Design and engineering of artificial oxygen-activating metalloenzymes

57Fe nuclear resonance vibrational spectroscopic studies of tetranuclear iron clusters bearing terminal iron(iii)-oxido/hydroxido moieties

57Fe nuclear resonance vibrational spectroscopy (NRVS) has been applied to study a series of tetranuclear iron ([Fe4]) clusters based on a multidentate ligand platform (L3-) anchored by a 1,3,5-triarylbenzene linker and pyrazolate or (tertbutylamino)pyrazolate ligand (PzNH t Bu-). These clusters bear a terminal Fe(iii)-O/OH moiety at the apical position and three additional iron centers forming the basal positions. The three basal irons are connected with the apical iron center via a μ4-oxido ligand. Detailed vibrational analysis via density functional theory calculations revealed that strong NRVS spectral features below 400 cm-1 can be used as an oxidation state marker for the overall [Fe4] cluster core. The terminal Fe(iii)-O/OH stretching frequencies, which were observed in the range of 500-700 cm-1, can be strongly modulated (energy shifts of 20-40 cm-1 were observed) upon redox events at the three remote basal iron centers of the [Fe4] cluster without the change of the terminal Fe(iii) oxidation state and its coordination environment. Therefore, the current study provides a quantitative vibrational analysis of how the remote iron centers within the same iron cluster exert exquisite control of the chemical reactivities and thermodynamic properties of the specific iron site that is responsible for small molecule activation.

Exploring the influence of H-bonding and ligand constraints on thiolate ligated non-heme iron mediated dioxygen activation

Converting triplet dioxygen into a powerful oxidant is fundamentally important to life. The study reported herein quantitatively examines the formation of a well-characterized, reactive, O2-derived thiolate ligated FeIII-superoxo using low-temperature stopped-flow kinetics. Comparison of the kinetic barriers to the formation of this species via two routes, involving either the addition of (a) O2 to [FeII(S2 Me2N3(Pr,Pr))] (1) or (b) superoxide to [FeIII(S2 Me2N3(Pr,Pr))]+ (3) is shown to provide insight into the mechanism of O2 activation. Route (b) was shown to be significantly slower, and the kinetic barrier 14.9 kJ mol-1 higher than route (a), implying that dioxygen activation involves inner-sphere, as opposed to outer sphere, electron transfer from Fe(ii). H-bond donors and ligand constraints are shown to dramatically influence O2 binding kinetics and reversibility. Dioxygen binds irreversibly to [FeII(S2 Me2N3(Pr,Pr))] (1) in tetrahydrofuran, but reversibly in methanol. Hydrogen bonding decreases the ability of the thiolate sulfur to stabilize the transition state and the FeIII-superoxo, as shown by the 10 kJ mol-1 increase in the kinetic barrier to O2 binding in methanol vs. tetrahydrofuran. Dioxygen release from [FeIII(S2 Me2N3(Pr,Pr))O2] (2) is shown to be 24 kJ mol-1 higher relative to previously reported [FeIII(SMe2N4(tren))(O2)]+ (5), the latter of which contains a more flexible ligand. These kinetic results afford an experimentally determined reaction coordinate that illustrates the influence of H-bonding and ligand constraints on the kinetic barrier to dioxygen activation an essential step in biosynthetic pathways critical to life.

Fresh Impetus in the Chemistry of Calcium Peroxides

Redox-inactive metal ions are essential in modulating the reactivity of various oxygen-containing metal complexes and metalloenzymes, including photosystem II (PSII). The heart of this unique membrane-protein complex comprises the Mn4CaO5 cluster, in which the Ca2+ ion acts as a critical cofactor in the splitting of water in PSII. However, there is still a lack of studies involving Ca-based reactive oxygen species (ROS) systems, and the exact nature of the interaction between the Ca2+ center and ROS in PSII still generates intense debate. Here, harnessing a novel Ca-TEMPO complex supported by the β-diketiminate ligand to control the activation of O2, we report the isolation and structural characterization of hitherto elusive Ca peroxides, a homometallic Ca hydroperoxide and a heterometallic Ca/K peroxide. Our studies indicate that the presence of K+ cations is a key factor controlling the outcome of the oxygenation reaction of the model Ca-TEMPO complex. Combining experimental observations with computational investigations, we also propose a mechanistic rationalization for the reaction outcomes. The designed approach demonstrates metal-TEMPO complexes as a versatile platform for O2 activation and advances the understanding of Ca/ROS systems.

Artificial Manganese Metalloenzymes with Laccase-like Activity: Design, Synthesis, and Characterization

Laccase is an oxidase of great industrial interest due to its ability to catalyze oxidation processes of phenols and persistent organic pollutants. However, it is susceptible to denaturation at high temperatures, sensitive to pH, and unstable in the presence of high concentrations of solvents, which is a issue for industrial use. To solve this problem, this work develops the synthesis in an aqueous medium of a new Mn metalloenzyme with laccase oxidase mimetic catalytic activity. Geobacillus thermocatenulatus lipase (GTL) was used as a scaffold enzyme, mixed with a manganese salt at 50 °C in an aqueous medium. This leads to the in situ formation of manganese(IV) oxide nanowires that interact with the enzyme, yielding a GTL-Mn bionanohybrid. On the other hand, its oxidative activity was evaluated using the ABTS assay, obtaining a catalytic efficiency 300 times higher than that of Trametes versicolor laccase. This new Mn metalloenzyme was 2 times more stable at 40 °C, 3 times more stable in the presence of 10% acetonitrile, and 10 times more stable in 20% acetonitrile than Novozym 51003 laccase. Furthermore, the site-selective immobilized GTL-Mn showed a much higher stability than the soluble form. The oxidase-like activity of this Mn metalloenzyme was successfully demonstrated against other substrates, such as l-DOPA or phloridzin, in oligomerization reactions.

