A Whole Cell E. coli Display Platform for Artificial Metalloenzymes: Poly(phenylacetylene) Production with a Rhodium–Nitrobindin Metalloprotein

Material-specific binding peptides empower sustainable innovations in plant health, biocatalysis, medicine and microplastic quantification

Material-binding peptides (MBPs) have emerged as a diverse and innovation-enabling class of peptides in applications such as plant-/human health, immobilization of catalysts, bioactive coatings, accelerated polymer degradation and analytics for micro-/nanoplastics quantification. Progress has been fuelled by recent advancements in protein engineering methodologies and advances in computational and analytical methodologies, which allow the design of, for instance, material-specific MBPs with fine-tuned binding strength for numerous demands in material science applications. A genetic or chemical conjugation of second (biological, chemical or physical property-changing) functionality to MBPs empowers the design of advanced (hybrid) materials, bioactive coatings and analytical tools. In this review, we provide a comprehensive overview comprising naturally occurring MBPs and their function in nature, binding properties of short man-made MBPs (<20 amino acids) mainly obtained from phage-display libraries, and medium-sized binding peptides (20-100 amino acids) that have been reported to bind to metals, polymers or other industrially produced materials. The goal of this review is to provide an in-depth understanding of molecular interactions between materials and material-specific binding peptides, and thereby empower the use of MBPs in material science applications. Protein engineering methodologies and selected examples to tailor MBPs toward applications in agriculture with a focus on plant health, biocatalysis, medicine and environmental monitoring serve as examples of the transformative power of MBPs for various industrial applications. An emphasis will be given to MBPs' role in detecting and quantifying microplastics in high throughput, distinguishing microplastics from other environmental particles, and thereby assisting to close an analytical gap in food safety and monitoring of environmental plastic pollution. In essence, this review aims to provide an overview among researchers from diverse disciplines in respect to material-(specific) binding of MBPs, protein engineering methodologies to tailor their properties to application demands, re-engineering for material science applications using MBPs, and thereby inspire researchers to employ MBPs in their research.

Benzylic C(sp3 )-H Bond Oxidation with Ketone Selectivity by a Cobalt(IV)-Oxo Embedded in a β-Barrel Protein

Artificial metalloenzymes have emerged as biohybrid catalysts that allow to combine the reactivity of a metal catalyst with the flexibility of protein scaffolds. This work reports the artificial metalloenzymes based on the β-barrel protein nitrobindin NB4, in which a cofactor [CoII X(Me3 TACD-Mal)]+ X- (X=Cl, Br; Me3 TACD=N,N' ,N''-trimethyl-1,4,7,10-tetraazacyclododecane, Mal=CH2 CH2 CH2 NC4 H2 O2 ) was covalently anchored via a Michael addition reaction. These biohybrid catalysts showed higher efficiency than the free cobalt complexes for the oxidation of benzylic C(sp3 )-H bonds in aqueous media. Using commercially available oxone (2KHSO5  ⋅ KHSO4  ⋅ K2 SO4 ) as oxidant, a total turnover number of up to 220 and 97 % ketone selectivity were achieved for tetralin. As catalytically active intermediate, a mononuclear terminal cobalt(IV)-oxo species [Co(IV)=O]2+ was generated by reacting the cobalt(II) cofactor with oxone in aqueous solution and characterized by ESI-TOF MS.

Spiers Memorial Lecture: Shielding the active site: a streptavidin superoxide-dismutase chimera as a host protein for asymmetric transfer hydrogenation

By anchoring a metal cofactor within a host protein, so-called artificial metalloenzymes can be generated. Such hybrid catalysts combine the versatility of transition metals in catalyzing new-to-nature reactions with the power of genetic-engineering to evolve proteins. With the aim of gaining better control over second coordination-sphere interactions between a streptavidin host-protein (Sav) and a biotinylated cofactor, we engineered a hydrophobic dimerization domain, borrowed from superoxide dismutase C (SOD), on Sav's biotin-binding vestibule. The influence of the SOD dimerization domain (DD) on the performance of an asymmetric transfer hydrogenase (ATHase) resulting from anchoring a biotinylated Cp*Ir-cofactor - [Cp*Ir(biot-p-L)Cl] (1-Cl) - within Sav-SOD is reported herein. We show that, depending on the nature of the residue at position Sav S112, the introduction of the SOD DD on the biotin-binding vestibule leads to an inversion of configuration of the reduction product, as well as a fivefold increase in catalytic efficiency. The findings are rationalized by QM/MM calculations, combined with X-ray crystallography.

