzed sulfimidation reaction could be an interesting starting point to unlock the potential of nitrene transfer catalysis in vivo.

Cross-Linked Crystals of Dirhodium Tetraacetate/RNase A Adduct Can Be Used as Heterogeneous Catalysts

Due to their unique coordination structure, dirhodium paddlewheel complexes are of interest in several research fields, like medicinal chemistry, catalysis, etc. Previously, these complexes were conjugated to proteins and peptides for developing artificial metalloenzymes as homogeneous catalysts. Fixation of dirhodium complexes into protein crystals is interesting to develop heterogeneous catalysts. Porous solvent channels present in protein crystals can benefit the activity by increasing the probability of substrate collisions at the catalytic Rh binding sites. Toward this goal, the present work describes the use of bovine pancreatic ribonuclease (RNase A) crystals with a pore size of 4 nm (P3₂21 space group) for fixing [Rh₂(OAc)₄] and developing a heterogeneous catalyst to perform reactions in an aqueous medium. The structure of the [Rh₂(OAc)₄]/RNase A adduct was investigated by X-ray crystallography: the metal complex structure remains unperturbed upon protein binding. Using a number of crystal structures, metal complex accumulation over time, within the RNase A crystals, and structures at variable temperatures were evaluated. We also report the large-scale preparation of microcrystals (∼10-20 μm) of the [Rh₂(OAc)₄]/RNase A adduct and cross-linking reaction with glutaraldehyde. The catalytic olefin cyclopropanation reaction and self-coupling of diazo compounds by these cross-linked [Rh₂(OAc)₄]/RNase A crystals were demonstrated. The results of this work reveal that these systems can be used as heterogeneous catalysts to promote reactions in aqueous solution. Overall, our findings demonstrate that the dirhodium paddlewheel complexes can be fixed in porous biomolecule crystals, like those of RNase A, to prepare biohybrid materials for catalytic applications.

Design of Artificial Enzymes: Insights into Protein Scaffolds

The design of artificial enzymes has emerged as a promising tool for the generation of potent biocatalysts able to promote new-to-nature reactions with improved catalytic performances, providing a powerful platform for wide-ranging applications and a better understanding of protein functions and structures. The selection of an appropriate protein scaffold plays a key role in the design process. This review aims to give a general overview of the most common protein scaffolds that can be exploited for the generation of artificial enzymes. Several examples are discussed and categorized according to the strategy used for the design of the artificial biocatalyst, namely the functionalization of natural enzymes, the creation of a new catalytic site in a protein scaffold bearing a wide hydrophobic pocket and de novo protein design. The review is concluded by a comparison of these different methods and by our perspective on the topic.

Dehaloperoxidase Catalyzed Stereoselective Synthesis of Cyclopropanol Esters

Chiral cyclopropanols are highly desirable building blocks for medicinal chemistry, but the stereoselective synthesis of these molecules remains challenging. Here, a novel strategy is reported for the diastereo- and enantioselective synthesis of cyclopropanol derivatives via the biocatalytic asymmetric cyclopropanation of vinyl esters with ethyl diazoacetate (EDA). A dehaloperoxidase enzyme from Amphitrite ornata was repurposed to catalyze this challenging cyclopropanation reaction, and its activity and stereoselectivity were optimized via protein engineering. Using this system, a broad range of electron-deficient vinyl esters were efficiently converted to the desired cyclopropanation products with up to 99.5:0.5 diastereomeric and enantiomeric ratios. In addition, the engineered dehaloperoxidase-based biocatalyst is able to catalyze a variety of other abiological carbene transfer reactions, including N-H/S-H carbene insertion with EDA as well as cyclopropanation with diazoacetonitrile, thus adding to the multifunctionality of this enzyme and defining it as a valuable new scaffold for the development of novel carbene transferases.