Biohybrid materials comprising an artificial peroxidase and differently shaped gold nanoparticles

The immobilization of biocatalysts on inorganic supports allows the development of bio-nanohybrid materials with defined functional properties. Gold nanomaterials (AuNMs) are the main players in this field, due to their fascinating shape-dependent properties that account for their versatility. Even though incredible progress has been made in the preparation of AuNMs, few studies have been carried out to analyze the impact of particle morphology on the behavior of immobilized biocatalysts. Herein, the artificial peroxidase Fe(iii)-Mimochrome VI*a (FeMC6*a) was conjugated to two different anisotropic gold nanomaterials, nanorods (AuNRs) and triangular nanoprisms (AuNTs), to investigate how the properties of the nanosupport can affect the functional behavior of FeMC6*a. The conjugation of FeMC6*a to AuNMs was performed by a click-chemistry approach, using FeMC6*a modified with pegylated aza-dibenzocyclooctyne (FeMC6*a-PEG4@DBCO), which was allowed to react with azide-functionalized AuNRs and AuNTs, synthesized from citrate-capped AuNMs. To this end, a literature protocol for depleting CTAB from AuNRs was herein reported for the first time to prepare citrate-AuNTs. The overall results suggest that the nanomaterial shape influences the nanoconjugate functional properties. Besides giving new insights into the effect of the surfaces on the artificial peroxidase properties, these results open up the way for creating novel nanostructures with potential applications in the field of sensing devices.

Design of a gold nanoparticles site in an engineered lipase: an artificial metalloenzyme with enantioselective reductase-like activity

The conjugation of gold complexes with proteins has proved to be interesting and effective in obtaining artificial metalloenzymes as catalysts with improved properties such as higher stability, activity and selectivity. However, the design and precise regulation of their structure as protein nanostructured forms level remains a challenge. Here, we have designed and constructed a gold nanoparticles-enzyme bioconjugate, by tailoring the in situ formation of gold nanoparticles (AuNPs) at two specific sites on the structure of an alkalophilic lipase from Geobacillus thermocatenulatus (GTL). For this purpose, two genetically modified variants of GTL were created by inserting a unique cysteine residue into the catalytic active site by replacing the active serine (GTL-114) and into the lid site (GTL-193). The enzyme, after a first protein-gold coordination, induced the in situ formation of AuNPs, generating a homogeneous artificial enzyme. The size and morphology of the nanoparticles in the AuNPs-enzyme conjugate have been controlled by specific pH conditions in synthesis and the specific protein region where they are formed. Reductase activity of all of them was confirmed in the hydrogenation of nitroarenes in aqueous media. The protein area seemed to be key for the AuNPs, with the best TOF values obtained for the bioconjugates with AuNPs in the lid site. Finally, the protein environment and the asymmetric properties of the AuNPs were tested in the reduction of acetophenone to 1-phenylethanol in aqueous medium at room temperature. A high reductive conversion and an enantiomeric excess of up to 39% towards (R)-1-phenylethanol was found using Au-Mt@GTL-114 pH 10 as a catalyst. Moderate enantioselectivity towards the opposite isomer was also observed using the Au-Mt@GTL-193 pH 10 conjugate.

Atomic-level design of metalloenzyme-like active pockets in metal-organic frameworks for bioinspired catalysis

Natural metalloenzymes with astonishing reaction activity and specificity underpin essential life transformations. Nevertheless, enzymes only operate under mild conditions to keep sophisticated structures active, limiting their potential applications. Artificial metalloenzymes that recapitulate the catalytic activity of enzymes can not only circumvent the enzymatic fragility but also bring versatile functions into practice. Among them, metal-organic frameworks (MOFs) featuring diverse and site-isolated metal sites and supramolecular structures have emerged as promising candidates for metalloenzymes to move toward unparalleled properties and behaviour of enzymes. In this review, we systematically summarize the significant advances in MOF-based metalloenzyme mimics with a special emphasis on active pocket engineering at the atomic level, including primary catalytic sites and secondary coordination spheres. Then, the deep understanding of catalytic mechanisms and their advanced applications are discussed. Finally, a perspective on this emerging frontier research is provided to advance bioinspired catalysis.

Heme-copper and Heme O2-derived synthetic (bioinorganic) chemistry toward an understanding of cytochrome c oxidase dioxygen chemistry

Cytochrome c oxidase (CcO), also widely known as mitochondrial electron-transport-chain complex IV, is a multi-subunit transmembrane protein responsible for catalyzing the last step of the electron transport chain, dioxygen reduction to water, which is essential to the establishment and maintenance of the membrane proton gradient that drives ATP synthesis. Although many intermediates in the CcO catalytic cycle have been spectroscopically and/or computationally authenticated, the specifics regarding the IP intermediate, hypothesized to be a heme-Cu (hydro)peroxo species whose O-O bond homolysis is supported by a hydrogen-bonding network of water molecules, are largely obscured by the fast kinetics of the A (FeIII-O2•-/CuI/Tyr) → PM (FeIV=O/CuII-OH/Tyr•) step. In this review, we have focused on the recent advancements in the design, development, and characterization of synthetic heme-peroxo‑copper model complexes, which can circumvent the abovementioned limitation, for the investigation of the formation of IP and its O-O cleavage chemistry. Novel findings regarding (a) proton and electron transfer (PT/ET) processes, together with their contributions to exogenous phenol induced O-O cleavage, (b) the stereo-electronic tunability of the secondary coordination sphere (especially hydrogen-bonding) on the geometric and spin state alteration of the heme-peroxo‑copper unit, and (c) a plausible mechanism for the Tyr-His cofactor biogenesis, are discussed in great detail. Additionally, since the ferric-superoxide and the ferryl-oxo (Compound II) species are critically involved in the CcO catalytic cycle, this review also highlights a few fundamental aspects of these heme-only (i.e., without copper) species, including the structural and reactivity influences of electron-donating trans-axial ligands and Lewis acid-promoted H-bonding.

An Artificial Miniaturized Peroxidase for Signal Amplification in Lateral Flow Immunoassays

Signal amplification strategies are widely used for improving the sensitivity of lateral flow immunoassays (LFiAs). Herein, the artificial miniaturized peroxidase Fe(III)-MimochromeVI*a (FeMC6*a), immobilized on gold nanoparticles (AuNPs), is used as a strategy to obtain catalytic signal amplification in sandwich immunoassays on lateral flow strips. The assay scheme uses AuNPs decorated with the mini-peroxidase FeMC6*a and anti-human-IgG as a detection antibody (dAb), for the detection of human-IgG, as a model analyte. Recognition of the analyte by the capture and detection antibodies is first evidenced by the appearance of a red color in the test line (TL), due to the accumulation of AuNPs. Subsequent addition of 3,3',5,5'-tetramethylbenzidine (TMB) induces an increase of the test line color, due to the TMB being converted into an insoluble colored product, catalyzed by FeMC6*a. This work shows that FeMC6*a acts as an efficient catalyst in paper, increasing the sensitivity of an LFiA up to four times with respect to a conventional LFiA. Furthermore, FeMC6*a achieves lower limits of detection that are found in control experiments where it is replaced with horseradish peroxidase (HRP), its natural counterpart. This study represents a significant proof-of-concept for the development of more sensitive LFiAs, for different analytes, based on properly designed artificial metalloenzymes.