Artificial Metalloenzyme-Catalyzed Enantioselective Amidation via Nitrene Insertion in Unactivated C(sp3)-H Bonds

Enantioselective C-H amidation offers attractive means to assemble C-N bonds to synthesize high-added value, nitrogen-containing molecules. In recent decades, complementary enzymatic and homogeneous-catalytic strategies for C-H amidation have been reported. Herein, we report on an artificial metalloenzyme (ArM) resulting from anchoring a biotinylated Ir-complex within streptavidin (Sav). The resulting ArM catalyzes the enantioselective amidation of unactivated C(sp3)-H bonds. Chemogenetic optimization of the Ir cofactor and Sav led to significant improvement in both the activity and enantioselectivity. Up to >700 TON and 92% ee for the amidation of unactivated C(sp3)-H bonds was achieved. The single crystal X-ray analysis of the artificial nitrene insertase (ANIase) combined with quantum mechanics-molecular mechanics (QM-MM) calculations sheds light on critical second coordination sphere contacts leading to improved catalytic performance.

Design of Artificial Enzymes: Insights into Protein Scaffolds

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

In Vivo Biocatalytic Cascades Featuring an Artificial-Enzyme-Catalysed New-to-Nature Reaction

Artificial enzymes utilizing the genetically encoded non-proteinogenic amino acid p-aminophenylalanine (pAF) as a catalytic residue are able to react with carbonyl compounds through an iminium ion mechanism to promote reactions that have no equivalent in nature. Herein, we report an in vivo biocatalytic cascade that is augmented with such an artificial enzyme-catalysed new-to-nature reaction. The artificial enzyme in this study is a pAF-containing evolved variant of the lactococcal multidrug-resistance regulator, designated LmrR_V15pAF_RMH, which efficiently converts benzaldehyde derivatives produced in vivo into the corresponding hydrazone products inside E. coli cells. These in vivo biocatalytic cascades comprising an artificial-enzyme-catalysed reaction are an important step towards achieving a hybrid metabolism.

Design of a Flexible, Zn-Selective Protein Scaffold that Displays Anti-Irving-Williams Behavior

Selective metal binding is a key requirement not only for the functions of natural metalloproteins but also for the potential applications of artificial metalloproteins in heterogeneous environments such as cells and environmental samples. The selection of transition-metal ions through protein design can, in principle, be achieved through the appropriate choice and the precise positioning of amino acids that comprise the primary metal coordination sphere. However, this task is made difficult by the intrinsic flexibility of proteins and the fact that protein design approaches generally lack the sub-Å precision required for the steric selection of metal ions. We recently introduced a flexible/probabilistic protein design strategy (MASCoT) that allows metal ions to search for optimal coordination geometry within a flexible, yet covalently constrained dimer interface. In an earlier proof-of-principle study, we used MASCoT to generate an artificial metalloprotein dimer, (AB)₂, which selectively bound Co^II and Ni^II over Cu^II (as well as other first-row transition-metal ions) through the imposition of a rigid octahedral coordination geometry, thus countering the Irving-Williams trend. In this study, we set out to redesign (AB)₂ to examine the applicability of MASCoT to the selective binding of other metal ions. We report here the design and characterization of a new flexible protein dimer, B₂, which displays Zn^II selectivity over all other tested metal ions including Cu^II both in vitro and in cellulo. Selective, anti-Irving-Williams Zn^II binding by B₂ is achieved through the formation of a unique trinuclear Zn coordination motif in which His and Glu residues are rigidly placed in a tetrahedral geometry. These results highlight the utility of protein flexibility in the design and discovery of selective binding motifs.

