Engineering and emerging applications of artificial metalloenzymes with whole cells

Recent advances in the design and optimization of artificial metalloenzymes

Embedding a catalytically competent transition metal into a protein scaffold affords an artificial metalloenzyme (ArM). Such hybrid catalysts display features that are reminiscent of both homogeneous and enzymatic catalysts. Pioneered by Whitesides and Kaiser in the late 1970s, this field of ArMs has expanded over the past two decades, marked by ever-increasing diversity in reaction types, cofactors, and protein scaffolds. Recent noteworthy developments include i) the use of earth-abundant metal cofactors, ii) concurrent cascade reactions, iii) synergistic catalysis, and iv) in vivo catalysis. Thanks to significant progress in computational protein design, ArMs based on de novo-designed proteins and tailored chimeric proteins promise a bright future for this exciting field.

Chemogenetic Evolution of Diversified Photoenzymes for Enantioselective [2 + 2] Cycloadditions in Whole Cells

Artificial photoenzymes with novel catalytic modes not found in nature are in high demand; yet, they also present significant challenges in the field of biocatalysis. In this study, a chemogenetic modification strategy is developed to facilitate the rapid diversification of photoenzymes. This strategy integrates site-specific chemical conjugation of various artificial photosensitizers into natural protein cavities and the iterative mutagenesis in cell lysates. Through rounds of directed evolution, prominent visible-light-activatable photoenzyme variants were developed, featuring a thioxanthone chromophore. They successfully enabled the enantioselective [2 + 2] photocycloaddition of 2-carboxamide indoles, a class of UV-sensitive substrates that are traditionally challenging for known photoenzymes. Furthermore, the versatility of this photoenzyme is demonstrated in enantioselective whole-cell photobiocatalysis, enabling the efficient synthesis of enantioenriched cyclobutane-fused indoline tetracycles. These findings significantly expand the photophysical properties of artificial photoenzymes, a critical factor in enhancing their potential for harnessing excited-state reactivity in stereoselective transformations.

Nanoscale and chiral metal-organic frameworks for asymmetric reactions in water: bridging Lewis acid catalysis and biological systems

Nowadays, stereoselective control over the sheer variety of chemical transformations benefits from the multipotency of chiral Lewis acids. Their use under biocompatible conditions has long posed a challenge because profuse amounts of biogenic nucleophiles readily deactivate them. To bridge the gap between chiral Lewis acid catalysis and biocompatible chemistry, the conversion of UiO(BPY)-type nanosized metal-organic frameworks (NMOFs) into chiral variants was herein exemplified. The combination of an elongated 2,2'-bipyridyl linker and scandium salt with a hydrophobic anion proved essential to implement traits such as robustness, biocompatibility, and catalytic activity. The catalyst could construct sufficiently hydrophobic environments sequestered within the framework, catalyzing asymmetric ring-opening reactions of meso-epoxide with low catalyst loading to afford β-amino acid alcohols in high yield (up to >99%) with high enantioselectivity (up to 88%). Most impressively, it exhibited a tolerance to the ex vivo poisoning of chiral Lewis acid catalysis by biogenic nucleophiles in sharp contrast to conventional water-compatible Lewis acids.

Biodegradation of the strobilurin fungicide pyraclostrobin by Burkholderia sp. Pyr-1: Characteristics, degradation pathway, water remediation, and toxicity assessment

Pyraclostrobin, a widely used fungicide, poses significant risks to both the environment and human health. However, research on the microbial degradation process of pyraclostrobin was scarce. Here, a pyraclostrobin-degrading strain, identified as Burkholderia sp. Pyr-1, was isolated from activated sludge. Pyraclostrobin was efficiently degraded by strain Pyr-1, and completely eliminated within 6 d in the presence of glucose. Additionally, pyraclostrobin degradation was significantly enhanced by the addition of divalent metal cations (Mn2+ and Cu2+). The degradation pathway involving ether bond and N-O bond cleavage was proposed by metabolite identification. The sodium alginate-immobilized strain Pyr-1 had a higher pyraclostrobin removal rate from contaminated lake water than the free cells. Moreover, the toxicity evaluation demonstrated that the metabolite 1-(4-chlorophenyl)-1H-pyrazol-3-ol significantly more effectively inhibited Chlorella ellipsoidea than pyraclostrobin, while its degradation products by strain Pyr-1 alleviated the growth inhibition of C. ellipsoidea, which confirmed that the low-toxic metabolites were generated from pyraclostrobin by strain Pyr-1. The study provides a potential strain Pyr-1 for the bioremediation in pyraclostrobin-contaminated aquatic environments.