Distinct chemical factors in hydrolytic reactions catalyzed by metalloenzymes and metal complexes

The selective hydrolysis of the extremely stable phosphoester, peptide and ester bonds of molecules by bio-inspired metal-based catalysts (metallohydrolases) is required in a wide range of biological, biotechnological and industrial applications. Despite the impressive advances made in the field, the ultimate goal of designing efficient enzyme mimics for these reactions is still elusive. Its realization will require a deeper understanding of the diverse chemical factors that influence the activities of both natural and synthetic catalysts. They include catalyst-substrate complexation, non-covalent interactions and the electronic nature of the metal ion, ligand environment and nucleophile. Based on our computational studies, their roles are discussed for several mono- and binuclear metallohydrolases and their synthetic analogues. Hydrolysis by natural metallohydrolases is found to be promoted by a ligand environment with low basicity, a metal bound water and a heterobinuclear metal center (in binuclear enzymes). Additionally, peptide and phosphoester hydrolysis is dominated by two competing effects, i.e. nucleophilicity and Lewis acid activation, respectively. In synthetic analogues, hydrolysis is facilitated by the inclusion of a second metal center, hydrophobic effects, a biological metal (Zn, Cu and Co) and a terminal hydroxyl nucleophile. Due to the absence of the protein environment, hydrolysis by these small molecules is exclusively influenced by nucleophile activation. The results gleaned from these studies will enhance the understanding of fundamental principles of multiple hydrolytic reactions. They will also advance the development of computational methods as a predictive tool to design more efficient catalysts for hydrolysis, Diels-Alder reaction, Michael addition, epoxide opening and aldol condensation.

Selective Oxidation of Halophenols Catalyzed by an Artificial Miniaturized Peroxidase

The development of artificial enzymes for application in sustainable technologies, such as the transformation of environmental pollutants or biomass, is one of the most challenging goals in metalloenzyme design. In this work, we describe the oxidation of mono-, di-, tri- and penta-halogenated phenols catalyzed by the artificial metalloenzyme Fe-MC6*a. It promoted the dehalogenation of 4-fluorophenol into the corresponding 1,4-benzoquinone, while under the same experimental conditions, 4-chloro, 4-bromo and 4-iodophenol were selectively converted into higher molecular weight compounds. Analysis of the 4-chlorophenol oxidation products clarified that oligomers based on C-O bonds were exclusively formed in this case. All results show that Fe-MC6*a holds intriguing enzymatic properties, as it catalyzes halophenol oxidation with substrate-dependent chemoselectivity.

Chimeric Biocatalyst Combining Peptidic and Nucleic Acid Components Overcomes the Performance and Limitations of the Native Horseradish Peroxidase

Chimeric peptide-DNAzyme (CPDzyme) is a novel artificial peroxidase that relies on the covalent assembly of DNA, peptides, and an enzyme cofactor in a single scaffold. An accurate control of the assembly of these different partners allows for the design of the CPDzyme prototype G4-Hemin-KHRRH, found to be >2000-fold more active (in terms of conversion number k_cat) than the corresponding but non-covalent G4/Hemin complex and, more importantly, >1.5-fold more active than the corresponding native peroxidase (horseradish peroxidase) when considering a single catalytic center. This unique performance originates in a series of gradual improvements, thanks to an accurate selection and arrangement of the different components of the CPDzyme, in order to benefit from synergistic interactions between them. The optimized prototype G4-Hemin-KHRRH is efficient and robust as it can be used under a wide range of non-physiologically relevant conditions [organic solvents, high temperature (95 °C), and in a wide range of pH (from 2 to 10)], thus compensating for the shortcomings of the natural enzymes. Our approach thus opens broad prospects for the design of ever more efficient artificial enzymes.

Versatile Synthesis of Multivalent Porphyrin-Peptide Conjugates by Direct Porphyrin Construction on Resin

Multifunctional porphyrin-peptide conjugates with different propensities for self-assembly into various supramolecular nanoarchitectures play important roles in advanced materials and biomedical research. However, preparing prefunctionalized core porphyrins by traditional low-yielding statistical synthesis and purifying them after peptide ligation through many rounds of HPLC purification is tedious and unsustainable. Herein, we report a novel integrated solid-phase synthetic protocol for the construction of porphyrin moieties from simple aldehydes and dipyrromethanes on resin-bound peptides directly to form mono-, cis/trans-di-, and trivalent porphyrin-peptide conjugates in a highly efficient and controllable manner; moreover, only single final-stage HPLC purification of the products is needed. This efficient strategy enables the rapid, greener, and substrate-controlled diversity-oriented synthesis of multivalent porphyrin-(long) peptide conjugate libraries for multifarious biological and materials applications.

Roles of Metal Ions in Foldamers and Other Conformationally Flexible Supramolecular Systems

Conformational control is a key prerequisite for much molecular function. As chemists seek to create complex molecules that have applications beyond the academic laboratory, correct spatial positioning is critical. This is particularly true of flexible systems. Conformationally flexible molecules show potential because they resemble in many cases naturally occurring analogues such as the secondary structures found in proteins and peptides such as α-helices and β-sheets. One of the ways in which conformation can be controlled in these molecules is through interaction with or coordination to metal ions. This review explores how secondary structure (i.e., controlled local conformation) in foldamers and other conformationally flexible systems can be enforced or modified through coordination to metal ions. We hope to provide examples that illustrate the power of metal ions to influence this structure toward multiple different outcomes.