Directed Evolution of Artificial Metalloenzymes in Whole Cells

Artificial metalloenzymes (ArMs), created by introducing synthetic cofactors into protein scaffolds, are an emerging class of catalyst for non-natural reactions. Although many classes of ArMs are known, in vitro reconstitution of cofactors and proteins has been a limiting step in the high-throughput screening and directed evolution of ArMs because purification of individual host proteins is time-consuming. We describe the application of a platform to combine mutants of the P450 enzyme CYP119 and the cofactor Ir(Me)MPIX in vivo, by coexpression of the CYP119 mutants with the heme transporter encoded by the hug operon, to the directed evolution of ArMs containing Ir(Me)MPIX in whole cells. We applied this platform to the development an ArMs catalyzing the insertion of the acyclic carbene from α-diazopropanoate esters (Me-EDA) into the N-H bonds of N-alkyl anilines, a combination of carbene and amine classes for which mutant enzymes of natural hemoproteins previously reacted with low enantioselectivity. The mutants of the artificial metalloenzyme Ir(Me)CYP119 identified by an evolution campaign involving more than 4000 mutants are shown to catalyze the reaction of Me-EDA with N-methyl anilines to form chiral chiral amino esters with high TON and good enantioselectivity, thereby demonstrating that the directed evolution of ArMs can rival that of natural enzymes in vivo.

Synthetic prodrug design enables biocatalytic activation in mice to elicit tumor growth suppression

Considering the intrinsic toxicities of transition metals, their incorporation into drug therapies must operate at minimal amounts while ensuring adequate catalytic activity within complex biological systems. As a way to address this issue, this study investigates the design of synthetic prodrugs that are not only tuned to be harmless, but can be robustly transformed in vivo to reach therapeutically relevant levels. To accomplish this, retrosynthetic prodrug design highlights the potential of naphthylcombretastatin-based prodrugs, which form highly active cytostatic agents via sequential ring-closing metathesis and aromatization. Structural adjustments will also be done to improve aspects related to catalytic reactivity, intrinsic bioactivity, and hydrolytic stability. The developed prodrug therapy is found to possess excellent anticancer activities in cell-based assays. Furthermore, in vivo activation by intravenously administered glycosylated artificial metalloenzymes can also induce significant reduction of implanted tumor growth in mice.

Unnatural biosynthesis by an engineered microorganism with heterologously expressed natural enzymes and an artificial metalloenzyme

Synthetic biology enables microbial hosts to produce complex molecules that are otherwise produced by organisms that are rare or difficult to cultivate, but the structures of these molecules are limited to those formed by chemical reactions catalyzed by natural enzymes. The integration of artificial metalloenzymes (ArMs) that catalyze unnatural reactions into metabolic networks could broaden the cache of molecules produced biosynthetically by microorganisms. We report an engineered microbial cell expressing a heterologous biosynthetic pathway, which contains both natural enzymes and ArMs, that produces an unnatural product with high diastereoselectivity. To create this hybrid biosynthetic organism, we engineered Escherichia coli (E. coli) with a heterologous terpene biosynthetic pathway and an ArM containing an iridium-porphyrin complex that was transported into the cell with a heterologous transport system. We improved the diastereoselectivity and product titer of the unnatural product by evolving the ArM and selecting the appropriate gene induction and cultivation conditions. This work shows that synthetic biology and synthetic chemistry can produce, together with natural and artificial enzymes in whole cells, molecules that were previously inaccessible to nature.