Catch-and-Release: The Assembly, Immobilization, and Recycling of Redox-Reversible Artificial Metalloenzymes

Technologies to improve the applicability of artificial metalloenzymes (ArMs) are gaining considerable interest; one such approach is the immobilization of these biohybrid catalysts on support materials to enhance stability and enable their retention, recovery, and reuse. Here, we describe the immobilization of polyhistidine-tagged ArMs that allow the redox-controlled replacement of catalytic cofactors that have lost activity, e.g., due to poisoning or decomposition, on immobilized metal affinity chromatography resins. By using periplasmic siderophore-binding protein scaffolds that originate from thermophilic bacteria (GstCeuE and PthCeuE) in combination with a siderophore-linked imine reduction catalyst, reaction rates were achieved that are about 3.5 times faster than those previously obtained with CjCeuE, the analogous protein of Campylobacter jejuni. Upon immobilization, the GstCeuE-derived ArM showed a decrease in turnover frequency in the reduction of dehydrosalsolidine by 3.4-fold, while retaining enantioselectivity (36%) and showing improved stability that allowed repeat recovery and recycling cycles. Catalytic activity was preserved over the initial four cycles. In subsequent cycles, a gradual reduction of activity was evident. Once the initial activity decreased to around 40% of the initial activity (23rd recycling cycle), the redox-triggered artificial cofactor release permitted the subsequent recharging of the immobilized protein scaffold with fresh, active cofactor, thereby restoring the initial catalytic activity of the immobilized ArM and allowing its reuse for several more cycles. Furthermore, the ArM could be assembled directly from protein present in crude cell extracts, avoiding time-consuming and costly protein purification steps. Overall, this study demonstrates that the immobilization of redox-reversible ArMs facilitates their "catch-and-release" assembly and disassembly and the recycling of their components, improving their potential commercial viability and environmental footprint.

Creating new-to-nature carbon fixation: A guide

Synthetic biology aims at designing new biological functions from first principles. These new designs allow to expand the natural solution space and overcome the limitations of naturally evolved systems. One example is synthetic CO2-fixation pathways that promise to provide more efficient ways for the capture and conversion of CO2 than natural pathways, such as the Calvin Benson Bassham (CBB) cycle of photosynthesis. In this review, we provide a practical guideline for the design and realization of such new-to-nature CO2-fixation pathways. We introduce the concept of "synthetic CO2-fixation", and give a general overview over the enzymology and topology of synthetic pathways, before we derive general principles for their design from their eight naturally evolved analogs. We provide a comprehensive summary of synthetic carbon-assimilation pathways and derive a step-by-step, practical guide from the theoretical design to their practical implementation, before ending with an outlook on new developments in the field.

Target-Specific Bioorthogonal Reactions for Precise Biomedical Applications

Bioorthogonal chemistry is a promising toolbox for dissecting biological processes in the native environment. Recently, bioorthogonal reactions have attracted considerable attention in the medical field for treating diseases, since this approach may lead to improved drug efficacy and reduced side effects via in situ drug synthesis. For precise biomedical applications, it is a prerequisite that the reactions should occur in the right locations and on the appropriate therapeutic targets. In this minireview, we highlight the design and development of targeted bioorthogonal reactions for precise medical treatment. First, we compile recent strategies for achieving target-specific bioorthogonal reactions. Further, we emphasize their application for the precise treatment of different therapeutic targets. Finally, a perspective is provided on the challenges and future directions of this emerging field for safe, efficient, and translatable disease treatment.

