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

Functional metalloenzymes based on myoglobin and neuroglobin that exploit covalent interactions

Globins, such as myoglobin (Mb) and neuroglobin (Ngb), are ideal protein scaffolds for the design of functional metalloenzymes. To date, numerous approaches have been developed for enzyme design. This review presents a summary of the progress made in the design of functional metalloenzymes based on Mb and Ngb, with a focus on the exploitation of covalent interactions, including coordination bonds and covalent modifications. These include the construction of a metal-binding site, the incorporation of a non-native metal cofactor, the formation of Cys/Tyr-heme covalent links, and the design of disulfide bonds, as well as other Cys-covalent modifications. As exemplified by recent studies from our group and others, the designed metalloenzymes have potential applications in biocatalysis and bioconversions. Furthermore, we discuss the current trends in the design of functional metalloenzymes and highlight the importance of covalent interactions in the design of functional metalloenzymes.

Zinc-Substituted Hemoglobin-Albumin Cluster as a Porphyrin-Carrier for Enhanced Photodynamic Therapy

Apohemoprotein is focused on the field of theranostics, serving as a porphyrin carrier. Hemoglobin (Hb) consists of α2β2 tetramer with iron(II)-protoporphyrin IX (heme) bound to each globin. However, heme-removed Hb (apoHb) causes dissociation at αβ interfaces and aggregation under physiological conditions. We synthesized a stable apoHb derivative comprising intramolecular-crosslinked apoHb (apoXHb) and human serum albumin (HSA), apoXHb-HSA3. ApoXHb-HSA3 engendered no aggregates in the physiological solutions. Moreover, apoXHb-HSA3 was reconstituted with zinc(II)-protoporphyrin IX (ZnP), generating ZnXHb-HSA3, a potent photosensitizer for photodynamic therapy (PDT). The photophysical properties of ZnXHb-HSA3 were identical to those of zinc-substituted XHb (ZnXHb). Cellular uptake behavior was evaluated using various cancer cell lines. ZnXHb-HSA3 released ZnP around the cells, and the free ZnP penetrated cell membranes. In contrast, protein units were not observed within the cells. ZnXHb-HSA3 showed no cytotoxicity under dark conditions and demonstrated superior PDT activity in comparison to naked ZnXHb. ZnXHb-HSA3 acts as an innovative porphyrin carrier for enhanced PDT.

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.

Regulating the Heme Active Site by Covalent Modifications: Two Case Studies of Myoglobin

Using myoglobin (Mb) as a model protein, we herein developed a facial approach to modifying the heme active site. A cavity was first generated in the heme distal site by F46 C mutation, and the thiol group of Cys46 was then used for covalently linked to exogenous ligands, 1H-1,2,4-triazole-3-thiol and 1-(4-hydroxyphenyl)-1H-pyrrole-2,5-dione. The engineered proteins, termed F46C-triazole Mb and F46C-phenol Mb, respectively, were characterized by X-ray crystallography, spectroscopic and stopped-flow kinetic studies. The results showed that both the heme coordination state and the protein function such as H2 O2 activation and peroxidase activity could be efficiently regulated, which suggests that this approach might be generally applied to the design of functional heme proteins.

Application of engineered myoglobins for biosynthesis of clofazimine by integration with chemical synthesis

Significant efforts have been made in the design of artificial metalloenzymes. Myoglobin (Mb), an O2 carrier, has been engineered to exhibit different functions. Herein, we applied a series of engineered Mb mutants with peroxidase activity for biosynthesis of clofazimine (CFZ), a potential drug with a broad-spectrum antiviral activity, by integration with chemical synthesis. Two of those mutants, F43Y Mb and F43Y/T67R Mb, have been shown to efficiently catalyze the oxidative coupling of 2-N-(4-chlorophenyl) benzene-1,2-diamine (N-4-CPBDA) in the presence of H2O2, with 97% yields. The overall catalytic efficiency (kcat/Km) is 46-fold and 82-fold higher than that of WT Mb, respectively. By further combination of this reaction with chemical synthesis, the production of CFZ was accomplished with an isolated yield of 72%. These results showed that engineered Mbs containing the Tyr-heme cross-link (F43Y Mb and F43Y/T67R Mb) exhibit enhanced activity in the oxidative coupling reaction. This study also indicates that the combination of biocatalysis and chemical synthesis avoids the need for the separation of intermediate products, which offers a convenient approach for the total synthesis of the biological compound CFZ.