Directed Evolution of a Surface-Displayed Artificial Allylic Deallylase Relying on a GFP Reporter Protein

Artificial metalloenzymes (ArMs) combine characteristics of both homogeneous catalysts and enzymes. Merging abiotic and biotic features allows for the implementation of new-to-nature reactions in living organisms. Here, we present the directed evolution of an artificial metalloenzyme based on Escherichia coli surface-displayed streptavidin (SavSD hereafter). Through the binding of a ruthenium-pianostool cofactor to SavSD, an artificial allylic deallylase (ADAse hereafter) is assembled, which displays catalytic activity toward the deprotection of alloc-protected 3-hydroxyaniline. The uncaged aminophenol acts as a gene switch and triggers the overexpression of a fluorescent green fluorescent protein (GFP) reporter protein. This straightforward readout of ADAse activity allowed the simultaneous saturation mutagenesis of two amino acid residues in Sav near the ruthenium cofactor, expediting the screening of 2762 individual clones. A 1.7-fold increase of in vivo activity was observed for SavSD S112T-K121G compared to the wild-type SavSD (wt-SavSD). Finally, the best performing Sav isoforms were purified and tested in vitro (SavPP hereafter). For SavPP S112M-K121A, a total turnover number of 372 was achieved, corresponding to a 5.9-fold increase vs wt-SavPP. To analyze the marked difference in activity observed between the surface-displayed and purified ArMs, the oligomeric state of SavSD was determined. For this purpose, crosslinking experiments of E. coli cells overexpressing SavSD were carried out, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot. The data suggest that SavSD is most likely displayed as a monomer on the surface of E. coli. We hypothesize that the difference between the in vivo and in vitro screening results may reflect the difference in the oligomeric state of SavSD vs soluble SavPP (monomeric vs tetrameric). Accordingly, care should be applied when evolving oligomeric proteins using E. coli surface display.

An artificial ruthenium-containing β-barrel protein for alkene-alkyne coupling reaction

A modified Cp*Ru complex, equipped with a maleimide group, was covalently attached to a cysteine of an engineered variant of Ferric hydroxamate uptake protein component: A (FhuA). This synthetic metalloprotein catalyzed the intermolecular alkene-alkyne coupling of 3-butenol with 5-hexynenitrile. When compared with the protein-free Cp*Ru catalyst, the biohybrid catalyst produced the linear product with higher regioselectivity.

In Vivo Assembly of Artificial Metalloenzymes and Application in Whole-Cell Biocatalysis*

We report the supramolecular assembly of artificial metalloenzymes (ArMs), based on the Lactococcal multidrug resistance regulator (LmrR) and an exogeneous copper(II)-phenanthroline complex, in the cytoplasm of E. coli cells. A combination of catalysis, cell-fractionation, and inhibitor experiments, supplemented with in-cell solid-state NMR spectroscopy, confirmed the in-cell assembly. The ArM-containing whole cells were active in the catalysis of the enantioselective Friedel-Crafts alkylation of indoles and the Diels-Alder reaction of azachalcone with cyclopentadiene. Directed evolution resulted in two different improved mutants for both reactions, LmrR_A92E_M8D and LmrR_A92E_V15A, respectively. The whole-cell ArM system required no engineering of the microbial host, the protein scaffold, or the cofactor to achieve ArM assembly and catalysis. We consider this a key step towards integrating abiological catalysis with biosynthesis to generate a hybrid metabolism.

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.

Chemogenetic engineering of nitrobindin toward an artificial epoxygenase

Chemogenetic engineering turned the heme protein nitrobindin into an artificial epoxygenase: MnPPIX was introduced and subsequent protein engineering increased the activity in the epoxidation of styrene derivatives by overall 7-fold.

High Throughput Screening with SAMDI Mass Spectrometry for Directed Evolution

Advances in directed evolution have led to an exploration of new and important chemical transformations; however, many of these efforts still rely on the use of low-throughput chromatography-based screening methods. We present a high-throughput strategy for screening libraries of enzyme variants for improved activity. Unpurified reaction products are immobilized to a self-assembled monolayer and analyzed by mass spectrometry, allowing for direct evaluation of thousands of variants in under an hour. The method was demonstrated with libraries of randomly mutated cytochrome P411 variants to identify improved catalysts for C-H alkylation. The technique may be tailored to evolve enzymatic activity for a variety of transformations where higher throughput is needed.

