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The field of artificial metalloenzymes (ArMs) is rapidly growing and ArMs are attracting increasing attention, for example, in the fields of biosensing and drug therapy. Protein-engineering methods that are commonly used to tailor the properties of natural enzymes are more frequently included in the design of ArMs. In particular, directed evolution allows the fine-tuning of ArMs, ultimately assisting in the development of their enormous potential. The integration of ArMs in whole cells enables their in vivo application and facilitates high-throughput directed-evolution methodologies. In this Review, we highlight the recent progress of whole-cell conversions and applications of ArMs and critically discuss their limitations and prospects. To focus on ArMs and their specific properties, advantages and challenges, the evolution of natural enzymes for non-natural reactions will not be covered. Artificial metalloenzymes (ArMs) combine the reaction scope of chemocatalysts with the selectivity of enzymes. This Review discusses the prospects and recent progress in utilizing ArMs in whole cells for applications in diverse areas such as drug therapy and integration with biosynthetic pathways.

Directed evolution has helped enzyme engineering to remarkable successes in the past. A main challenge in directed evolution is to find the most suitable starting point, that is, an enzyme that all...

Chemogenetic engineering of metalloproteins emerges as a powerful strategy to generate proteins that catalyze non-natural reactions or convert non-natural substrates. Here, we report on an artificial metalloenzyme (ArM) based on the β-barrel protein nitrobindin (NB) equipped with a manganese protoporphyrin IX catalyst (MnPPIX@NB) for the epoxidation of aromatic alkenes. After exchanging the metal cofactor and two rounds of directed evolution, our ArM shows improved activity (>7-fold increase) with an ee of 20%. The evolution campaign also revealed the importance of the proximal ligand for peroxide activation and subsequent oxygen atom transfer. By utilizing cell adhesion-promoting peptides, a facile strategy is presented to immobilize ArMs on the surface of E. coli cells for on-cell catalysis and chemogenetic engineering of ArMs.

We report a facile and reversible method to immobilize a broad range of His6-tagged proteins on the E. coli cell surface through Fe(iii)-metal complexes. A His6-tagged eGFP and four His6-tagged enzymes were successfully immobilized on the cell surface. Additionally, a hydrogel sheath around E. coli cells was generated by immobilized His6-tagged HRP.

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.

Incorporation of metal catalysts into protein scaffolds results in biohybrid catalysts, which have been shown to catalyze various non-natural reactions. Until recently, catalysis with biohybrid cat...

Cell reports / Physical science 3(10), 101054 (2022). doi:10.1016/j.xcrp.2022.101054, Published by Elsevier, [New York, NY]

Sulfation is an important way for detoxifying xenobiotics and endobiotics including catechols. Enzymatic sulfation occurs usually with high chemo- and/or regioselectivity under mild reaction conditions. In this study, a two-step p-NPS-4-AAP screening system for laboratory evolution of aryl sulfotransferase B (ASTB) was developed in 96-well microtiter plates to improve the sulfate transfer efficiency toward catechols. Increased transfer efficiency and improved sulfation stoichiometry are achieved through the two-step screening procedure in a one-pot reaction. In the first step, the p-NPS assay is used (detection of the colorimetric by-product, p-nitrophenol) to determine the apparent ASTB activity. The sulfated product, 3-chlorocatechol-1-monosulfate, is quantified by the 4-aminoantipyrine (4-AAP) assay in the second step. Comparison of product formation to p-NPS consumption ensures successful directed evolution campaigns of ASTB. Optimization yielded a coefficient of variation below 15% for the two-step screening system (p-NPS-4-AAP). In total, 1760 clones from an ASTB-SeSaM library were screened toward the improved sulfation activity of 3-chlorocatechol. The turnover number (kcat = 41 ± 2 s-1) and catalytic efficiency (kcat/KM = 0.41 μM-1 s-1) of the final variant ASTB-M5 were improved 2.4- and 2.3-fold compared with ASTB-WT. HPLC analysis confirmed the improved sulfate stoichiometry of ASTB-M5 with a conversion of 58% (ASTB-WT 29%; two-fold improvement). Mass spectrometry (MS) and nuclear magnetic resonance spectroscopy (NMR) confirmed the chemo- and regioselectivity, which yielded exclusively 3-chlorocatechol-1-monosulfate. For all five additionally investigated catechols, the variant ASTB-M5 achieved an improved kcat value of up to 4.5-fold and sulfate transfer efficiency was also increased (up to 2.3-fold).

