C(sp3)–H Bond Hydroxylation Catalyzed by Myoglobin Reconstituted with Manganese Porphycene

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.

An artificial metalloenzyme that can oxidize water photocatalytically: design, synthesis, and characterization

In nature, light-driven water oxidation (WO) catalysis is performed by photosystem II via the delicate interplay of different cofactors positioned in its protein scaffold. Artificial systems for homogeneous photocatalytic WO are based on small molecules that often have limited solubility in aqueous solutions. In this work, we alleviated this issue and present a cobalt-based WO-catalyst containing artificial metalloenzyme (ArM) that is active in light-driven, homogeneous WO catalysis in neutral-pH aqueous solutions. A haem-containing electron transfer protein, cytochrome B5 (CB5), served to host a first-row transition-metal-based WO catalyst, CoSalen (CoIISalen, where H2Salen = N,N'-bis(salicylidene)ethylenediamine), thus producing an ArM capable of driving photocatalytic WO. The CoSalen ArM formed a water-soluble pre-catalyst in the presence of [Ru(bpy)3](ClO4)2 as photosensitizer and Na2S2O8 as the sacrificial electron acceptor, with photocatalytic activity similar to that of free CoSalen. During photocatalysis, the CoSalen-protein interactions were destabilized, and the protein partially unfolded. Rather than forming tens of nanometer sized CoOx nanoparticles as free CoSalen does under photocatalytic WO conditions, the CB5 : CoSalen ArM showed limited protein cross-linking and remained soluble. We conclude that a weak, dynamic interaction between a soluble cobalt species and apoCB5 was formed, which generated a catalytically active adduct during photocatalysis. A detailed analysis was performed on protein stability and decomposition processes during the harsh oxidizing reaction conditions of WO, which will serve for the future design of WO ArMs with improved activity and stability.

Intramolecular C-H bond amination catalyzed by myoglobin reconstituted with iron porphycene

C-H bond amination is an effective way to obtain nitrogen-containing products. In this work, we demonstrate that myoglobin reconstituted with iron porphycene (rMb(FePc)) catalyzes intramolecular C(sp3)-H bond amination of arylsulfonyl azides to yield corresponding sultam analogs. The total turnover number of rMb(FePc) is up to 5.7 × 104 for the C-H bond amination of 2,4,6-triisopropylbenzenesulfonyl azide. Moreover, rMb(FePc) exhibits higher selectivity for the desired C-H bond amination than the competing azide reduction compared to native myoglobin. Kinetic studies reveal that the kcat value of rMb(FePc) is 4-fold higher than that of native myoglobin. Furthermore, H64A, H64V and H64I mutants of rMb(FePc) enhance the turnover number (TON) and enantioselectivity for the C-H bond amination of 2,4,6-triethylbenzenesulfonyl azide. The present findings indicate that iron porphycene is an attractive artificial cofactor for myoglobin toward the C-H bond amination reaction.

Effect of β-Substitution and β,β'-Fusion on the Formation of Boron Complexes of Porphycenes

Boron(III) complexation was investigated in a series of β-substituted porphycenes. Unlike meso-arylporphycenes, these macrocycles undergo a facile complexation reaction. Upon fusion of the β,β'-positions of the porphycene ligand, the complexation resulted in fast insertion of boron, forming the cisoid-B2OF2 complex. However, in the case of the other β-substituted porphycenes, only bis-BF2 complexes formed. The effect of these substituents on the core geometry and photophysical properties are elaborated here.

Manganese Transfer Hydrogenases Based on the Biotin-Streptavidin Technology

Artificial (transfer) hydrogenases have been developed for organic synthesis, but they rely on precious metals. Native hydrogenases use Earth-abundant metals, but these cannot be applied for organic synthesis due, in part, to their substrate specificity. Herein, we report the design and development of manganese transfer hydrogenases based on the biotin-streptavidin technology. By incorporating bio-mimetic Mn(I) complexes into the binding cavity of streptavidin, and through chemo-genetic optimization, we have obtained artificial enzymes that hydrogenate ketones with nearly quantitative yield and up to 98 % enantiomeric excess (ee). These enzymes exhibit broad substrate scope and high functional-group tolerance. According to QM/MM calculations and X-ray crystallography, the S112Y mutation, combined with the appropriate chemical structure of the Mn cofactor plays a critical role in the reactivity and enantioselectivity of the artificial metalloenzyme (ArMs). Our work highlights the potential of ArMs incorporating base-meal cofactors for enantioselective organic synthesis.