Protein-Porphyrin Complex Photosensitizers for Anticancer and Antimicrobial Photodynamic Therapies

Photodynamic therapy (PDT) efficiently induces apoptosis through visible-light irradiation of photosensitizers (PSs) within tumors and microbial cells. Porphyrin analogues serve as widely utilized photosensitizing agents with their therapeutic abilities being governed by molecular structures and central metal ions. However, these macrocyclic compounds tend to agglutinate and form stacks in aqueous environments, resulting in a loss of photochemical activity. To overcome this limitation, encapsulation within liposomes and polymer micelles enables the dispersion of porphyrins as monomolecular entities in aqueous solutions, preventing undesirable deactivation. Recently, the use of reconstituted hemoproteins containing various metal-porphyrins and protein cages incorporating porphyrins has garnered significant interest as a new generation of biocompatible PSs. In this concept paper, we provide a comprehensive review of recent developments and trends of protein-porphyrin complex PSs for applications in anticancer and antimicrobial PDTs.

A polyacrylamide gel containing an engineered hexameric hemoprotein as a cross-linking unit toward redox-responsive materials

Hydrogels containing synthetic polymers and supramolecular cross-linking units are expected to exhibit unique functions and properties. The heme-heme pocket interaction in hemeproteins may be useful for development of a cross-linking unit because heme binding depends on the redox states of the iron center. In this work, hexameric tyrosine-coordinated hemoprotein (HTHP) is employed as a cross-linking unit in a polyacrylamide gel to create redox-responsive hydrogels. First, redox-dependent stability of the heme-heme pocket interaction in HTHP was evaluated, and it was found that the heme affinity dramatically decreases in the Fe(ii) state. Second, the polymerization of acrylamide and engineered HTHP possessing acryloyl group-tethering heme moieties provided a polyacrylamide gel containing HTHP as a cross-linking unit. A reduction-triggered gel-sol transition in the presence of apomyoglobin was observed. Furthermore, the mechanical properties of the gels containing the engineered HTHP and methylene bisacrylamide were evaluated by a tensile test, and the Young's modulus value was determined to be 14 kPa, which is higher than that of the control gel containing only methylene bisacrylamide (8.5 kPa). Compression tests of the gels revealed redox-responsive mechanical behavior, resulting in a decrease in the compressive modulus upon the addition of a reductant. This behavior is qualitatively consistent with the redox-responsive heme binding of HTHP in a solution state. This finding is expected to contribute to the development of redox-responsive materials for biomedical and biological applications.

An engineered thermally tolerant apo-cytochrome scaffold for metal-less incorporation of heme derivative

Cytochrome c552 from Thermus thermophilus is one of the hot topics for creating smart biomaterials as it possesses remarkable stability, is tolerant to multiple mutations and has therefore been recently reported for a number of functionalizations upon substitution of the original prosthetic group with an artificial prosthetic group. However, all of the substitutions were driven by the coordination through the axial ligands followed by complete reconstitution with a metal-porphyrin complex. This limits the scope of the cytochrome c for incorporating a metal-less non-natural heme species that could improve the versatility of cytochrome c for a new generation of engineered cytochrome proteins for further enhancement in their functionalities such as biocatalysts. In this connection, a new variant of Cytochrome c (rC552 C14A) from Thermus thermophilus was reported, where an easy approach to remove the original prosthetic group was achieved, followed by the incorporation of a number of metal-PPIX derivatives that ultimately led to the formation of artificial c-type cytochromes through covalent bonding. The apo-cytochrome was found to be thermally tolerant and to possess a distinctive overall structure as that of the wild type, as was evident from the corresponding CD spectra, which ultimately encouraged reconstitution with a metal-less protoporphyrin derivative for better understanding the role of axial ligands in the reconstitution process. Successful reconstitution was achieved, resulting in a new type of Cytochrome b-type artificial protein without the metal in its active site, indicating the non-involvement of the axial ligand. In order to prove the non-involvement of the axial ligand, a subsequent double mutant (C14A/M69A) was constructed, replacing the methionine at 69 position with non-coordinating alanine residue. Accordingly, the apo-C14A/M69A was prepared and found to be extremely stable as the earlier mutants and the WT showed no signs of denaturation, even at the elevated temperature of 98°C. Subsequently, heme b was successfully incorporated into the apo-C14A/M69A, which demonstrated itself as a highly thermally tolerant protein scaffold for incorporating a metal-less artificial prosthetic group in the absence of the axial ligand. Further improvement in the reconstitution process is achieved by replacing the methionine at 69 position with phenyl alanine (C14A/M69F mutant), resulting in further stabilization of heme species, possibly through non-covalent π-interactions, as corroborated by molecular docking.