A one-pot cascade reaction sequence to obtain 1,2-diarylethanes from styrenes was developed. The cascade reaction comprises an olefin cross metathesis and a hydrogenation step with two biohybrid catalysts. One biohybrid catalyst is based on the engineered transmembrane protein FhuA harboring a Grubbs–Hoveyda-type ruthenium carbene complex and the other one harbors a rhodium(I) complex. Both active sites are covalently anchored via a modified N-heterocyclic carbene (NHC).

We report the preparation of symmetrical stilbene derivatives in a two-step one-pot cascade reaction based on enzymatic decarboxylation of cinnamic acid followed by olefin cross metathesis. Embedment of the metathesis catalyst in a protein scaffold enabled the cascade reaction to symmetric stilbenes and furthermore very efficient removal of metal impurities (

Small : nano micro 17(46), 2102975 (2021). doi:10.1002/smll.202102975, Published by Wiley-VCH, Weinheim

Enzymatic oxidative decarboxylation is an up‐and‐coming reaction yet lacking efficient screening methods for the directed evolution of decarboxylases. Here, we describe a simple photoclick assay for the detection of decarboxylation products and its application in a proof‐of‐principle directed evolution study on the decarboxylase OleT. The assay was compatible with two frequently used OleT operation modes (directly using hydrogen peroxide as the enzyme's co‐substrate or using a reductase partner) and the screening of saturation mutagenesis libraries identified two enzyme variants shifting the enzyme's substrate preference from long chain fatty acids toward styrene derivatives. Overall, this photoclick assay holds promise to speed‐up the directed evolution of OleT and other decarboxylases., Enzymatic decarboxylation is synthetically important, but so far, directed evolution of decarboxylases was impeded by the lack of a high‐throughput screening (HTS). Here, a simple photoclick‐based HTS was developed and used to evolve the decarboxylase OleT toward the acceptance of small aromatic substrates. The improved enzyme variants efficiently convert small aromatic substrates while the parent enzyme favors long‐chain fatty acids.

Copper efflux oxidase (CueO) from Escherichia coli is a special bacterial laccase due to its fifth copper binding site. Herein, it is discovered that the fifth Cu occupancy plays a crucial and favorable role of electron relay in bioelectrocatalytic oxygen reduction. By substituting the residues at the four coordinated positions of the fifth Cu, 11 beneficial variants are identified with ≥2.5‐fold increased currents at −250 mV (up to 6.13 mA cm−2). Detailed electrocatalytic characterization suggests the microenvironment of the fifth Cu binding site governs the electrocatalytic current of CueO. Additionally, further electron transfer analysis assisted by molecular dynamics (MD) simulation demonstrates that an increase in localized structural stability and a decrease of distance between the fifth Cu and the T1 Cu are two main factors contributing to the improved kinetics of CueO variants. It may guide a novel way to tailor laccases and perhaps other oxidoreductases for bioelectrocatalytic applications., As a promising cathodic biocatalyst in enzymatic biofuel cells, laccase CueO catalyzes oxygen reduction efficiently thanks to the supplemental Cu binding. Substituting at the four coordinated positions of the fifth Cu binding site was shown to enable the manipulation of electrocatalytic current, which is more remarkable than substitution at the conventional Cu active sites.