Organometallic catalysis in aqueous and biological environments: harnessing the power of metal carbenes

Translating the power of transition metal catalysis to the native habitats of enzymes can significantly expand the possibilities of interrogating or manipulating natural biological systems, including living cells and organisms. This is especially relevant for organometallic reactions that have shown great potential in the field of organic synthesis, like the metal-catalyzed transfer of carbenes. While, at first sight, performing metal carbene chemistry in aqueous solvents, and especially in biologically relevant mixtures, does not seem obvious, in recent years there has been a growing number of reports demonstrating the feasibility of the task. Either using small molecule metal catalysts or artificial metalloenzymes, a number of carbene transfer reactions that tolerate aqueous and biorelevant media are being developed. This review intends to summarize the most relevant contributions, and establish the state of the art in this emerging research field.

Properties of the Reactants and Their Interactions within and with the Enzyme Binding Cavity Determine Reaction Selectivities. The Case of Fe(II)/2-Oxoglutarate Dependent Enzymes

Fe(II)/2-oxoglutarate dependent dioxygenases (ODDs) share a double stranded beta helix (DSBH) fold and utilise a common reactive intermediate, ferryl species, to catalyse oxidative transformations of substrates. Despite the structural similarities, ODDs accept a variety of substrates and facilitate a wide range of reactions, that is hydroxylations, desaturations, (oxa)cyclisations and ring rearrangements. In this review we present and discuss the factors contributing to the observed (regio)selectivities of ODDs. They span from inherent properties of the reactants, that is, substrate molecule and iron cofactor, to the interactions between the substrate and the enzyme's binding cavity; the latter can counterbalance the effect of the former. Based on results of both experimental and computational studies dedicated to ODDs, we also line out the properties of the reactants which promote reaction outcomes other than the "default" hydroxylation. It turns out that the reaction selectivity depends on a delicate balance of interactions between the components of the investigated system.

Highly stereoselective and enantiodivergent synthesis of cyclopropylphosphonates with engineered carbene transferases

Organophosphonate compounds have represented a rich source of biologically active compounds, including enzyme inhibitors, antibiotics, and antimalarial agents. Here, we report the development of a highly stereoselective strategy for olefin cyclopropanation in the presence of a phosphonyl diazo reagent as carbene precursor. In combination with a ‘substrate walking’ protein engineering strategy, two sets of efficient and enantiodivergent myoglobin-based biocatalysts were developed for the synthesis of both (1R,2S) and (1S,2R) enantiomeric forms of the desired cyclopropylphosphonate ester products. This methodology enables the efficient transformation of a broad range of vinylarene substrates at a preparative scale (i.e. gram scale) with up to 99% de and ee. Mechanistic studies provide insights into factors that contribute to make this reaction inherently more challenging than hemoprotein-catalyzed olefin cyclopropanation with ethyl diazoacetate investigated previously. This work expands the range of synthetically useful, enzyme-catalyzed transformations and paves the way to the development of metalloprotein catalysts for abiological carbene transfer reactions involving non-canonical carbene donor reagents.

Two enantiocomplementary myoglobin-based carbene transfer biocatalysts were developed for the synthesis of cyclopropylphosphonate esters with high diastereo- and enantioselectivity and in high yields.

Tuning Through-Space Interactions via the Secondary Coordination Sphere of an Artificial Metalloenzyme Leads to Enhanced Rh(III)-Catalysis

We report computationally-guided protein engineering of monomeric streptavidin Rh(iii) artificial metalloenzyme to enhance catalysis of the enantioselective coupling of acrylamide hydroxamate esters and styrenes. Increased TON correlates with calculated distances between the Rh(iii) metal and surrounding residues, underscoring an artificial metalloenzyme's propensity for additional control in metal-catalyzed transformations by through-space interactions.

We report computationally-guided protein engineering of monomeric streptavidin Rh(iii) artificial metalloenzyme to enhance catalysis of the enantioselective coupling of acrylamide hydroxamate esters and styrenes.