Isolating Fe-O2 Intermediates in Dioxygen Activation by Iron Porphyrin Complexes

Dioxygen (O2) is an environmentally benign and abundant oxidant whose utilization is of great interest in the design of bioinspired synthetic catalytic oxidation systems to reduce energy consumption. However, it is unfortunate that utilization of O2 is a significant challenge because of the thermodynamic stability of O2 in its triplet ground state. Nevertheless, nature is able to overcome the spin state barrier using enzymes, which contain transition metals with unpaired d-electrons facilitating the activation of O2 by metal coordination. This inspires bioinorganic chemists to synthesize biomimetic small-molecule iron porphyrin complexes to carry out the O2 activation, wherein Fe-O2 species have been implicated as the key reactive intermediates. In recent years, a number of Fe-O2 intermediates have been synthesized by activating O2 at iron centers supported on porphyrin ligands. In this review, we focus on a few examples of these advances with emphasis in each case on the particular design of iron porphyrin complexes and particular reaction environments to stabilize and isolate metal-O2 intermediates in dioxygen activation, which will provide clues to elucidate structures of reactive intermediates and mechanistic insights in biological processes.

Designing Artificial Metalloenzymes by Tuning of the Environment beyond the Primary Coordination Sphere

Metalloenzymes catalyze a variety of reactions using a limited number of natural amino acids and metallocofactors. Therefore, the environment beyond the primary coordination sphere must play an important role in both conferring and tuning their phenomenal catalytic properties, enabling active sites with otherwise similar primary coordination environments to perform a diverse array of biological functions. However, since the interactions beyond the primary coordination sphere are numerous and weak, it has been difficult to pinpoint structural features responsible for the tuning of activities of native enzymes. Designing artificial metalloenzymes (ArMs) offers an excellent basis to elucidate the roles of these interactions and to further develop practical biological catalysts. In this review, we highlight how the secondary coordination spheres of ArMs influence metal binding and catalysis, with particular focus on the use of native protein scaffolds as templates for the design of ArMs by either rational design aided by computational modeling, directed evolution, or a combination of both approaches. In describing successes in designing heme, nonheme Fe, and Cu metalloenzymes, heteronuclear metalloenzymes containing heme, and those ArMs containing other metal centers (including those with non-native metal ions and metallocofactors), we have summarized insights gained on how careful controls of the interactions in the secondary coordination sphere, including hydrophobic and hydrogen bonding interactions, allow the generation and tuning of these respective systems to approach, rival, and, in a few cases, exceed those of native enzymes. We have also provided an outlook on the remaining challenges in the field and future directions that will allow for a deeper understanding of the secondary coordination sphere a deeper understanding of the secondary coordintion sphere to be gained, and in turn to guide the design of a broader and more efficient variety of ArMs.

Supramolecular Hybrids from Cyanometallate Complexes and Diblock Copolypeptide Amphiphiles in Water

The self-assembly of discrete cyanometallates has attracted significant interest due to the potential of these materials to undergo soft metallophilic interactions as well as their optical properties. Diblock copolypeptide amphiphiles have also been investigated concerning their capacity for self-assembly into morphologies such as nanostructures. The present work combined these two concepts by examining supramolecular hybrids comprising cyanometallates with diblock copolypeptide amphiphiles in aqueous solutions. Discrete cyanometallates such as [Au(CN)2]-, [Ag(CN)2]-, and [Pt(CN)4]2- dispersed at the molecular level in water cannot interact with each other at low concentrations. However, the results of this work demonstrate that the addition of diblock copolypeptide amphiphiles such as poly-(L-lysine)-block-(L-cysteine) (Lysm-b-Cysn) to solutions of these complexes induces the supramolecular assembly of the discrete cyanometallates, resulting in photoluminescence originating from multinuclear complexes with metal-metal interactions. Electron microscopy images confirmed the formation of nanostructures of several hundred nanometers in size that grew to form advanced nanoarchitectures, including those resembling the original nanostructures. This concept of combining diblock copolypeptide amphiphiles with discrete cyanometallates allows the design of flexible and functional supramolecular hybrid systems in water.

Second Sphere Effects on Oxygen Reduction and Peroxide Activation by Mononuclear Iron Porphyrins and Related Systems

Activation and reduction of O₂ and H₂O₂ by synthetic and biosynthetic iron porphyrin models have proved to be a versatile platform for evaluating second-sphere effects deemed important in naturally occurring heme active sites. Advances in synthetic techniques have made it possible to install different functional groups around the porphyrin ligand, recreating artificial analogues of the proximal and distal sites encountered in the heme proteins. Using judicious choices of these substituents, several of the elegant second-sphere effects that are proposed to be important in the reactivity of key heme proteins have been evaluated under controlled environments, adding fundamental insight into the roles played by these weak interactions in nature. This review presents a detailed description of these efforts and how these have not only demystified these second-sphere effects but also how the knowledge obtained resulted in functional mimics of these heme enzymes.

Strategies for the application of metal-organic frameworks in catalytic reactions

Efficient catalysts play crucial roles in various organic reactions and polymerization. Metal–organic frameworks (MOFs) have the merits of ultrahigh porosity, large surface area, dispersed polymetallic sites and modifiable linkers, which make them promising candidates for catalyzation. This review primarily summarizes the recent research progress on diverse strategies for tailoring MOFs that are endowed with excellent catalytic behavior. These strategies include utilizing MOFs as nanosized reaction channels, metal nodes decorated as catalytic active sites and the modification of ligands or linkers. All these make them highly attractive to various applications, especially in catalyzing organic reactions or polymerizations and they have proven to be effective catalysts for a wide variety of reactions. MOFs are still an evolving field with tremendous prospects; therefore, through the research and development of more modification and regulation strategies, MOFs will realize their wider practical application in the future.

Metal–organic frameworks (MOFs) are promising candidates for catalyzation. This review primarily summarized the recent research progress in diverse strategies for tailoring MOFs which are endowed with more excellent catalytic behavior.