Diversifying the functions of heme proteins with non-porphyrin cofactors

Heme proteins perform diverse biochemical functions using a single iron porphyrin cofactor. This versatility makes them attractive platforms for the development of new functional proteins. While directed evolution and metal substitution have expanded the properties, reactivity, and applications of heme proteins, the incorporation of porphyrin analogs remains an underexplored approach. This review discusses the replacement of heme with non-porphyrin cofactors, such as porphycene, corrole, tetradehydrocorrin, phthalocyanine, and salophen, and the attendant properties of these conjugates. While structurally similar, each ligand exhibits distinct optical and redox properties, as well as unique chemical reactivity. These hybrids serve as model systems to elucidate the effects of the protein environment on the electronic structure, redox potentials, optical properties, or other features of the porphyrin analog. Protein encapsulation can confer distinct chemical reactivity or selectivity of artificial metalloenzymes that cannot be achieved with the small molecule catalyst alone. Additionally, these conjugates can interfere with heme acquisition and uptake in pathogenic bacteria, providing an inroad to innovative antibiotic strategies. Together, these examples illustrate the diverse functionality that can be achieved by cofactor substitution. The further expansion of this approach will access unexplored chemical space, enabling the development of superior catalysts and the creation of heme proteins with emergent properties.

Artificial Small Molecules as Cofactors and Biomacromolecular Building Blocks in Synthetic Biology: Design, Synthesis, Applications, and Challenges

Enzymes are essential catalysts for various chemical reactions in biological systems and often rely on metal ions or cofactors to stabilize their structure or perform functions. Improving enzyme performance has always been an important direction of protein engineering. In recent years, various artificial small molecules have been successfully used in enzyme engineering. The types of enzymatic reactions and metabolic pathways in cells can be expanded by the incorporation of these artificial small molecules either as cofactors or as building blocks of proteins and nucleic acids, which greatly promotes the development and application of biotechnology. In this review, we summarized research on artificial small molecules including biological metal cluster mimics, coenzyme analogs (mNADs), designer cofactors, non-natural nucleotides (XNAs), and non-natural amino acids (nnAAs), focusing on their design, synthesis, and applications as well as the current challenges in synthetic biology.

Hijacking Chemical Reactions of P450 Enzymes for Altered Chemical Reactions and Asymmetric Synthesis

Cytochrome P450s are heme-containing enzymes capable of the oxidative transformation of a wide range of organic substrates. A protein scaffold that coordinates the heme iron, and the catalytic pocket residues, together, determine the reaction selectivity and regio- and stereo-selectivity of the P450 enzymes. Different substrates also affect the properties of P450s by binding to its catalytic pocket. Modulating the redox potential of the heme by substituting iron-coordinating residues changes the chemical reaction, the type of cofactor requirement, and the stereoselectivity of P450s. Around hundreds of P450s are experimentally characterized, therefore, a mechanistic understanding of the factors affecting their catalysis is increasingly vital in the age of synthetic biology and biotechnology. Engineering P450s can enable them to catalyze a variety of chemical reactions viz. oxygenation, peroxygenation, cyclopropanation, epoxidation, nitration, etc., to synthesize high-value chiral organic molecules with exceptionally high stereo- and regioselectivity and catalytic efficiency. This review will focus on recent studies of the mechanistic understandings of the modulation of heme redox potential in the engineered P450 variants, and the effect of small decoy molecules, dual function small molecules, and substrate mimetics on the type of chemical reaction and the catalytic cycle of the P450 enzymes.

Searching for correlations between geometric and spectroscopic parameters of intramolecular hydrogen bonds in porphyrin-like macrocycles

Chemical bond lengths and angles are characteristic structural parameters of a molecule. Similarly, the frequencies of the vibrational modes and the NMR chemical shifts are unique "chemical fingerprints" specific to a compound. These are the basic parameters describing newly obtained compounds and enabling their identification. Intramolecular hydrogen bonding significantly influences the physicochemical properties of macrocyclic compounds with a porphyrin-like structure. This work presents the verification for correlations between geometric and spectroscopic parameters related to hydrogen bonds in this type of macrocyclic compounds. In particular, such relationships were investigated for a large group of porphyrin, porphycene, and dibenzotetraaza[14]annulene derivatives and a group of other macrocycles with similar structure. A very strong linear correlation was found only between the vibrational frequencies of the NH groups involved in a hydrogen bond and the length of this bond, which applied to all macrocyclic compounds of this type. Several other relationships were found between spectroscopic (IR, Raman, NMR) and geometric (X-ray) parameters, highlighting differences and similarities between different families of macrocycles. Apart from providing a better understanding of the nature of hydrogen bonds and their characteristics in porphyrin-like macrocyclic compounds, these relationships will facilitate the identification of new macrocycles and the extrapolation of their spectroscopic properties.