Heteroleptic dirhodium(II,II) paddlewheel complexes as carbene transfer catalysts

This review highlights the applications of dirhodium(II,II) paddlewheel complexes with a heteroleptic scaffold. Dirhodium(II,II) paddlewheel complexes are well known as highly efficient and selective carbene transfer catalysts. While the majority of described complexes are homoleptic, comparatively fewer studies have concerned heteroleptic complexes. Here, we emphasise the use of heteroleptic complexes in order to highlight their benefits as carbene transfer catalysts and spur future research. Methods to synthesise heteroleptic dirhodium(II,II) paddlewheel complexes are discussed as well as a categorical review of their types of heteroleptic complexes and the carbene reactions in which they have been used.

Biocatalytic Strategy for the Highly Stereoselective Synthesis of CHF2 -Containing Trisubstituted Cyclopropanes

The difluoromethyl (CHF2 ) group has attracted significant attention in drug discovery and development efforts, owing to its ability to serve as fluorinated bioisostere of methyl, hydroxyl, and thiol groups. Herein, we report an efficient biocatalytic method for the highly diastereo- and enantioselective synthesis of CHF2 -containing trisubstituted cyclopropanes. Using engineered myoglobin catalysts, a broad range of α-difluoromethyl alkenes are cyclopropanated in the presence of ethyl diazoacetate to give CHF2 -containing cyclopropanes in high yield (up to >99 %, up to 3000 TON) and with excellent stereoselectivity (up to >99 % de and ee). Enantiodivergent selectivity and extension of the method to the stereoselective cyclopropanation of mono- and trifluoromethylated olefins was also achieved. This methodology represents a powerful strategy for the stereoselective synthesis of high-value fluorinated building blocks for medicinal chemistry, as exemplified by the formal total synthesis of a CHF2 isostere of a TRPV1 inhibitor.

Modular Design of G-Quadruplex MetalloDNAzymes for Catalytic C-C Bond Formations with Switchable Enantioselectivity

Metal-binding DNA structures with catalytic function are receiving increasing interest. Although a number of metalloDNAzymes have been reported to be highly efficient, the exact coordination/position of their catalytic metal center is often unknown. Here, we present a new approach to rationally develop metalloDNAzymes for Lewis acid-catalyzed reactions such as enantioselective Michael additions. Our strategy relies on the predictable folding patterns of unimolecular DNA G-quadruplexes, combined with the concept of metal-mediated base-pairing. Transition-metal coordination environments were created in G-quadruplex loop regions, accessible by substrates. Therefore, protein-inspired imidazole ligandoside L was covalently incorporated into a series of G-rich DNA strands by solid-phase synthesis. Iterative rounds of DNA sequence design and catalytic assays allowed us to select tailored metalloDNAzymes giving high conversions and excellent enantioselectivities (≥99%). Based on their primary sequence, folding pattern, and metal coordination mode, valuable information on structure-activity relationships could be extracted. Variation of the number and position of ligand L within the sequence allowed us to control the formation of (S) and (R) enantiomeric reaction products, respectively.

Construction of a whole-cell biohybrid catalyst using a Cp*Rh(III)-dithiophosphate complex as a precursor of a metal cofactor

A whole-cell biohybrid catalyst where a (pentamethylcyclopentadienyl)rhodium(III) (Cp*Rh(III)) complex was covalently incorporated into the cavity of nitrobindin (NB), a β-barrel protein, was prepared on an E. coli cell surface to produce isoquinolines via C(sp²)-H bond activation. In this whole-cell biohybrid system, the Cp*Rh(III)-dithiophosphate complex with latent catalytic activity was utilized as a precursor of the metal cofactor. Strong chelation of the dithiophosphate ligands protects the rhodium complex from being deactivated by abundant nucleophiles in cellular environments during conjugation of the cofactor with the protein scaffold. The whole-cell biohybrid catalyst was then activated upon addition of Ag⁺ ion to dissociate the dithiophosphate ligands and promoted cycloaddition of acetophenone oxime with diphenylacetylene. Furthermore, the activity of the Cp*Rh(III)-linked whole-cell biohybrid catalyst was enhanced 2.1-fold by introducing glutamate residues at positions adjacent to the Cp*Rh(III) cofactor. These results indicate that the use of the Cp*Rh(III)-dithiophosphate complex with switchable activity from a "latent" form to an "active" form provides a new strategy for generating whole-cell biohybrid catalysts.