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.

Evolutionary Engineering of a Cp*Rh(III) Complex-Linked Artificial Metalloenzyme with a Chimeric β-Barrel Protein Scaffold

Evolutionary engineering of our previously reported Cp*Rh(III)-linked artificial metalloenzyme was performed based on a DNA recombination strategy to improve its catalytic activity toward C(sp2)-H bond functionalization. Improved scaffold design was achieved with α-helical cap domains of fatty acid binding protein (FABP) embedded within the β-barrel structure of nitrobindin (NB) as a chimeric protein scaffold for the artificial metalloenzyme. After optimization of the amino acid sequence by directed evolution methodology, an engineered variant, designated NBHLH1(Y119A/G149P) with enhanced performance and enhanced stability was obtained. Additional rounds of metalloenzyme evolution provided a Cp*Rh(III)-linked NBHLH1(Y119A/G149P) variant with a >35-fold increase in catalytic efficiency (kcat/KM) for cycloaddition of oxime and alkyne. Kinetic studies and MD simulations revealed that aromatic amino acid residues in the confined active-site form a hydrophobic core which binds to aromatic substrates adjacent to the Cp*Rh(III) complex. The metalloenzyme engineering process based on this DNA recombination strategy will serve as a powerful method for extensive optimization of the active-sites of artificial metalloenzymes.

Systematic engineering of Saccharomyces cerevisiae for efficient synthesis of hemoglobins and myoglobins

Hemoglobin (Hb) and myoglobin (Mb) are kinds of heme-binding proteins that play crucial physiological roles in different organisms. With rapid application development in food processing and biocatalysis, the requirement of biosynthetic Hb and Mb is increasing. However, the production of Hb and Mb is limited by the lower expressional level of globins and insufficient or improper heme supply. After selecting an inducible strategy for the expression of globins, removing the spatial barrier during heme synthesis, increasing the synthesis of 5-aminolevulinate and moderately enhancing heme synthetic rate-limiting steps, the microbial synthesis of bovine and porcine Hb was firstly achieved. Furthermore, an engineered Saccharomyces cerevisiae obtained a higher titer of soybean (108.2 ± 3.5 mg/L) and clover (13.7 ± 0.5 mg/L) Hb and bovine (68.9 ± 1.6 mg/L) and porcine (85.9 ± 5.0 mg/L) Mb. Therefore, this systematic engineering strategy will be useful to produce other hemoproteins or hemoenzymes with high activities.

Zinc Substituted Myoglobin-Albumin Fusion Protein: A Photosensitizer for Cancer Therapy

Myoglobin combined with human serum albumin (Mb-HSA) can be produced using yeast Pichia pastoris as a host strain, with secretion into the culture medium. This Mb-HSA fusion protein possesses identical O₂ binding affinity to that of naked Mb. The Mb unit is reconstituted with a zinc(II) protoporphyrin IX, yielding (zinc substituted Mb)-HSA, ZnMb-HSA. The photophysical property and singlet O₂ generation ability of ZnMb-HSA are equivalent to those of ZnMb. In vitro cell experiments revealed that ZnMb-HSA acts as a superior photosensitizer for photodynamic cancer therapy. It is noteworthy that ZnMb-HSA shows long circulation lifetime in vivo.

Phenoxazinone Synthase-like Activity of Rationally Designed Heme Enzymes Based on Myoglobin

The design of functional metalloenzymes is attractive for the biosynthesis of biologically important compounds, such as phenoxazinones and phenazines catalyzed by native phenoxazinone synthase (PHS). To design functional heme enzymes, we used myoglobin (Mb) as a model protein and introduced an artificial CXXC motif into the heme distal pocket by F46C and L49C mutations, which forms a de novo disulfide bond, as confirmed by the X-ray crystal structure. We further introduced a catalytic Tyr43 into the heme distal pocket and found that the F43Y/F46C/L49C Mb triple mutant and the previously designed F43Y/F46S Mb exhibit PHS-like activity (80-98% yields in 5-15 min), with the catalytic efficiency exceeding those of natural metalloenzymes, including o-aminophenol oxidase, laccase, and dye-decolorizing peroxidase. Moreover, we showed that the oxidative coupling product of 1,6-disulfonic-2,7-diaminophenazine is a potential pH indicator, with the orange-magenta color change at pH 4-5 (pK_a = 4.40). Therefore, this study indicates that functional heme enzymes can be rationally designed by structural modifications of Mb, exhibiting the functionality of the native PHS for green biosynthesis.