Enhancing the Cellular Uptake of Macromolecules via Enzyme-Instructed Self-Assembly

Poor solubility, low cellular uptake, and poor cell selectivity are the main obstacles hampering the therapeutic potential and clinic application of macromolecules. To overcome these limitations, here we propose a chemical modification strategy of macromolecules based on enzyme-instructed self-assembly (EISA). By using protoporphyrin IX (PpIX) and its metal complex Zn-PpIX as the modification objects, we demonstrated that the integration of enzymatic transformation and molecular self-assembly of macromolecules successfully improved the solubility of macromolecules, enhancing their intracellular uptake selectively against cancer cells. The proposed strategy is potentially applicable as a general tool for the development of macromolecule-based nanomedicine.

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

Confusion is a novel concept of isomerism in porphyrin chemistry, delivering a steady stream of new chemistry since the discovery of N-confused porphyrin, a porphyrin mutant, in 1994. These days, the number of confused porphyrinoids is increasing, and confusion and associated fusion are found in various fields such as supramolecular chemistry, materials chemistry, biological chemistry, and catalysts. In this review, the birth and growth of confused porphyrinoids in the last three decades are described.

Corrole–protein interactions in H-NOX and HasA

Mutagenesis was utilised to reveal corrole–protein interactions in H-NOX and HasA. The key interaction is a hydrogen bond between the PO unit of the corrole and a protonated histidine residue.

Engineered Myoglobin as a Catalyst for Atom Transfer Radical Cyclisation

Myoglobin was subjected to site-directed mutagenesis and transformed into a catalyst able to perform atom transfer radical cyclisation reactions, i.e. intramolecular atom transfer radical additions. Replacing the iron-coordinating histidine with serine, or introducing small changes inside or at the entrance of the active site, transformed the completely inactive wild-type myoglobin into an artificial metalloenzyme able to catalyse the 5-exo cyclisation of halogenated unsaturated compounds for the synthesis of γ-lactams. This new-to-nature activity was achieved not only with purified protein but also in crude cell lysate and in whole cells.

Myoglobin was subjected to site-directed mutagenesis and transformed into a catalyst able to perform the atom transfer radical reaction.

De novo biosynthesis of a nonnatural cobalt porphyrin cofactor in E. coli and incorporation into hemoproteins

Enzymes that bear a nonnative or artificially introduced metal center can engender novel reactivity and enable new spectroscopic and structural studies. In the case of metal-organic cofactors, such as metalloporphyrins, no general methods exist to build and incorporate new-to-nature cofactor analogs in vivo. We report here that a common laboratory strain, Escherichia coli BL21(DE3), biosynthesizes cobalt protoporphyrin IX (CoPPIX) under iron-limited, cobalt-rich growth conditions. In supplemented minimal media containing CoCl₂, the metabolically produced CoPPIX is directly incorporated into multiple hemoproteins in place of native heme b (FePPIX). Five cobalt-substituted proteins were successfully expressed with this new-to-nature cobalt porphyrin cofactor: myoglobin H64V V68A, dye decolorizing peroxidase, aldoxime dehydratase, cytochrome P450 119, and catalase. We show conclusively that these proteins incorporate CoPPIX, with the CoPPIX making up at least 95% of the total porphyrin content. In cases in which the native metal ligand is a sulfur or nitrogen, spectroscopic parameters are consistent with retention of native metal ligands. This method is an improvement on previous approaches with respect to both yield and ease-of-implementation. Significantly, this method overcomes a long-standing challenge to incorporate nonnatural cofactors through de novo biosynthesis. By utilizing a ubiquitous laboratory strain, this process will facilitate spectroscopic studies and the development of enzymes for CoPPIX-mediated biocatalysis.