Directed Evolution of a Cp*RhIII -Linked Biohybrid Catalyst Based on a Screening Platform with Affinity Purification

Directed evolution of Cp*Rh^III‐linked nitrobindin (NB), a biohybrid catalyst, was performed based on an in vitro screening approach. A key aspect of this effort was the establishment of a high‐throughput screening (HTS) platform that involves an affinity purification step employing a starch‐agarose resin for a maltose binding protein (MBP) tag. The HTS platform enables efficient preparation of the purified MBP‐tagged biohybrid catalysts in a 96‐well format and eliminates background influence of the host E. coli cells. Three rounds of directed evolution and screening of more than 4000 clones yielded a Cp*Rh^III‐linked NB(T98H/L100K/K127E) variant with a 4.9‐fold enhanced activity for the cycloaddition of acetophenone oximes with alkynes. It is confirmed that this HTS platform for directed evolution provides an efficient strategy for generating highly active biohybrid catalysts incorporating a synthetic metal cofactor.

Enhanced cis- and enantioselective cyclopropanation of styrene catalysed by cytochrome P450BM3 using decoy molecules

We report the enhanced cis- and enantioselective cyclopropanation of styrene catalysed by cytochrome P450BM3 in the presence of dummy substrates, i.e. decoy molecules. With the aid of the decoy molecule R-Ibu-Phe, diastereoselectivity for the cis diastereomers reached 91%, and the enantiomeric ratio for the (1S,2R) isomer reached 94%. Molecular dynamics simulations underpin the experimental data, revealing the mechanism of how enantioselectivity is controlled by the addition of decoy molecules.

Highly Stereoselective Synthesis of Fused Cyclopropane-γ-Lactams via Biocatalytic Iron-Catalyzed Intramolecular Cyclopropanation

We report the development of an iron-based biocatalytic strategy for the asymmetric synthesis of fused cyclopropane-γ-lactams, which are key structural motifs found in synthetic drugs and bioactive natural products. Using a combination of mutational landscape and iterative site-saturation mutagenesis, sperm whale myoglobin was evolved into a biocatalyst capable of promoting the cyclization of a diverse range of allyl diazoacetamide substrates into the corresponding bicyclic lactams in high yields and with high enantioselectivity (up to 99% ee). These biocatalytic transformations can be performed in whole cells and could be leveraged to enable the efficient (chemo)enzymatic construction of chiral cyclopropane-γ-lactams as well as β-cyclopropyl amines and cyclopropane-fused pyrrolidines, as valuable building blocks and synthons for medicinal chemistry and natural product synthesis.

Unlocking the therapeutic potential of artificial metalloenzymes

In order to harness the functionality of metals, nature has evolved over billions of years to utilize metalloproteins as key components in numerous cellular processes. Despite this, transition metals such as ruthenium, palladium, iridium, and gold are largely absent from naturally occurring metalloproteins, likely due to their scarcity as precious metals. To mimic the evolutionary process of nature, the field of artificial metalloenzymes (ArMs) was born as a way to benefit from the unique chemoselectivity and orthogonality of transition metals in a biological setting. In its current state, numerous examples have successfully incorporated transition metals into a variety of protein scaffolds. Using these ArMs, many examples of new-to-nature reactions have been carried out, some of which have shown substantial biocompatibility. Given the rapid rate at which this field is growing, this review aims to highlight some important studies that have begun to take the next step within this field; namely the development of ArM-centered drug therapies or biotechnological tools.