Here we report a chemoenzymatic approach to synthesize 1-octene from carbohydrates via ethenolysis of rhamnolipids. Rhamnolipids synthesized by P. putida contain a double bond between carbon five and six, which is experimentally confirmed via olefin cross metathesis. Utilizing these lipids in the ethenolysis catalyzed by a Grubbs&minus, Hoveyda-type catalyst selectively generates 1-octene and with good conversions. This study shows the potential of chemoenzymatic approaches to produce compounds for the chemical industry from renewable resources.

Cetyl-trimethylammonium bromide (CTAB) is a widely used cationic surfactant that is biodegradable in nature. CTAB biodegradation requires hydroxylation in the first step, which is rate-limiting and crucial for solubility in water. In this study, the OmniChange multi-site mutagenesis method was applied to reengineer the P450 BM3 substrate specificity towards the hydroxylation of CTAB by simultaneous mutagenesis of four previously reported positions (R47, Y51, F87, and L188). 1740 clones from the P450 BM3 OmniChange library were screened with the NADPH depletion assay. A total of 696 clones were rescreened with the NADPH depletion and an Ampliflu™ Red/ horseradish peroxidase based H2 O2 detection assay. Several improved P450 BM3 variants were identified and finally four were kinetically characterized with respect to CTAB hydroxylation, based on both performance and coupling efficiency. Based on NADPH consumption, the P450 BM3 variant P3A8 (R47E/Y51M/F87V/L188E) displayed an initial activity (64.9±4.8 s-1 , 13.5-fold increased activity compared with wild-type P450 BM3), which nearly matches the specific activity for its natural fatty acid substrate (palmitic acid (32-122 s-1 )). Variant P3A8 showed high coupling efficiency (92.5 %), whereas wild-type P450 BM3 displayed a low coupling efficiency (0.5 %). HPLC-MS/MS detection confirmed that P3A8 and P2E7 (R47D/Y51L/F87V/L188A) form 13 and 35 times more 2-hydroxylated CTAB than P450 BM3. In addition, di-hydroxylated CTAB products were detected for all four investigated P450 BM3 variants (up to a yield of 77 %; P3A8). Di-hydroxylated quaternary amines are highly interesting bolaform surfactants with a high hydrophilicity (surface contact angle: θ=16.7°).

Incorporation of a synthetic metal catalyst into a protein scaffold yields a biohybrid catalyst, with a remarkable performance in aqueous media and the broad reaction scope of organometallic catalysts. A major challenge for efficient catalysis is the design of the interface between the protein scaffold and the metal catalyst. Until now, protein scaffolds have primarily been engineered by exchanging individual amino acids to anchor metal catalysts and alter their immediate environment. Here, cavity size engineering of the β-barrel protein nitrobindin was performed by duplicating multiple β-strands to generate an expanded variant. The approach of cavity size engineering enabled covalent incorporation of bulky catalysts at excellent coupling efficiencies and yielded excellent conversions in olefin metathesis, including ring-closing metathesis, ring-opening metathesis polymerization, and cross metathesis (conversions up to 99% and turnover numbers up to 10000).

Whole cell catalysis is, in many cases, a prerequisite for the cost-effective production of chemicals by biotechnological means. Synthetic metal catalysts for bioorthogonal reactions can be inactivated within cells due to abundant thiol derivatives. Here, a cell surface display-based whole cell biohybrid catalyst system (termed ArMt bugs) is reported as a generally applicable platform to unify cost-effective whole cell catalysis with biohybrid catalysis. An inactivated esterase autotransporter is employed to display the nitrobindin protein scaffold with a Rh catalyst on the E. coli surface. Stereoselective polymerization of phenylacetylene yielded a high turnover number (TON) (39 × 106 cell–1) for the ArMt bugs conversion platform.