Myoglobins engineered with artificial cofactors serve as artificial metalloenzymes and models of natural enzymes

Metalloenzymes naturally achieve various reactivities by assembling limited types of cofactors with endogenous amino acid residues. Enzymes containing metal porphyrinoid cofactors such as heme, cobalamin and F430 exert precise control over the reactivities of the cofactors with protein matrices. This perspective article focuses on our recent efforts to assemble metal complexes of non-natural porphyrinoids within the protein matrix of myoglobin, an oxygen storage hemoprotein. Engineered myoglobins with suitable metal complexes as artificial cofactors demonstrate unique reactivities toward C-H bond hydroxylation, olefin cyclopropanation, methyl group transfer and methane generation. In these cases, the protein matrix enhances the catalytic activities of the cofactors and allows us to monitor the active intermediates. The present findings indicate that placing artificial cofactors in protein matrices provides a useful strategy for creating artificial metalloenzymes that catalyse otherwise unfavourable reactions and providing enzyme models for elucidating the complicated reaction mechanisms of natural enzymes.

De Novo Design, Solution Characterization, and Crystallographic Structure of an Abiological Mn-Porphyrin-Binding Protein Capable of Stabilizing a Mn(V) Species

De novo protein design offers the opportunity to test our understanding of how metalloproteins perform difficult transformations. Attaining high-resolution structural information is critical to understanding how such designs function. There have been many successes in the design of porphyrin-binding proteins; however, crystallographic characterization has been elusive, limiting what can be learned from such studies as well as the extension to new functions. Moreover, formation of highly oxidizing high-valent intermediates poses design challenges that have not been previously implemented: (1) purposeful design of substrate/oxidant access to the binding site and (2) limiting deleterious oxidation of the protein scaffold. Here we report the first crystallographically characterized porphyrin-binding protein that was programmed to not only bind a synthetic Mn-porphyrin but also maintain binding site access to form high-valent oxidation states. We explicitly designed a binding site with accessibility to dioxygen units in the open coordination site of the Mn center. In solution, the protein is capable of accessing a high-valent Mn(V)-oxo species which can transfer an O atom to a thioether substrate. The crystallographic structure is within 0.6 Å of the design and indeed contained an aquo ligand with a second water molecule stabilized by hydrogen bonding to a Gln side chain in the active site, offering a structural explanation for the observed reactivity.

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.

Systematic engineering of artificial metalloenzymes for new-to-nature reactions

Artificial metalloenzymes (ArMs) catalyzing new-to-nature reactions could play an important role in transitioning toward a sustainable economy. While ArMs have been created for various transformations, attempts at their genetic optimization have been case specific and resulted mostly in modest improvements. To realize their full potential, methods to rapidly discover active ArM variants for ideally any reaction of interest are required. Here, we introduce a reaction-independent, automation-compatible platform, which relies on periplasmic compartmentalization in Escherichia coli to rapidly and reliably engineer ArMs based on the biotin-streptavidin technology. We systematically assess 400 ArM mutants for five bioorthogonal transformations involving different metals, reaction mechanisms, and reactants, which include novel ArMs for gold-catalyzed hydroamination and hydroarylation. Activity enhancements up to 15-fold highlight the potential of the systematic approach. Furthermore, we suggest smart screening strategies and build machine learning models that accurately predict ArM activity from sequence, which has crucial implications for future ArM development.

Artificial Metalloprotein Nanoanalogues: In Situ Catalytic Production of Oxygen to Enhance Photoimmunotherapeutic Inhibition of Primary and Abscopal Tumor Growth

Photoimmunotherapy (PIT) has shown enormous potential in not only eliminating primary tumors, but also inhibiting abscopal tumor growth. However, the efficacy of PIT is greatly limited by tumor hypoxia, which causes the attenuation of phototherapeutic efficacy and is a feature of the immunosuppressive tumor microenvironment (TME). In this study, one type of brand-new artificial metalloprotein nanoanalogues is developed via reasonable integration of a "phototherapy-enzymatic" RuO₂ and a model antigen, ovalbumin (OVA) for enhanced PIT of cancers, namely, RuO₂ -hybridized OVA nanoanalogues (RuO₂ @OVA NAs). The RuO₂ @OVA NAs exhibit remarkable photothermal/photodynamic capabilities under the near-infrared light irradiation. More importantly, the photoacoustic imaging and immunofluorescence staining confirm that RuO₂ @OVA NAs can remarkably alleviate hypoxia via in situ catalysis of hydrogen peroxide overexpressed in the TME to produce oxygen (O₂ ). This ushers a prospect of concurrently enhancing photodynamic therapy and reversing the immunosuppressive TME. Also, OVA, as a supplement to the immune stimulation induced by phototherapy, can activate immune responses. Finally, further combination with the cytotoxic T-lymphocyte-associated protein 4 checkpoint blockade is reported to effectively eliminate the primary tumor and inhibit distant tumor growth via the abscopal effect of antitumor immune responses, prolonging the survival.