A Unique P450 Peroxygenase System Facilitated by a Dual-Functional Small Molecule: Concept, Application, and Perspective

Cytochrome P450 monooxygenases (P450s) are promising versatile oxidative biocatalysts. However, the practical use of P450s in vitro is limited by their dependence on the co-enzyme NAD(P)H and the complex electron transport system. Using H₂O₂ simplifies the catalytic cycle of P450s; however, most P450s are inactive in the presence of H₂O₂. By mimicking the molecular structure and catalytic mechanism of natural peroxygenases and peroxidases, an artificial P450 peroxygenase system has been designed with the assistance of a dual-functional small molecule (DFSM). DFSMs, such as N-(ω-imidazolyl fatty acyl)-l-amino acids, use an acyl amino acid as an anchoring group to bind the enzyme, and the imidazolyl group at the other end functions as a general acid-base catalyst in the activation of H₂O₂. In combination with protein engineering, the DFSM-facilitated P450 peroxygenase system has been used in various oxidation reactions of non-native substrates, such as alkene epoxidation, thioanisole sulfoxidation, and alkanes and aromatic hydroxylation, which showed unique activities and selectivity. Moreover, the DFSM-facilitated P450 peroxygenase system can switch to the peroxidase mode by mechanism-guided protein engineering. In this short review, the design, mechanism, evolution, application, and perspective of these novel non-natural P450 peroxygenases for the oxidation of non-native substrates are discussed.

Functional Conversion of Acetyl-Coenzyme a Synthase to a Nickel Superoxide Dismutase via Rational Design of Coordination Microenvironment for the Nid-Site

The Ni_d site coordination microenvironment of a truncated acetyl-coenzyme A synthase has been designed systematically for functional conversion to a Ni-SOD-like enzyme. To this end, the first strategy is to introduce an axial histidine ligand, using mutations F598H, S594H and S594H-GP individually. The resulting three mutants obtained Ni-SOD-like activity successfully, although the catalytic activity was about 10-fold lower than in native Ni-SOD. The second strategy is to mimic the H-bond network in the second sphere coordination microenvironment of the native Ni-SOD. Two mutations based on F598H (EFG-F598H and YGP-F598H) were designed. The successful EFG-F598H exhibited ~3-fold Ni-SOD-like activity of F598H. These designed Ni-SOD-like metalloproteins were characterized by UV/Vis, EPR and Cyclic voltammetry while F598H was also characterized by X-ray protein crystallography. The pH titrations were performed to reveal the source of the two protons required for forming H₂O₂ in the SOD catalytic reaction. Based on all of the results, a proposed catalytic mechanism for the Ni-SOD-like metalloproteins is presented.

Directed Evolution of a Nonheme Diiron N-oxygenase AzoC for Improving Its Catalytic Efficiency toward Nitrogen Heterocycle Substrates

The azoxy compounds with an intriguing chemical bond [-N=N⁺(-O⁻)-] are known to have broad applications in many industries. Our previous work revealed that a nonheme diiron N-oxygenase AzoC catalyzed the oxidization of amino-group to its nitroso analogue in the formation of azoxy bond in azoxymycins biosynthesis. However, except for the reported pyridine alkaloid azoxy compounds, most azoxy bonds of nitrogen heterocycles have not been biosynthesized so far, and the substrate scope of AzoC is limited to p-aminobenzene-type compounds. Therefore, it is very meaningful to use AzoC to realize the biosynthesis of azoxy nitrogen heterocycles compounds. In this work, we further studied the catalytic potential of AzoC toward nitrogen heterocycle substrates including 5-aminopyrimidine and 5-aminopyridine compounds to form new azoxy compounds through directed evolution. We constructed a double mutant L101I/Q104R via molecular engineering with improved catalytic efficiency toward 2-methoxypyrimidin-5-amine. These mutations also proved to be beneficial for N-oxygenation of methyl 5-aminopyrimidine-2-carboxylate. The structural analysis showed that relatively shorter distance between the substrate and the diiron center and amino acid residues of the active center may be responsible for the improvement of catalytic efficiency in L101I/Q104R. Our results provide a molecular basis for broadening the AzoC catalytic activity and its application in the biosynthesis of azoxy six-membered nitrogen catenation compounds.

Ferritin-Like Proteins: A Conserved Core for a Myriad of Enzyme Complexes

Ferritin-like proteins share a common fold, a four α-helix bundle core, often coordinating a pair of metal ions. Although conserved, the ferritin fold permits a diverse set of reactions, and is central in a multitude of macromolecular enzyme complexes. Here, we emphasize this diversity through three members of the ferritin-like superfamily: the soluble methane monooxygenase, the class I ribonucleotide reductase and the aldehyde deformylating oxygenase. They all rely on dinuclear metal cofactors to catalyze different challenging oxygen-dependent reactions through the formation of multi-protein complexes. Recent studies using cryo-electron microscopy, serial femtosecond crystallography at an X-ray free electron laser source, or single-crystal X-ray diffraction, have reported the structures of the active protein complexes, and revealed unprecedented insights into the molecular mechanisms of these three enzymes.

Oxidative dehalogenation of trichlorophenol catalyzed by a promiscuous artificial heme-enzyme

The miniaturized metalloenzyme Fe(iii)-mimochrome VI*a (Fe(iii)-MC6*a) acts as an excellent biocatalyst in the H₂O₂-mediated oxidative dehalogenation of the well-known pesticide and biocide 2,4,6-trichlorophenol (TCP). The artificial enzyme can oxidize TCP with a catalytic efficiency (k_cat/K^TCP_m = 150 000 mM⁻¹ s⁻¹) up to 1500-fold higher than the most active natural metalloenzyme horseradish peroxidase (HRP). UV-visible and EPR spectroscopies were used to provide indications of the catalytic mechanism. One equivalent of H₂O₂ fully converts Fe(iii)-MC6*a into the oxoferryl-porphyrin radical cation intermediate [(Fe(iv) Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O)por˙⁺], similarly to peroxidase compound I (Cpd I). Addition of TCP to Cpd I rapidly leads to the formation of the corresponding quinone, while Cpd I decays back to the ferric resting state in the absence of substrate. EPR data suggest a catalytic mechanism involving two consecutive one-electron reactions. All results highlight the value of the miniaturization strategy for the development of chemically stable, highly efficient artificial metalloenzymes as powerful catalysts for the oxidative degradation of toxic pollutants.

The artificial metalloenzyme FeMC6*a is able to perform the H₂O₂-mediated dechlorination of 2,4,6-trichlorophenol with unrivalled catalytic efficiency, highlighting its potential application for the removal of toxic pollutants.