Tailored Transition-Metal Coordination Environments in Imidazole-Modified DNA G-Quadruplexes

Two types of imidazole ligands were introduced both at the end of tetramolecular and into the loop region of unimolecular DNA G‐quadruplexes. The modified oligonucleotides were shown to complex a range of different transition‐metal cations including Ni^II, Cu^II, Zn^II and Co^II, as indicated by UV/Vis absorption spectroscopy and ion mobility mass spectrometry. Molecular dynamics simulations were performed to obtain structural insight into the investigated systems. Variation of ligand number and position in the loop region of unimolecular sequences derived from the human telomer region (htel) allows for a controlled design of distinct coordination environments with fine‐tuned metal affinities. It is shown that Cu^II, which is typically square‐planar coordinated, has a higher affinity for systems offering four ligands, whereas Ni^II prefers G‐quadruplexes with six ligands. Likewise, the positioning of ligands in a square‐planar versus tetrahedral fashion affects binding affinities of Cu^II and Zn^II cations, respectively. Gaining control over ligand arrangement patterns will spur the rational development of transition‐metal‐modified DNAzymes. Furthermore, this method is suited to combine different types of ligands, for example, those typically found in metalloenzymes, inside a single DNA architecture.

Aminoquinoline-Rhodium(II) Conjugates as Src-Family SH3 Ligands

High-affinity, selective ligands are sought for a variety of biomolecules but are particularly difficult to generate in the protein-protein interaction space. Rhodium(II) conjugates provide a structure-based approach to improved affinity and specificity for targeting protein-protein interactions such as SH3 domains. In this study of small-molecule-rhodium conjugates, we report a potent ligand 4b (K _d of 27 nM) for the Lyn SH3 domain, based on an aminoquinoline fragment. The results demonstrate robust affinity gains possible from even modest small-molecule leads through cooperative inorganic-organic binding, based on specific histidine interactions. A docking study sheds light on the structural basis of binding and supports a previously proposed binding model.

Artificial Metalloenzymes Based on the Biotin-Streptavidin Technology: Enzymatic Cascades and Directed Evolution

Artificial metalloenzymes (ArMs) result from anchoring a metal-containing moiety within a macromolecular scaffold (protein or oligonucleotide). The resulting hybrid catalyst combines attractive features of both homogeneous catalysts and enzymes. This strategy includes the possibility of optimizing the reaction by both chemical (catalyst design) and genetic means leading to achievement of a novel degree of (enantio)selectivity, broadening of the substrate scope, or increased activity, among others. In the past 20 years, the Ward group has exploited, among others, the biotin–(strept)avidin technology to localize a catalytic moiety within a well-defined protein environment. Streptavidin has proven versatile for the implementation of ArMs as it offers the following features: (i) it is an extremely robust protein scaffold, amenable to extensive genetic manipulation and mishandling, (ii) it can be expressed in E. coli to very high titers (up to >8 g·L^–1 in fed-batch cultures), and (iii) the cavity surrounding the biotinylated cofactor is commensurate with the size of a typical metal-catalyzed transition state. Relying on a chemogenetic optimization strategy, varying the orientation and the nature of the biotinylated cofactor within genetically engineered streptavidin, 12 reactions have been reported by the Ward group thus far. Recent efforts within our group have focused on extending the ArM technology to create complex systems for integration into biological cascade reactions and in vivo.