An Artificial Hemoprotein with Inducible Peroxidase- and Monooxygenase-Like Activities

A novel inducible artificial metalloenzyme obtained by covalent attachment of a manganese(III)-tetraphenylporphyrin (MnTPP) to the artificial bidomain repeat protein, (A3A3')Y26C, is reported. The protein is part of the αRep family. The biohybrid was fully characterized by MALDI-ToF mass spectrometry, circular dichroism and UV/Vis spectroscopies. The peroxidase and monooxygenase activities were evaluated on the original and modified scaffolds including those that have a) an additional imidazole, b) a specific αRep bA3-2 that is known to induce the opening of the (A3A3') interdomain region and c) a derivative of the αRep bA3-2 inducer extended with a His₆ -Tag (His₆ -bA3-2). Catalytic profiles are highly dependent on the presence of co-catalysts with the best activity obtained with His₆ -bA3-2. The entire mechanism was rationalized by an integrative molecular modeling study that includes protein-ligand docking and large-scale molecular dynamics. This constitutes the first example of an entirely artificial metalloenzyme with inducible peroxidase and monooxygenase activities, reminiscent of allosteric regulation of natural enzymatic pathways.

Incorporation of a Cp*Rh(III)-dithiophosphate Cofactor with Latent Activity into a Protein Scaffold Generates a Biohybrid Catalyst Promoting C(sp2)-H Bond Functionalization

A Cp*Rh(III)-dithiophosphate cofactor with "latent" catalytic activity was developed to construct an artificial metalloenzyme representing a new type of biohybrid catalyst which is capable of promoting C(sp²)-H bond functionalization within the β-barrel structure of nitrobindin (NB). To covalently conjugate the Cp*Rh(III) cofactor into a specific position of the hydrophobic cavity of NB via a maleimide-Cys linkage, strong chelation of the dithiophosphate ligand is employed to protect the rhodium metal center against attack by nucleophilic amino acid residues in the protein. It is found that subsequent addition of the Ag⁺ ion induces dissociation of the dithiophosphate ligands, thereby activating the catalytic activity of the Cp*Rh(III) cofactor. The resulting "active" biohybrid catalyst promotes cycloaddition of acetophenone oxime with diphenylacetylene via C(sp²)-H bond activation. This catalytic activity is enhanced 2.3-fold with the introduction of two glutamate residues (A100E/L125E) adjacent to the Cp*Rh(III) cofactor. The Cp*Rh(III) cofactor with switchable activity from a "latent" form to an "active" form provides a new strategy for generating biohybrid catalysts incorporating a variety of highly reactive transition metal complexes specifically within its protein scaffolds.

Artificial Metalloenzymes: From Selective Chemical Transformations to Biochemical Applications

Artificial metalloenzymes (ArMs) comprise a synthetic metal complex in a protein scaffold. ArMs display performances combining those of both homogeneous catalysts and biocatalysts. Specifically, ArMs selectively catalyze non-natural reactions and reactions inspired by nature in water under mild conditions. In the past few years, the construction of ArMs that possess a genetically incorporated unnatural amino acid and the directed evolution of ArMs have become of great interest in the field. Additionally, biochemical applications of ArMs have steadily increased, owing to the fact that compartmentalization within a protein scaffold allows the synthetic metal complex to remain functional in a sea of inactivating biomolecules. In this review, we present updates on: 1) the newly reported ArMs, according to their type of reaction, and 2) the unique biochemical applications of ArMs, including chemoenzymatic cascades and intracellular/in vivo catalysis. We believe that ArMs have great potential as catalysts for organic synthesis and as chemical biology tools for pharmaceutical applications.

Artificial Metalloenzymes based on TetR Proteins and Cu(II) for Enantioselective Friedel-Crafts Alkylation Reactions

The supramolecular approach is among the most convenient methodologies for creating artificial metalloenzymes (ArMs). Usually this approach involves the binding of a transition metal ion complex to a biomolecular scaffold via its ligand, which also modulates the catalytic properties of the metal ion. Herein, we report ArMs based on the proteins CgmR, RamR and QacR from the TetR family of multidrug resistance regulators (MDRs) and Cu2+ ions, assembled without the need of a ligand. These ArMs catalyze the enantioselective vinylogous Friedel-Crafts alkylation reaction with up to 75 % ee. Competition experiments with ethidium and rhodamine 6G confirm that the reactions occur in the chiral environment of the hydrophobic pocket. It is proposed that the Cu2+-substrate complex is bound via a combination of electrostatic and π-stacking interactions provided by the second coordination sphere. This approach constitutes a fast and straightforward way to assemble metalloenzymes and may facilitate future optimization of the protein scaffolds via mutagenesis or directed evolution approaches.