Cell mimicry as a bottom-up strategy for hierarchical engineering of nature-inspired entities

Artificial biology is an emerging concept that aims to design and engineer the structure and function of natural cells, organelles, or biomolecules with a combination of biological and abiotic building blocks. Cell mimicry focuses on concepts that have the potential to be integrated with mammalian cells and tissue. In this feature article, we will emphasize the advancements in the past 3-4 years (2017-present) that are dedicated to artificial enzymes, artificial organelles, and artificial mammalian cells. Each aspect will be briefly introduced, followed by highlighting efforts that considered key properties of the different mimics. Finally, the current challenges and opportunities will be outlined. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

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

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

Artificial Enzymes for Diels-Alder Reactions

The Diels-Alder (DA) reaction is a cycloaddition of a conjugated diene and an alkene (dienophile) leading to the formation of a cyclohexene derivative through a concerted mechanism. As DA reactions generally proceed with a high degree of regio- and stereoselectivity, they are widely used in synthetic organic chemistry. Considering eco-conscious public and governmental movements, efforts are now directed towards the development of synthetic processes that meet environmental concerns. Artificial enzymes, which can be developed to catalyze abiotic reactions, appear to be important synthetic tools in the synthetic biology field. This review describes the different strategies used to develop protein-based artificial enzymes for DA reactions, including for in cellulo approaches.

Display of functional nucleic acid polymerase on Escherichia coli surface and its application in directed polymerase evolution

We report a first of its kind functional cell surface display of nucleic acid polymerase and its directed evolution to efficiently incorporate 2'-O-methyl nucleotide triphosphates (2'-OMe-NTPs). In the development of polymerase cell surface display, two autotransporter proteins (Escherichia coli adhesin involved in diffuse adherence and Pseudomonas aeruginosa esterase A [EstA]) were employed to transport and anchor the 68-kDa Klenow fragment (KF) of E. coli DNA polymerase I on the surface of E. coli. The localization and function of the displayed KF were verified by analysis of cell outer membrane fractions, immunostaining, and fluorometric detection of synthesized DNA products. The EstA cell surface display system was applied to evolve KF for the incorporation of 2'-OMe-NTPs and a KF variant with a 50.7-fold increased ability to successively incorporate 2'-OMe-NTPs was discovered. Expanding the scope of cell-surface displayable proteins to the realm of polymerases provides a novel screening tool for tailoring polymerases to diverse application demands in a polymerase chain reaction and sequencing-based biotechnological and medical applications. Especially, cell surface display enables novel polymerase screening strategies in which the heat-lysis step is bypassed and thus allows the screening of mesophilic polymerases with broad application potentials ranging from diagnostics and DNA sequencing to replication of synthetic genetic polymers.

Designer metalloenzymes for synthetic biology: Enzyme hybrids for catalysis

Combining organometallics and biology has generated broad interest from scientists working on applications from in situ drug release to biocatalysis. Engineered enzymes and biohybrid catalysts (also referred to as artificial enzymes) have introduced a wide range of abiotic chemistry into biocatalysis. Predominantly, this work has concentrated on using these catalysts for single step in vitro reactions. However, the promise of using these hybrid catalysts in vivo and combining them with synthetic biology and metabolic engineering is vast. This report will briefly review recent advances in artificial metalloenzyme design, followed by summarising recent studies that have looked at the use of these hybrid catalysts in vivo and in enzymatic cascades, therefore exploring their potential for synthetic 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.

Self-supported rhodium catalysts based on a microporous metal–organic framework for polymerization of phenylacetylene and its derivatives

Highly efficient, cis-selective, stable, recyclable and reusable heterogeneous single-site MOF-supported rhodium catalysts polymerize PA and its derivatives, producing cis-transoidal PPAs and their functional derivatives having a helical configuration or AIE properties.