An (η5-cyclopentadienyl)cobalt(i) complex was covalently incorporated into an engineered variant of the transmembrane protein ferric hydroxamate uptake protein component: A, FhuA ΔCVFtev, using a thiol-ene reaction. A CD spectrum shows the structural integrity of the biohybrid catalyst. MALDI-TOF of the segment containing the anchoring site for the cobalt complex Cys545 confirmed successful conjugation. This biohybrid catalyst catalyzed the cyclotrimerization of phenylacetylene to give a mixture of regioisomeric 1,2,4- and 1,3,5-triphenylbenzene in aqueous medium.

Adhesion promoting peptides have been reported to enable efficient enzyme immobilization on various material surfaces. Here we report the first immobilization of a synthetic Grubbs-Hoveyda (GH) type catalyst on two different materials (silica and polypropylene). To this end, the GH catalyst was coupled to an engineered (F16C) variant of the adhesion promoting peptide LCI through thiol-maleimide “click” reaction. Immobilization was performed in an oriented manner through the adhesion promoting peptide by simple incubation with the materials in water and subsequent washing with water and tetrahydrofuran. The immobilized GH catalyst was probed in ring-opening metathesis polymerization of a norbornene derivative to alter the surface properties in a layer-by-layer fashion.

European journal of organic chemistry 2019(37), 6341-6346 (2019). doi:10.1002/ejoc.201900904, Published by Wiley-VCH Verl., Weinheim

A biohybrid catalyst where a metal complex is embedded in a cavity of a protein can be regarded as an artificial metalloenzyme. In particular, incorporation of synthetic organometallic species in a protein matrix will promote catalytic reactions which cannot be seen in nature. This chapter focuses on olefin metathesis via C=C bond formation/cleavage catalyzed by biohybrid catalysts consisting of Hoveyda-Grubbs (HG) type Ru complex attached to protein. Robust β-barrel proteins, nitrobindin and FhuA, and α-chymotrypsin with a deep cleft are considered as the protein scaffold in this chapter. The HG-type complexes introduced into the scaffolds are found to catalyze ring-opening metathesis polymerization and ring-closing metathesis with good chemoselectivity. This strategy will serve as a new method to construct an attractive biohybrid catalyst for abiological transformations in aqueous media.

Triphenylborane (BPh3 ) in highly polar, aprotic solvents catalyzes hydrosilylation of CO2 effectively under mild conditions to provide silyl formates with high chemoselectivity (>95 %) and without over-reduction. This system also promotes reductive hydrosilylation of tertiary amides as well as dehydrogenative coupling of silane with alcohols.

The incorporation of organometallic catalyst precursors in proteins results in so-called artificial metalloenzymes. The protein structure will control activity, selectivity and stability of the organometallic site in aqueous medium and allow non-natural reactions in biological settings. Grubbs-Hoveyda type ruthenium catalysts with an N-heterocyclic carbene (NHC) as ancillary ligand, known to be active in olefin metathesis, have recently been incorporated in various proteins. An overview of these artificial metalloproteins and their potential application in olefin metathesis is given.

Incorporating artificial metal-cofactors into protein scaffolds results in a new class of catalysts, termed biohybrid catalysts or artificial metalloenzymes. Biohybrid catalysts can be modified chemically at the first coordination sphere of the metal complex, as well as at the second coordination sphere provided by the protein scaffold. Protein-scaffold reengineering by directed evolution exploits the full power of nature's diversity, but requires validated screening and sophisticated metal cofactor conjugation to evolve biohybrid catalysts. In this Minireview, we summarize the recent efforts in this field to establish high-throughput screening methods for biohybrid catalysts and we show how non-chiral catalysts catalyze reactions enantioselectively by highlighting the first successes in this emerging field. Furthermore, we shed light on the potential of this field and challenges that need to be overcome to advance from biohybrid catalysts to true artificial metalloenzymes.