The Red Color of Life Transformed - Synthetic Advances and Emerging Applications of Protoporphyrin IX in Chemical Biology

Protoporphyrin IX (PPIX) is the porphyrin scaffold of heme b, a ubiquitous prosthetic group of proteins responsible for oxygen binding (hemoglobin, myoglobin), electron transfer (cytochrome c) and catalysis (cytochrome P450, catalases, peroxidases). PPIX and its metallated derivatives frequently find application as therapeutic agents, imaging tools, catalysts, sensors and in light harvesting. The vast toolkit of accessible porphyrin functionalization reactions enables easy synthetic modification of PPIX to meet the requirements for its multiple uses. In the past few years, particular interest has arisen in exploiting the interaction of PPIX and its synthetic derivatives with biomolecules such as DNA and heme-binding proteins to evolve molecular devices with new functions as well as to uncover potential therapeutic toeholds. This review strives to shine a light on the most recent developments in the synthetic chemistry of PPIX and its uses in selected fields of chemical biology.

Enantioselective Hydroxylation of Benzylic C(sp3)-H Bonds by an Artificial Iron Hydroxylase Based on the Biotin-Streptavidin Technology

The selective hydroxylation of C–H bonds is of great interest to the synthetic community. Both homogeneous catalysts and enzymes offer complementary means to tackle this challenge. Herein, we show that biotinylated Fe(TAML)-complexes (TAML = Tetra Amido Macrocyclic Ligand) can be used as cofactors for incorporation into streptavidin to assemble artificial hydroxylases. Chemo-genetic optimization of both cofactor and streptavidin allowed optimizing the performance of the hydroxylase. Using H₂O₂ as oxidant, up to ∼300 turnovers for the oxidation of benzylic C–H bonds were obtained. Upgrading the ee was achieved by kinetic resolution of the resulting benzylic alcohol to afford up to >98% ee for (R)-tetralol. X-ray analysis of artificial hydroxylases highlights critical details of the second coordination sphere around the Fe(TAML) cofactor.

The Quest for Xenobiotic Enzymes: From New Enzymes for Chemistry to a Novel Chemistry of Life

Enzyme engineering has made impressive progress in the past decades, paving the way for the widespread use of enzymes for various purposes. In contrast to "classical" enzyme engineering, which focuses on optimizing specific properties of natural enzymes, a more recent trend towards the creation of artificial enzymes that catalyze fundamentally distinct, new-to-nature reactions is observable. While approaches for creating such enzymes differ significantly, they share the common goal of enabling biocatalytic novelty to broaden the range of applications for enzymes. Although most artificial enzymes reported to date are only moderately active and barely function in vivo, they have the potential to endow cells with capabilities that were previously out of reach and thus herald a new wave of "functional xenobiology". Herein, we highlight recent developments in the field of artificial enzymes with a particular focus on challenges and opportunities for their use in xenobiology.

A de novo peroxidase is also a promiscuous yet stereoselective carbene transferase

While the bottom-up design of enzymes appears to be an intractably complex problem, a minimal approach that combines elementary, de novo-designed proteins with intrinsically reactive cofactors offers a simple means to rapidly access sophisticated catalytic mechanisms. Not only is this method proven in the reproduction of powerful oxidative chemistry of the natural peroxidase enzymes, but we show here that it extends to the efficient, abiological—and often asymmetric—formation of strained cyclopropane rings, nitrogen–carbon and carbon–carbon bonds, and the ring expansion of a simple cyclic molecule to form a precursor for NAD+, a fundamentally important biological cofactor. That the enzyme also functions in vivo paves the way for its incorporation into engineered biosynthetic pathways within living organisms.

By constructing an in vivo-assembled, catalytically proficient peroxidase, C45, we have recently demonstrated the catalytic potential of simple, de novo-designed heme proteins. Here, we show that C45’s enzymatic activity extends to the efficient and stereoselective intermolecular transfer of carbenes to olefins, heterocycles, aldehydes, and amines. Not only is this a report of carbene transferase activity in a completely de novo protein, but also of enzyme-catalyzed ring expansion of aromatic heterocycles via carbene transfer by any enzyme.