An artificial metalloenzyme biosensor can detect ethylene gas in fruits and Arabidopsis leaves

Enzyme biosensors are useful tools that can monitor rapid changes in metabolite levels in real-time. However, current approaches are largely constrained to metabolites within a limited chemical space. With the rising development of artificial metalloenzymes (ArM), a unique opportunity exists to design biosensors from the ground-up for metabolites that are difficult to detect using current technologies. Here we present the design and development of the ArM ethylene probe (AEP), where an albumin scaffold is used to solubilize and protect a quenched ruthenium catalyst. In the presence of the phytohormone ethylene, cross metathesis can occur to produce fluorescence. The probe can be used to detect both exogenous- and endogenous-induced changes to ethylene biosynthesis in fruits and leaves. Overall, this work represents an example of an ArM biosensor, designed specifically for the spatial and temporal detection of a biological metabolite previously not accessible using enzyme biosensors.

Existing methods to detect ethylene in plant tissue typically require gas chromatography or use ethylene-dependent gene expression as a proxy. Here Vong et al. show that an artificial metalloenzyme-based ethylene probe can be used to detect ethylene in plants with improved spatiotemporal resolution.

Artificial Metalloenzymes: Challenges and Opportunities

Artificial metalloenzymes (ArMs) result from the incorporation of an abiotic metal cofactor within a protein scaffold. From the earliest techniques of transition metals adsorbed on silk fibers, the field of ArMs has expanded dramatically over the past 60 years to encompass a range of reaction classes and inspired approaches: Assembly of the ArMs has taken multiple forms with both covalent and supramolecular anchoring strategies, while the scaffolds have been intuitively selected and evolved, repurposed, or designed in silico. Herein, we discuss some of the most prominent recent examples of ArMs to highlight the challenges and opportunities presented by the field.

Chemical Optimization of Whole-Cell Transfer Hydrogenation Using Carbonic Anhydrase as Host Protein

Artificial metalloenzymes combine a synthetic metallocofactor with a protein scaffold and can catalyze abiotic reactions in vivo. Herein, we report on our efforts to valorize human carbonic anhydrase II as a scaffold for whole-cell transfer hydrogenation. Two platforms were tested: periplasmic compartmentalization and surface display in Escherichia coli. A chemical optimization of an IrCp* cofactor was performed. This led to 90 turnovers in the cell, affording a 69-fold increase in periplasmic product formation over the previously reported, sulfonamide-bearing IrCp* cofactor. These findings highlight the versatility of carbonic anhydrase as a promising scaffold for whole-cell catalysis with artificial metalloenzymes.

Synthesis and Reactivity of an Early-Transition-Metal Alkynyl Cubane Mn4 C4 Cluster

While the coordination chemistry of monometallic complexes and the surface properties of extended metal particles are well understood, the control of metal nanocluster formation has remained challenging. The isolation of discrete metal clusters provides an especially rare snapshot at the nanoscale of cluster growth. The synthesis and full characterization of the first early-transition-metal alkynyl cubane and the first μ³ -alkynyl Mn₃ motif are reported.

Metallopolymers for advanced sustainable applications

Since the development of metallopolymers, there has been tremendous interest in the applications of this type of materials. The interest in these materials stems from their potential use in industry as catalysts, biomedical agents in healthcare, energy storage and production as well as climate change mitigation. The past two decades have clearly shown exponential growth in the development of many new classes of metallopolymers that address these issues. Today, metallopolymers are considered to be at the forefront for discovering new and sustainable heterogeneous catalysts, therapeutics for drug-resistant diseases, energy storage and photovoltaics, molecular barometers and thermometers, as well as carbon dioxide sequesters. The focus of this review is to highlight the advances in design of metallopolymers with specific sustainable applications.

Biohybrid catalysts for sequential one-pot reactions based on an engineered transmembrane protein

A two-step one pot reaction sequence consisting of artificial metalloprotein olefin metathesis and hydrogenation was performed yielding 1,2-diphenylethane derivatives.