Beilstein journal of organic chemistry 14, 2861-2871 (2018). doi:10.3762/bjoc.14.265, Published by Beilstein-Institut zur Förderung der Chemischen Wissenschaften, Frankfurt, Main

The direct hydroxylation of benzene to hydroquinone (HQ) under mild reaction conditions is a challenging task for chemical catalysts. Cytochrome P450 (CYP) monooxygenases are known to catalyze the oxidation of a variety of aromatic compounds with atmospheric dioxygen. Protein engineering campaigns led to the identification of novel P450 variants, which yielded improvements in respect to activity, specificity, and stability. An effective screening strategy is crucial for the identification of improved enzymes with desired characteristics in large mutant libraries. Here, we report a first screening system designed for screening of P450 variants capable to produce hydroquinones. The hydroquinone quantification assay is based on the interaction of 4-nitrophenylacetonitrile (NpCN) with hydroquinones under alkaline conditions. In the 96-well plate format, a low detection limit (5 μM) and a broad linear detection range (5 to 250 μM) were obtained. The NpCN assay can be used for the quantification of dihydroxylated aromatic compounds such as hydroquinones, catechols, and benzoquinones. We chose the hydroxylation of pseudocumene by P450 BM3 as a target reaction and screened for improved trimethylhydroquinone (TMHQ) formation. The new P450 BM3 variant AW2 (R47Q, Y51F, I401M, A330P) was identified by screening a saturation mutagenesis library of amino acid position A330 with the NpCN assay. In summary, a 70-fold improved TMHQ formation was achieved with P450 BM3 AW2 when compared to the wild type (WT) and a 1.8-fold improved TMHQ formation compared to the recently reported P450 BM3 M3 (R47S, Y51W, A330F, I401M). Electronic supplementary material The online version of this article (10.1007/s00253-018-9328-3) contains supplementary material, which is available to authorized users.

A series of Grubbs–Hoveyda type catalyst precursors for olefin metathesis containing a maleimide moiety in the backbone of the NHC ligand was covalently incorporated in the cavity of the β-barrel protein nitrobindin. By using two protein mutants with different cavity sizes and choosing the suitable spacer length, an artificial metalloenzyme for olefin metathesis reactions in water in the absence of any organic cosolvents was obtained. High efficiencies reaching TON > 9000 in the ROMP of a water-soluble 7-oxanorbornene derivative and TON > 100 in ring-closing metathesis (RCM) of 4,4-bis(hydroxymethyl)-1,6-heptadiene in water under relatively mild conditions (pH 6, T = 25–40 °C) were observed.

Covering hydrophobic regions with stabilization agents to solubilize purified transmembrane proteins is crucial for their application in aqueous media. The small molecule 2-methyl-2,4-pentanediol (MPD) was used to stabilize the transmembrane protein Ferric hydroxamate uptake protein component A (FhuA) utilized as host for the construction of a rhodium-based biohybrid catalyst. Unlike commonly used detergents such as sodium dodecyl sulfate or polyethylene polyethyleneglycol, MPD does not form micelles in solution. Molecular dynamics simulations revealed the effect and position of stabilizing MPD molecules. The advantage of the amphiphilic MPD over micelle-forming detergents is demonstrated in the polymerization of phenylacetylene, showing a ten-fold increase in yield and increased molecular weights.

The incorporation of organometallic catalyst precursors in proteins results in so-called artificial metalloenzymes. The protein structure will control activity, selectivity and stability of the organometallic site in aqueous medium and allow non-natural reactions in biological settings. Grubbs-Hoveyda type ruthenium catalysts with an N-heterocyclic carbene (NHC) as ancillary ligand, known to be active in olefin metathesis, have recently been incorporated in various proteins. An overview of these artificial metalloproteins and their potential application in olefin metathesis is given.