De novo metalloprotein design

Natural metalloproteins perform many functions - ranging from sensing to electron transfer and catalysis - in which the position and property of each ligand and metal, is dictated by protein structure. De novo protein design aims to define an amino acid sequence that encodes a specific structure and function, providing a critical test of the hypothetical inner workings of (metallo)proteins. To date, de novo metalloproteins have used simple, symmetric tertiary structures - uncomplicated by the large size and evolutionary marks of natural proteins - to interrogate structure-function hypotheses. In this Review, we discuss de novo design applications, such as proteins that induce complex, increasingly asymmetric ligand geometries to achieve function, as well as the use of more canonical ligand geometries to achieve stability. De novo design has been used to explore how proteins fine-tune redox potentials and catalyse both oxidative and hydrolytic reactions. With an increased understanding of structure-function relationships, functional proteins including O2-dependent oxidases, fast hydrolases, and multi-proton/multi-electron reductases, have been created. In addition, proteins can now be designed using xeno-biological metals or cofactors and principles from inorganic chemistry to derive new-to-nature functions. These results and the advances in computational protein design suggest a bright future for the de novo design of diverse, functional metalloproteins.

Protein Assembly by Design

Proteins are nature's primary building blocks for the construction of sophisticated molecular machines and dynamic materials, ranging from protein complexes such as photosystem II and nitrogenase that drive biogeochemical cycles to cytoskeletal assemblies and muscle fibers for motion. Such natural systems have inspired extensive efforts in the rational design of artificial protein assemblies in the last two decades. As molecular building blocks, proteins are highly complex, in terms of both their three-dimensional structures and chemical compositions. To enable control over the self-assembly of such complex molecules, scientists have devised many creative strategies by combining tools and principles of experimental and computational biophysics, supramolecular chemistry, inorganic chemistry, materials science, and polymer chemistry, among others. Owing to these innovative strategies, what started as a purely structure-building exercise two decades ago has, in short order, led to artificial protein assemblies with unprecedented structures and functions and protein-based materials with unusual properties. Our goal in this review is to give an overview of this exciting and highly interdisciplinary area of research, first outlining the design strategies and tools that have been devised for controlling protein self-assembly, then describing the diverse structures of artificial protein assemblies, and finally highlighting the emergent properties and functions of these assemblies.

Modular Assembly of Tumor-Penetrating and Oligomeric Nanozyme Based on Intrinsically Self-Assembling Protein Nanocages

Biomimetic design of nanomaterials with enzyme-like characteristics has emerged as a promising method for the generation of novel therapeutics. However, synthesis of nanomaterials while maintaining a high degree of control over both geometry and valency poses a prominent challenge. Herein, the authors introduce a nanomaterial-based synthetic biology strategy for accurate and quantitative tailoring of high-ordered nanostructures that uses a "bottom-up" hierarchical incorporation of protein building blocks. The assembled nano-oligomers possessed tunable protein motifs and multivalent binding domains, which facilitated prolonged blood circulation time, accumulation within tumor cells through direct targeting of cell receptors, and deep tumor tissue penetration via a transcytosis mechanism. Using these protein/protein nano-oligomers as scaffolds, the authors created a new series of artificial nano-scaled metalloenzymes (nanozymes) by the in situ incorporation of metal nanoclusters within the cavity of the protein nanocages. Nanozymes were capable of mimicking peroxidase-like activity and generated cytotoxic free radicals. Compared to nanozyme alone, the systemic delivery of oligomeric nanozymes demonstrated significantly enhanced therapeutic and anti-tumor benefits. This study shows a new insight into nanotechnology by taking advantage of synthetic biotechnology.

Chemical modifications of proteins and their applications in metalloenzyme studies

Protein chemical modifications are important tools for elucidating chemical and biological functions of proteins. Several strategies have been developed to implement these modifications, including enzymatic tailoring reactions, unnatural amino acid incorporation using the expanded genetic codes, and recognition-driven transformations. These technologies have been applied in metalloenzyme studies, specifically in dissecting their mechanisms, improving their enzymatic activities, and creating artificial enzymes with non-natural activities. Herein, we summarize some of the recent efforts in these areas with an emphasis on a few metalloenzyme case studies.

Biomimetic Design of Mitochondria-Targeted Hybrid Nanozymes as Superoxide Scavengers

Development of enzyme mimics for the scavenging of excessive mitochondrial superoxide (O₂ ^•- ) can serve as an effective strategy in the treatment of many diseases. Here, protein reconstruction technology and nanotechnology is taken advantage of to biomimetically create an artificial hybrid nanozyme. These nanozymes consist of ferritin-heavy-chain-based protein as the enzyme scaffold and a metal nanoparticle core as the enzyme active center. This artificial cascade nanozyme possesses superoxide dismutase- and catalase-like activities and also targets mitochondria by overcoming multiple biological barriers. Using cardiac ischemia-reperfusion animal models, the protective advantages of the hybrid nanozymes are demonstrated in vivo during mitochondrial oxidative injury and in the recovery of heart functionality following infarction via systemic delivery and localized release from adhesive hydrogels (i.e., cardiac patch), respectively. This study illustrates a de novo design strategy in the development of enzyme mimics and provides a promising therapeutic option for alleviating oxidative damage in regenerative medicine.

Molecular understanding of heteronuclear active sites in heme-copper oxidases, nitric oxide reductases, and sulfite reductases through biomimetic modelling

Heme-copper oxidases (HCO), nitric oxide reductases (NOR), and sulfite reductases (SiR) catalyze the multi-electron and multi-proton reductions of O2, NO, and SO32-, respectively. Each of these reactions is important to drive cellular energy production through respiratory metabolism and HCO, NOR, and SiR evolved to contain heteronuclear active sites containing heme/copper, heme/nonheme iron, and heme-[4Fe-4S] centers, respectively. The complexity of the structures and reactions of these native enzymes, along with their large sizes and/or membrane associations, make it challenging to fully understand the crucial structural features responsible for the catalytic properties of these active sites. In this review, we summarize progress that has been made to better understand these heteronuclear metalloenzymes at the molecular level though study of the native enzymes along with insights gained from biomimetic models comprising either small molecules or proteins. Further understanding the reaction selectivity of these enzymes is discussed through comparisons of their similar heteronuclear active sites, and we offer outlook for further investigations.