Mechanism and stereoselectivity of benzylic C-H hydroxylation by Ru-porphyrin: a computational study

The mechanism and origin of the stereoselectivity of asymmetric benzylic C-H hydroxylation by Ru-porphyrin were elucidated with density functional theory calculations. The reaction proceeds via a hydrogen-atom abstraction/oxygen-rebound pathway, wherein a high-valent ruthenium-oxo species abstracts a hydrogen atom from ethylbenzene to generate a radical pair intermediate, followed by the oxygen-rebound process to form 1-phenylethanol. The hydrogen-atom abstraction step is the rate- and stereoselectivity-determining step. Based on the mechanistic model, the computed stereoselectivity is in agreement with the experimental observations. Analysis of the distortion/interaction model suggests that stereoselectivity is determined by both the distortion energy of the ethylbenzene and the interaction energy between the ethylbenzene and the chiral Ru-porphyrin. The steric repulsion between the phenyl group of ethylbenzene and the bulky substituent of Ru-porphyrin is the leading cause of chiral induction.

Functionalized mesoporous silica nanoparticles with 2,7,12,17-tetraphenylporphycene

Mesoporous silica nanoparticles decorated with 2,7,12,17-tetraphenylporphycene have been prepared by reaction of aminated nanoparticles with 9-(glutaric [Formula: see text]-succinimidylesteramide)-2,7,12,17-tetraphenylporphycene. The as-prepared nanoparticles were characterized by UV-vis spectroscopy, DLS and TEM.

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.

Stereoselective Cyclopropanation of Electron-Deficient Olefins with a Cofactor Redesigned Carbene Transferase Featuring Radical Reactivity

Engineered myoglobins and other hemoproteins have recently emerged as promising catalysts for asymmetric olefin cyclopropanation reactions via carbene transfer chemistry. Despite this progress, the transformation of electron-poor alkenes has proven very challenging using these systems. Here, we describe the design of a myoglobin-based carbene transferase incorporating a non-native iron-porphyrin cofactor and axial ligand, as an efficient catalyst for the asymmetric cyclopropanation of electron-deficient alkenes. Using this metalloenzyme, a broad range of both electron-rich and electron-deficient alkenes are cyclopropanated with high efficiency and high diastereo- and enantioselectivity (up to >99% de and ee). Mechanistic studies revealed that the expanded reaction scope of this carbene transferase is dependent upon the acquisition of metallocarbene radical reactivity as a result of the reconfigured coordination environment around the metal center. The radical-based reactivity of this system diverges from the electrophilic reactivity of myoglobin and most of known organometallic carbene transfer catalysts. This work showcases the value of cofactor redesign toward tuning and expanding the reactivity of metalloproteins in abiological reactions and it provides a biocatalytic solution to the asymmetric cyclopropanation of electrodeficient alkenes. The metallocarbene radical reactivity exhibited by this biocatalyst is anticipated to prove useful in the context of a variety of other synthetic transformations.

Combining enzymes and organometallic complexes: novel artificial metalloenzymes and hybrid systems for C-H activation chemistry

This review describes the recent advances in the design of novel artificial metalloenzymes and their application in C-H activation reactions. The combination of enzymes and metal or organometallic complexes for the creation of new artificial metalloenzymes has represented a very exciting research line. In particular, the development of proteins with the ability to perform C-H functionalization presents a significant challenge. Here we discuss the development of these processes on natural metalloenzymes by using directed evolution, biotin-(strept)avidin technologies, photocatalytic hybrids or reconstitution of heme-protein technology.

2,7,12,17-Tetra(2,5-thienylene)-substituted porphycenes

We report syntheses of thiophene and dithiophene-substituted porphycenes (ThPc and DThPc) at 2,7,12,17-positions by McMurry coupling. The crystal structure of ThPc revealed that the porphycene plane shows a highly planar structure, and the dihedral angles between the porphycene core and thiophene are relatively small at 21[Formula: see text] and 18[Formula: see text]. ThPc and DThPc exhibit red-shifted and broadened absorption because of the extension of [Formula: see text] conjugations through porphycene to the substituted thiophenes. We found that introduction of thiophene units onto porphycene results in decreasing the HOMO–LUMO differences effectively.