With the long-term goal of complementing in vivo natural enzymes with ArMs, we summarize herein three complementary research lines: (i) With the aim of mimicking complex cross-regulation mechanisms prevalent in metabolism, we have engineered enzyme cascades, including cross-regulated reactions, that rely on ArMs. These efforts highlight the remarkable (bio)compatibility and complementarity of ArMs with natural enzymes. (ii) Additionally, multiple-turnover catalysis in the cytoplasm of aerobic organisms was achieved with ArMs that are compatible with a glutathione-rich environment. This feat is demonstrated in HEK-293T cells that are engineered with a gene switch that is upregulated by an ArM equipped with a cell-penetrating module. (iii) Finally, ArMs offer the fascinating prospect of “endowing organometallic chemistry with a genetic memory.” With this goal in mind, we have identified E. coli’s periplasmic space and surface display to compartmentalize an ArM, while maintaining the critical phenotype–genotype linkage. This strategy offers a straightforward means to optimize by directed evolution the catalytic performance of ArMs. Five reactions have been optimized following these compartmentalization strategies: ruthenium-catalyzed olefin metathesis, ruthenium-catalyzed deallylation, iridium-catalyzed transfer hydrogenation, dirhodium-catalyzed cyclopropanation and carbene insertion in C–H bonds. Importantly, >100 turnovers were achieved with ArMs in E. coli whole cells, highlighting the multiple turnover catalytic nature of these systems.

Beyond the Second Coordination Sphere: Engineering Dirhodium Artificial Metalloenzymes To Enable Protein Control of Transition Metal Catalysis

Transition metal catalysis is a powerful tool for chemical synthesis, a standard by which understanding of elementary chemical processes can be measured, and a source of awe for those who simply appreciate the difficulty of cleaving and forming chemical bonds. Each of these statements is amplified in cases where the transition metal catalyst controls the selectivity of a chemical reaction. Enantioselective catalysis is a challenging but well-established phenomenon, and regio- or site-selective catalysis is increasingly common. On the other hand, transition-metal-catalyzed reactions are typically conducted under highly optimized conditions. Rigorous exclusion of air and water is common, and it is taken for granted that only a single substrate (of a particular class) will be present in a reaction, a desired site selectivity can be achieved by installing a directing group, and undesired reactivity can be blocked with protecting groups. These are all reasonable synthetic strategies, but they also highlight limits to catalyst control. The utility of transition metal catalysis could be greatly expanded if catalysts possessed the ability to regulate which molecules they encounter and the relative orientation of those molecules. The rapid and widespread adoption of stoichiometric bioorthogonal reactions illustrates the utility of robust reactions that proceed with high selectivity and specificity under mild reaction conditions. Expanding this capability beyond preprogrammed substrate pairs via catalyst control could therefore have an enormous impact on molecular science. Many metalloenzymes exhibit this level of catalyst control, and directed evolution can be used to rapidly improve the catalytic properties of these systems. On the other hand, the range of reactions catalyzed by enzymes is limited relative to that developed by chemists. The possibility of imparting enzyme-like activity, selectivity, and evolvability to reactions catalyzed by synthetic transition metal complexes has inspired the creation of artificial metalloenzymes (ArMs). The increasing levels of catalyst control exhibited by ArMs developed to date suggest that these systems could constitute a powerful platform for bioorthogonal transition metal catalysis and for selective catalysis in general. This Account outlines the development of a new class of ArMs based on a prolyl oligopeptidase (POP) scaffold. Studies conducted on POP ArMs containing a covalently linked dirhodium cofactor have shown that POP can impart enantioselectivity to a range of dirhodium-catalyzed reactions, increase reaction rates, and improve the specificity for reaction of dirhodium carbene intermediates with targeted organic substrates over components of cell lysate, including bulk water. Several design features of these ArMs enabled their evolution via random mutagenesis, which revealed that mutations throughout the POP scaffold, beyond the second sphere of the dirhodium cofactor, were important for ArM activity and selectivity. While it was anticipated that the POP scaffold would be capable of encapsulating and thus controlling the selectivity of bulky cofactors, molecular dynamics studies also suggest that POP conformational dynamics plays a role in its unique efficacy. These advances in scaffold selection, bioconjugation, and evolution form the basis of our ongoing efforts to control transition metal reactivity using protein scaffolds with the goal of enabling unique synthetic capabilities, including bioorthogonal catalysis.