Artificial Metalloproteins with Dinuclear Iron-Hydroxido Centers

Dinuclear iron centers with a bridging hydroxido or oxido ligand form active sites within a variety of metalloproteins. A key feature of these sites is the ability of the protein to control the structures around the Fe centers, which leads to entatic states that are essential for function. To simulate this controlled environment, artificial proteins have been engineered using biotin-streptavidin (Sav) technology in which Fe complexes from adjacent subunits can assemble to form [Fe^III-(μ-OH)-Fe^III] cores. The assembly process is promoted by the site-specific localization of the Fe complexes within a subunit through the designed mutation of a tyrosinate side chain to coordinate the Fe centers. An important outcome is that the Sav host can regulate the Fe···Fe separation, which is known to be important for function in natural metalloproteins. Spectroscopic and structural studies from X-ray diffraction methods revealed uncommonly long Fe···Fe separations that change by less than 0.3 Å upon the binding of additional bridging ligands. The structural constraints imposed by the protein host on the di-Fe cores are unique and create examples of active sites having entatic states within engineered artificial metalloproteins.

Transfer hydrogenation catalysis in cells

Hydrogenation reactions in biology are usually carried out by enzymes with nicotinamide adenine dinucleotide (NAD(P)H) or flavin mononucleotide (FAMH2)/flavinadenine dinucleotide (FADH2) as cofactors and hydride sources. Industrial scale chemical transfer hydrogenation uses small molecules such as formic acid or alcohols (e.g. propanol) as hydride sources and transition metal complexes as catalysts. We focus here on organometallic half-sandwich RuII and OsII η6-arene complexes and RhIII and IrIII η5-Cp x complexes which catalyse hydrogenation of biomolecules such as pyruvate and quinones in aqueous media, and generate biologically important species such as H2 and H2O2. Organometallic catalysts can achieve enantioselectivity, and moreover can be active in living cells, which is surprising on account of the variety of poisons present. Such catalysts can induce reductive stress using formate as hydride source or oxidative stress by accepting hydride from NAD(P)H. In some cases, photocatalytic redox reactions can be induced by light absorption at metal or flavin centres. These artificial transformations can interfere in biochemical pathways in unusual ways, and are the basis for the design of metallodrugs with novel mechanisms of action.

Rational design of metal-binding sites in domain-swapped myoglobin dimers

The metal active site is precisely designed in metalloproteins. Here we applied 3D domain swapping, a phenomenon in which a partial protein structure is exchanged between molecules, to introduce metal sites in proteins. We designed multiple metal-binding sites specific to domain-swapped myoglobin (Mb) with His mutation. Stable dimeric Mbs with metal-binding sites were obtained by shifting the His position and introducing two Ala residues in the hinge region (K78H/G80A/H82A and K79H/G80A/H81A Mbs). The absorption and circular dichroism spectra of the monomer and dimer of K78H/G80A/H82A and K79H/G80A/H81A Mbs were similar to the corresponding spectra, respectively, of wild-type Mb. No negative peak due to dimer-to-monomer dissociation was observed below the denaturation temperature in the differential scanning calorimetry thermograms of K78H/G80A/H82A and K79H/G80A/H81A Mbs, whereas the dimer dissociates into monomers at 68 °C for wild-type Mb. These results show that the two mutants were stable in the dimer state. Metal ions bound to the metal-binding sites containing the introduced His in the domain-swapped Mb dimers. Co²⁺-bound and Ni²⁺-bound K78H/G80A/H82A Mb exhibited octahedral metal-coordination structures, where His78, His81, Glu85, and three H₂O/OH^- molecules coordinated to the metal ion. On the other hand, Co²⁺-bound and Zn²⁺-bound K79H/G80A/H81A Mb exhibited tetrahedral metal-coordination structures, where His79, His82, Asp141, and a H₂O/OH^- molecule coordinated to the metal ion. The Co²⁺-bound site exists deep inside the protein in the K79H/G80A/H81A Mb dimer, which may allow the unique tetrahedral coordination for the Co²⁺ ion. These results show that we can utilize domain swapping to construct artificial metalloproteins.

The pH-Induced Selectivity Between Cysteine or Histidine Coordinated Heme in an Artificial α-Helical Metalloprotein

De Novo metalloprotein design assesses the relationship between metal active site architecture and catalytic reactivity. Herein, we use an α-helical scaffold to control the iron coordination geometry when a heme cofactor is allowed to bind to either histidine or cysteine ligands, within a single artificial protein. Consequently, we uncovered a reversible pH-induced switch of the heme axial ligation within this simplified scaffold. Characterization of the specific heme coordination modes was done by using UV/Vis and Electron Paramagnetic Resonance spectroscopies. The penta- or hexa-coordinate thiolate heme (9≤pH≤11) and the penta-coordinate imidazole heme (6≤pH≤8.5) reproduces well the heme ligation in chloroperoxidases or cyt P450 monooxygenases and peroxidases, respectively. The stability of heme coordination upon ferric/ferrous redox cycling is a crucial property of the construct. At basic pHs, the thiolate mini-heme protein can catalyze O₂ reduction when adsorbed onto a pyrolytic graphite electrode.

Intermediates involved in serotonin oxidation catalyzed by Cu bound Aβ peptides

The degradation of neurotransmitters is a hallmark feature of Alzheimer's disease (AD). Copper bound Aβ peptides, invoked to be involved in the pathology of AD, are found to catalyze the oxidation of serotonin (5-HT) by H₂O₂. A combination of EPR and resonance Raman spectroscopy reveals the formation of a Cu(ii)–OOH species and a dimeric, EPR silent, Cu₂O₂ bis-μ-oxo species under the reaction conditions. The Cu(ii)–OOH species, which can be selectively formed in the presence of excess H₂O₂, is the reactive intermediate responsible for 5-HT oxidation. H₂O₂ produced by the reaction of O₂ with reduced Cu(i)–Aβ species can also oxidize 5-HT. Both these pathways are physiologically relevant and may be involved in the observed decay of neurotransmitters as observed in AD patients.

The mononuclear copper hydroperoxo species (Cu(ii)–OOH) of Cu–Aβ is the active oxidant responsible for serotonin oxidation by Cu–Aβ in the presence of physiologically relevant oxidants like O₂ and H₂O₂, which can potentially cause oxidative degradation of neurotransmitters, a marker of Alzheimer's disease.

Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase

The copper(II) complexes [Cu(L)NO₃] (1-9) of newer N₃O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04-0.11). The Cu-O_Phenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1-9 appeared around -0.102 to -0.428 V versus Ag/Ag⁺ in water. The electronic spectra of the complexes showed the d-d transitions around 643-735 nm and axial EPR parameter (g_||, 2.243-2.270; A_||, 164-179 × 10^-4 cm^-1) that corresponds to square pyramidal geometry. The bonding parameters α², 0.760-0.825; β², 0.761-0.994; γ², 0.504-0.856 and K_||, 0.698-0.954 and K_⊥, 0.383-0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41-61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98-15.65 × 10^-3 s^-1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (k_cat) 9.11 × 10⁵ h^-1 for 1 and 4.66 × 10⁵ h^-1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.

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.

How Does Replacement of the Axial Histidine Ligand in Cytochrome c Peroxidase by Nδ-Methyl Histidine Affect Its Properties and Functions? A Computational Study

Heme peroxidases have important functions in nature related to the detoxification of H₂O₂. They generally undergo a catalytic cycle where, in the first stage, the iron(III)-heme-H₂O₂ complex is converted into an iron(IV)-oxo-heme cation radical species called Compound I. Cytochrome c peroxidase Compound I has a unique electronic configuration among heme enzymes where a metal-based biradical is coupled to a protein radical on a nearby Trp residue. Recent work using the engineered N_δ-methyl histidine-ligated cytochrome c peroxidase highlighted changes in spectroscopic and catalytic properties upon axial ligand substitution. To understand the axial ligand effect on structure and reactivity of peroxidases and their axially N_δ-methyl histidine engineered forms, we did a computational study. We created active site cluster models of various sizes as mimics of horseradish peroxidase and cytochrome c peroxidase Compound I. Subsequently, we performed density functional theory studies on the structure and reactivity of these complexes with a model substrate (styrene). Thus, the work shows that the N_δ-methyl histidine group has little effect on the electronic configuration and structure of Compound I and little changes in bond lengths and the same orbital occupation is obtained. However, the N_δ-methyl histidine modification impacts electron transfer processes due to a change in the reduction potential and thereby influences reactivity patterns for oxygen atom transfer. As such, the substitution of the axial histidine by N_δ-methyl histidine in peroxidases slows down oxygen atom transfer to substrates and makes Compound I a weaker oxidant. These studies are in line with experimental work on N_δ-methyl histidine-ligated cytochrome c peroxidases and highlight how the hydrogen bonding network in the second coordination sphere has a major impact on the function and properties of the enzyme.

Contributions of primary coordination ligands and importance of outer sphere interactions in UFsc, a de novo designed protein with high affinity for metal ions

Metalloproteins constitute nearly half of all proteins and catalyze some of the most complex chemical reactions. Recently, we reported a design of 4G-UFsc (Uno Ferro single chain), a single chain four-helical bundle with extraordinarily high (30 pM) affinity for zinc. We evaluated the contribution of different side chains to binding of Co(II), Ni(II), Zn(II) and Mn(II) using systematic mutagenesis of the amino acids that constitute the primary metal coordination and outer spheres. The binding affinity of proteins for metals was then measured using isothermal titration calorimetry. Our results show that both primary metal coordination environment and side chains in the outer sphere of UFsc are highly sensitive to even slight changes and can be adapted to binding different 3d metals, including hard-to-tightly bind metal ions such as Mn(II). The studies on the origins of tight metal binding will guide future metalloprotein design efforts.

Clickable artificial heme-peroxidases for the development of functional nanomaterials

Artificial metalloenzymes as catalysts are promising candidates for their use in different technologies, such as bioremediation, biomass transformation, or biosensing. Despite this, their practical exploitation is still at an early stage. Immobilized natural enzymes have been proposed to enhance their applicability. Immobilization may offer several advantages: (i) catalyst reuse; (ii) easy separation of the enzyme from the reaction medium; (iii) better tolerance to harsh temperature and pH conditions. Here, we report an easy immobilization procedure of an artificial peroxidase on different surfaces, by means of click chemistry. FeMC6*a, a recently developed peroxidase mimic, has been functionalized with a pegylated aza-dibenzocyclooctyne to afford a "clickable" biocatalyst, namely FeMC6*a-PEG₄@DBCO, which easily reacts with azide-functionalized molecules and/or nanomaterials to afford functional bioconjugates. The clicked biocatalyst retains its structural and, to some extent, its functional behaviors, thus housing high potential for biotechnological applications.

Artificial Iron Proteins: Modeling the Active Sites in Non-Heme Dioxygenases

An important class of non-heme dioxygenases contains a conserved Fe binding site that consists of a 2-His-1-carboxylate facial triad. Results from structural biology show that, in the resting state, these proteins are six-coordinate with aqua ligands occupying the remaining three coordination sites. We have utilized biotin-streptavidin (Sav) technology to design new artificial Fe proteins (ArMs) that have many of the same structural features found within active sites of these non-heme dioxygenases. An Sav variant was isolated that contains the S112E mutation, which installed a carboxylate side chain in the appropriate position to bind to a synthetic FeII complex confined within Sav. Structural studies using X-ray diffraction (XRD) methods revealed a facial triad binding site that is composed of two N donors from the biotinylated ligand and the monodentate coordination of the carboxylate from S112E. Two aqua ligands complete the primary coordination sphere of the FeII center with both involved in hydrogen bond networks within Sav. The corresponding FeIII protein was also prepared and structurally characterized to show a six-coordinate complex with two exogenous acetato ligands. The FeIII protein was further shown to bind an exogenous azido ligand through replacement of one acetato ligand. Spectroscopic studies of the ArMs in solution support the results found by XRD.

Designing minimalist membrane proteins

The construction of artificial membrane proteins from first principles is of fundamental interest and holds considerable promise for new biotechnologies. This review considers the potential advantages of adopting a strictly minimalist approach to the process of membrane protein design. As well as the practical benefits of miniaturisation and simplicity for understanding sequence-structure-function relationships, minimalism should also support the abstract conceptualisation of membrane proteins as modular components for synthetic biology. These ideas are illustrated with selected examples that focus upon α-helical membrane proteins, and which demonstrate how such minimalist membrane proteins might be integrated into living biosystems.