O2 Carrier Myoglobin Also Exhibits β-Lactamase Activity That Is Regulated by the Heme Coordination State

Heme proteins perform a variety of biological functions and also play significant roles in the field of bio-catalysis. The β-lactamase activity of heme proteins has rarely been reported. Herein, we found, for the first time, that myoglobin (Mb), an O₂ carrier, also exhibits novel β-lactamase activity by catalyzing the hydrolysis of ampicillin. The catalytic proficiency ((k_cat/K_M)/k_uncat) was determined to be 6.25 × 10¹⁰, which is much higher than the proficiency reported for designed metalloenzymes, although it is lower than that of natural β-lactamases. Moreover, we found that this activity could be regulated by an engineered disulfide bond, such as Cys46-Cys61 in F46C/L61C Mb or by the addition of imidazole to directly coordinate to the heme center. These results indicate that the heme active site is responsible for the β-lactamase activity of Mb. Therefore, the study suggests the potential of heme proteins acting as β-lactamases, which broadens the diversity of their catalytic functions.

Reactivity of nitrogen species with inorganic and organic compounds in water

Many studies on the reactive nitrogen species (RNS, ^●NO₂, ^●NO and ^●NH₂) with pollutants in water have been performed to understand the abatement of inorganic and organic compounds by these species, and the mechanisms of the formation of oxidative transformation products, especially nitrogenous oxidized byproducts. In this review, approaches to generate RNS in aqueous solution is first presented, followed by a summary of their reactivity with a wide range of compounds. The second-order rate constants (k, M^-1 s^-1) for the reactivity of ^●NO₂ and ^●NO with a wide range of inorganic radical and nonradical species were correlated with thermodynamic one-electron oxidation potentials (E⁰). The positive correlation between log(k) versus E⁰ suggests one-electron transfer reactions. The Hammett-type correlations were developed for the reactions of ^●NO₂ and ^●NH₂ with organic compounds, using the unsubstituted benzene as a reference molecule (i.e., Σσ_o,p,m = 0) to calculate Σσ_o,p,m = σ_o + σ_p + σ_m for each organic molecule. Linear negative correlations of log(k) with Σσ_o,p,m were obtained for both ^●NO₂ and ^●NH₂, suggesting electrophilic substitution mechanism. The correlations presented herein may assist in eliminating organic micropollutants in water treatment and reuse processes.

Functionalised Cofactor Mimics for Interactome Discovery and Beyond

Cofactors are required for almost half of all enzyme reactions, but their functions and binding partners are not fully understood even after decades of research. Functionalised cofactor mimics that bind in place of the unmodified cofactor can provide answers, as well as expand the scope of cofactor activity. Through chemical proteomics approaches such as activity-based protein profiling, the interactome and localisation of the native cofactor in its physiological environment can be deciphered and previously uncharacterised proteins annotated. Furthermore, cofactors that supply functional groups to substrate biomolecules can be hijacked by mimics to site-specifically label targets and unravel the complex biology of post-translational protein modification. The diverse activity of cofactors has inspired the design of mimics for use as inhibitors, antibiotic therapeutics, and chemo- and biosensors, and cofactor conjugates have enabled the generation of novel enzymes and artificial DNAzymes.

Reactivity of Myoglobin Reconstituted with Cobalt Corrole toward Hydrogen Peroxide

The protein matrix of natural metalloenzymes regulates the reactivity of metal complexes to establish unique catalysts. We describe the incorporation of a cobalt complex of corrole (CoCor), a trianionic porphyrinoid metal ligand, into an apo-form of myoglobin to provide a reconstituted protein (rMb(CoCor)). This protein was characterized by UV-vis, EPR, and mass spectroscopic measurements. The reaction of rMb(CoCor) with hydrogen peroxide promotes an irreversible oxidation of the CoCor cofactor, whereas the same reaction in the presence of a phenol derivative yields the cation radical form of CoCor. Detailed kinetic investigations indicate the formation of a transient hydroperoxo complex of rMb(CoCor) which promotes the oxidation of the phenol derivatives. This mechanism is significantly different for native heme-dependent peroxidases, which generate a metal-oxo species as an active intermediate in a reaction with hydrogen peroxide. The present findings of unique reactivity will contribute to further design of artificial metalloenzymes.