Copper(I) and copper(II) complexes were covalently linked to an engineered variant of the transmembrane protein Ferric hydroxamate uptake protein component A (FhuA ΔCVFtev). Copper(I) was incorporated using an N-heterocyclic carbene (NHC) ligand equipped with a maleimide group on the side arm at the imidazole nitrogen. Copper(II) was attached by coordination to a terpyridyl ligand. The spacer length was varied in the back of the ligand framework. These biohybrid catalysts were shown to be active in the Diels–Alder reaction of a chalcone derivative with cyclopentadiene to preferentially give the endo product.

A hybrid biocatalyst containing a metal terpyridine (tpy) complex within a rigid β-barrel protein nitrobindin (NB) is constructed. A tpy ligand with a maleimide group, N-[2-([2,2':6',2''-terpyridin]-4'-yloxy)ethyl]maleimide (1), was covalently linked to Cys96 inside the cavity of NB to prepare a conjugate NB-1. Binding of Cu(2+), Zn(2+), or Co(2+) ion to the tpy ligand in NB-1 was confirmed by UV-vis spectroscopy and ESI-TOF MS measurements. Cu(2+)-bound NB-1 is found to catalyze a Diels-Alder reaction between azachalcone and cyclopentadiene in 22% yield, which is higher than that of the Cu(2+)-tpy complex without the NB matrix. The results suggest that the hydrophobic cavity close to the copper active site within the NB scaffold supports the binding of the two substrates, dienophile and diene, to promote the reaction.

The ferric hydroxamate uptake protein component A, FhuA, is a large monomeric transmembrane protein. FhuA functions as a receptor for ferrichrome and the structurally closely related antibiotic albomycin. In addition to its biological importance, FhuA is a robust protein scaffold that can be genetically modified and is stable under a broad range of conditions. By removing the globular cork domain (deletion of amino acids 1–160), FhuA became a large passive diffusion channel (FhuA Δ1–160) with an inner diameter of about 2.0 nm. FhuA was reconstituted in liposomes and polymersomes for controlled compound release responding to reducing agents and UV light. FhuA was also re-engineered to increase its length, enlarge its diameter and harbour single functional groups (–SH and –NH2). FhuA Δ1–159 Ext with an increased hydrophobic region was generated and inserted more efficiently into polymer membranes. FhuA Δ1–159 Exp has an enlarged diameter and shows increased diffusion kinetics. The remarkable resistance of FhuA variants to organic solvents and high temperatures makes it suitable as a scaffold for accommodating hybrid catalysts to perform chemical reactions. By substituting the amino acid residues surrounding the coupling site in the interior of the FhuA channel, one can also optimize the accessibility of the coupling site and the enantioselectivity.

Anfang September fanden an der RWTH Aachen der eintagige Workshop zu kunstlichen Metallenzymen und das zweitagige Aachen-Osaka-Joint-Symposium statt, im Rahmen des internationalen und interdisziplinaren Graduiertenkollegs Selectivity in Chemo- and Biocatalysis (Seleca). Es wurde von der Universitat Osaka und der RWTH Aachen im Jahr 2000 eingerichtet. Es soll dazu beitragen, katalytische Reaktionen auf molekularer Ebene besser zu verstehen. Verschiedene Aspekte der Chemo- und Biokatalyse werden verwendet um in einem einzigartigen interdisziplinaren Ansatz funktionalisierte Verbindungen selektiv zu synthetisieren. Es kooperieren zwolf Gruppen der RWTH Aachen, drei assoziierte Gruppen des Forschungszentrums Julich und elf Gruppen der Universitat Osaka.

A biohybrid ring-opening olefin metathesis polymerization catalyst based on the reengineered β-barrel protein FhuA ΔCVF(tev) was chemically modified with respect to the covalently anchored Grubbs-Hoveyda type catalyst. Shortening of the spacer (1,3-propanediyl to methylene) between the N-heterocyclic carbene ligand and the cysteine site 545 increased the ROMP activity toward a water-soluble 7-oxanorbornene derivative. The cis/trans ratio of the double bond in the polymer was influenced by the hybrid catalyst.