Highly malleable haem-binding site of the haemoprotein HasA permits stable accommodation of bulky tetraphenylporphycenes

Iron(iii)- and cobalt(iii)-9,10,19,20-tetraphenylporphycenes, which possess bulky phenyl groups at the four meso positions of porphycene, were successfully incorporated into the haem acquisition protein HasA secreted by Pseudomonas aeruginosa. Crystal structure analysis revealed that loops surrounding the haem-binding site are highly flexible, remodelling themselves to accommodate bulky metal complexes with significantly different structures from the native haem cofactor.

Hemoproteins Reconstituted with Artificial Metal Complexes as Biohybrid Catalysts

In nature, heme cofactor-containing proteins participate not only in electron transfer and O₂ storage and transport but also in biosynthesis and degradation. The simplest and representative cofactor, heme b, is bound within the heme pocket via noncovalent interaction in many hemoproteins, suggesting that the cofactor is removable from the protein, leaving a unique cavity. Since the cavity functions as a coordination sphere for heme, it is of particular interest to investigate replacement of native heme with an artificial metal complex, because the substituted metal complex will be stabilized in the heme pocket while providing alternative chemical properties. Thus, cofactor substitution has great potential for engineering of hemoproteins with alternative functions. For these studies, myoglobin has been a focus of our investigations, because it is a well-known oxygen storage hemoprotein. However, the heme pocket of myoglobin has been only arranged for stabilizing the heme-bound dioxygen, so the structure is not suitable for activation of small molecules such as H₂O₂ and O₂ as well as for binding an external substrate. Thus, the conversion of myoglobin to an enzyme-like biocatalyst has presented significant challenges. The results of our investigations have provided useful information for chemists and biologists. Our own efforts to develop functionalized myoglobin have focused on the incorporation of a chemically modified cofactor into apomyoglobin in order to (1) construct an artificial substrate-binding site near the heme pocket, (2) increase cofactor reactivity, or (3) promote a new reaction that has never before been catalyzed by a native heme enzyme. In pursuing these objectives, we first found that myoglobin reconstituted with heme having a chemically modified heme-propionate side chain at the exit of the heme pocket has peroxidase activity with respect to oxidation of phenol derivatives. Our recent investigations have succeeded in enhancing oxidation and oxygenation activities of myoglobin as well as promoting new reactions by reconstitution of myoglobin with new porphyrinoid metal complexes. Incorporation of suitable metal porphyrinoids into the heme pocket has produced artificial enzymes capable of efficiently generating reactive high valent metal-oxo and metallocarbene intermediates to achieve the catalytic hydroxylation of C(sp³)-H bonds and cyclopropanation of olefin molecules, respectively. In other efforts, we have focused on nitrobindin, an NO-binding hemoprotein, because aponitrobindin includes a β-barrel cavity, which provides a robust structure highly similar to that of the native holoprotein. It was expected that the aponitrobindin would be suitable for development as a protein scaffold for a metal complex. Recently, it was confirmed that several organometallic complexes can bind to this scaffold and function as catalysts promoting hydrogen evolution or C-C bond formation. The hydrophobic β-barrel structure plays a significant role in substrate binding as well as controlling the stereoselectivity of the reactions. Furthermore, these catalytic activities and stereoselectivities are remarkably improved by mutation-dependent modifications of the cavity structure for the artificial cofactor. This Account demonstrates how apoproteins of hemoproteins can provide useful protein scaffolds for metal complexes. Further development of these concepts will provide a useful strategy for generation of robust and useful artificial metalloenzymes.

Electrochemical Formation of Fe(IV)=O Derived from H2 O2 on a Hematite Electrode as an Active Catalytic Site for Selective Hydrocarbon Oxidation Reactions

The high-valence iron species (Fe(IV)=O) in the cytochrome P450 enzyme superfamily is generated via the activation of O₂ , and serves as the active center of selective hydrocarbon oxidation reactions. Furthermore, P450 can employ an alternate route to produce Fe(IV)=O, even from H₂ O₂ without O₂ activation. Meanwhile, Fe(IV)=O has recently been revealed to be the reactive intermediate during H₂ O oxidation to O₂ on hematite electrodes. Herein, we demonstrated the generation of Fe(IV)=O on hematite electrodes during the electrochemical oxidative decomposition of H₂ O₂ using in situ UV-visible absorption spectra. The generation of Fe(IV)=O on hematite electrodes from H₂ O₂ exhibited 100 mV lower overpotential than that from H₂ O. This is because H₂ O₂ serves not only as the oxygen source of Fe(IV)=O, but also as the additional oxidant. Finally, we confirmed that the Fe(IV)=O generated on hematite electrodes can serve as the catalytic site for styrene epoxidation reactions.