Focusing on a nickel hydrocorphinoid in a protein matrix: methane generation by methyl-coenzyme M reductase with F430 cofactor and its models

Methyl-coenzyme M reductase (MCR) containing a nickel hydrocorphinoid cofactor, F430, is an essential enzyme that catalyzes anaerobic methane generation and oxidation. The active Ni(I) species in MCR converts methyl-coenzyme M (CH₃S-CoM) and coenzyme B (HS-CoB) to methane and heterodisulfide (CoM-S-S-CoB). Extensive experimental and theoretical studies focusing on the substrate-binding cavity including the F430 cofactor in MCR have suggested two principally different reaction mechanisms involving an organonickel CH₃-Ni(III) species or a transient methyl radical species. In parallel with research on native MCR itself, the functionality of MCR has been investigated in the context of model complexes of F430 and recent protein-based functional models, which include a nickel complex. In the latter case, hemoproteins reconstituted with tetradehydro- and didehydrocorrinoid nickel complexes have been found to represent useful model systems that are responsible for methane generation. These efforts support the proposed mechanism of the enzymatic reaction and provide important insight into replicating the MCR-like methane-generation process. Furthermore, the modeling of MCR described here is expected to lead to understanding of protein-supported nickel porphyrinoid chemistry as well as the creation of MCR-inspired catalysis.

Controlling the optical and catalytic properties of artificial metalloenzyme photocatalysts using chemogenetic engineering

Visible light photocatalysis enables a broad range of organic transformations that proceed via single electron or energy transfer. Metal polypyridyl complexes are among the most commonly employed visible light photocatalysts. The photophysical properties of these complexes have been extensively studied and can be tuned by modifying the substituents on the pyridine ligands. On the other hand, ligand modifications that enable substrate binding to control reaction selectivity remain rare. Given the exquisite control that enzymes exert over electron and energy transfer processes in nature, we envisioned that artificial metalloenzymes (ArMs) created by incorporating Ru(ii) polypyridyl complexes into a suitable protein scaffold could provide a means to control photocatalyst properties. This study describes approaches to create covalent and non-covalent ArMs from a variety of Ru(ii) polypyridyl cofactors and a prolyl oligopeptidase scaffold. A panel of ArMs with enhanced photophysical properties were engineered, and the nature of the scaffold/cofactor interactions in these systems was investigated. These ArMs provided higher yields and rates than Ru(Bpy)3 2+ for the reductive cyclization of dienones and the [2 + 2] photocycloaddition between C-cinnamoyl imidazole and 4-methoxystyrene, suggesting that protein scaffolds could provide a means to improve the efficiency of visible light photocatalysts.

Nanosized porous artificial enzyme as a pH-sensitive doxorubicin delivery system for joint enzymatic and chemotherapy towards tumor treatment

A porous spherical artificial nanozyme (HF-900) prepared via pyrolysis of a porous organic polymer was used as drug carrier for efficient loading and highly selective pH-responsive delivery of doxorubicin (DOX) for the tumor joint nanotherapy.

3,6,13,16-Tetraalkylporphycenes: Synthesis and Exploration of Effect of Alkyl Groups on Structure, Photophysical Properties, and Basicity

Two new 3,6,13,16-tetraalkylporphycenes were synthesized following rational approach. The reason behind lower yield of the desired β,β'-bipyrroles was unraveled. The σ-donating effect of alkyl-substituents was more profound than reported positional...

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.

Nitrene transfers mediated by natural and artificial iron enzymes

Amines are ubiquitous in biology and pharmacy. As a consequence, introducing N functionalities in organic molecules is attracting strong continuous interest. The past decade has witnessed the emergence of very efficient and selective catalytic systems achieving this goal thanks to engineered hemoproteins. In this review, we examine how these enzymes have been engineered focusing rather on the rationale behind it than the methodology employed. These studies are put in perspective with respect to in vitro and in vivo nitrene transfer processes performed by cytochromes P450. An emphasis is put on mechanistic aspects which are confronted to current molecular knowledge of these reactions. Forthcoming developments are delineated.