Binding Interactions and Inhibition Mechanisms of Gold Complexes in Thiamine Diphosphate-Dependent Enzymes

Thiamine diphosphate (ThDP)-dependent enzymes possess the unique ability to generate a carbene within their active site. In this study, we sought to harness this carbene to produce a Au(I) N-heterocyclic complex directly in the active site of ThDP enzymes, thereby establishing a novel platform for artificial metalloenzymes. Because direct metalation of ThDP proved challenging, we synthesized a ThDP mimic that acts as a competitive inhibitor with a high affinity (Ki = 1.5 μM). Upon metalation with Au(I), we observed that the complex became a more potent inhibitor (Ki = 0.7 μM). However, detailed analysis of the inhibition mode, native mass spectrometry, and size exclusion experiments revealed that the complex does not bind specifically to the active site of ThDP enzymes. Instead, it exhibits unspecific binding and exceeds the 1:1 stoichiometry. Similar binding patterns were observed for other Au(I) species. These findings prompt an important question regarding the inherent propensity of ThDP enzymes to bind strongly to Au. If this phenomenon holds true, it could pave the way for the development of Au-based drugs targeting these enzymes.

Engineering of Therapeutic and Detoxifying Enzymes

Therapeutic enzymes present excellent opportunities for the treatment of human disease, modulation of metabolic pathways and system detoxification. However, current use of enzyme therapy in the clinic is limited as naturally occurring enzymes are seldom optimal for such applications and require substantial improvement by protein engineering. Engineering strategies such as design and directed evolution that have been successfully implemented for industrial biocatalysis can significantly advance the field of therapeutic enzymes, leading to biocatalysts with new-to-nature therapeutic activities, high selectivity, and suitability for medical applications. This minireview highlights case studies of how state-of-the-art and emerging methods in protein engineering are explored for the generation of therapeutic enzymes and discusses gaps and future opportunities in the field of enzyme therapy.

FhuA: From Iron-Transporting Transmembrane Protein to Versatile Scaffolds through Protein Engineering

ConspectusProtein engineering has emerged as a powerful methodology to tailor the properties of proteins. It empowers the design of biohybrid catalysts and materials, thereby enabling the convergence of materials science, chemistry, and medicine. The choice of a protein scaffold is an important factor for performance and potential applications. In the past two decades, we utilized the ferric hydroxamate uptake protein FhuA. FhuA is, from our point of view, a versatile scaffold due to its comparably large cavity and robustness toward temperature as well as organic cosolvents. FhuA is a natural iron transporter located in the outer membrane of Escherichia coli (E. coli). Wild-type FhuA consists of 714 amino acids and has a β-barrel structure composed of 22 antiparallel β-sheets, closed by an internal globular "cork" domain (amino acids 1-160). FhuA is robust in a broad pH range and toward organic cosolvents; therefore, we envisioned FhuA to be a suitable platform for various applications in (i) biocatalysis, (ii) materials science, and (iii) the construction of artificial metalloenzymes.(i) Applications in biocatalysis were achieved by removing the globular cork domain (FhuA_Δ1-160), thereby creating a large pore for the passive transport of otherwise difficult-to-import molecules through diffusion. Introducing this FhuA variant into the outer membrane of E. coli facilitates the uptake of substrates for downstream biocatalytic conversion. Furthermore, removing the globular "cork" domain without structural collapse of the ß-barrel protein allowed the use of FhuA as a membrane filter, exhibiting a preference for d-arginine over l-arginine.(ii) FhuA is a transmembrane protein, which makes it attractive to be used for applications in non-natural polymeric membranes. Inserting FhuA into polymer vesicles yielded so-called synthosomes (i.e., catalytic synthetic vesicles in which the transmembrane protein acted as a switchable gate or filter). Our work in this direction enables polymersomes to be used in biocatalysis, DNA recovery, and the controlled (triggered) release of molecules. Furthermore, FhuA can be used as a building block to create protein-polymer conjugates to generate membranes.(iii) Artificial metalloenzymes (ArMs) are formed by incorporating a non-native metal ion or metal complex into a protein. This combines the best of two worlds: the vast reaction and substrate scope of chemocatalysis and the selectivity and evolvability of enzymes. With its large inner diameter, FhuA can harbor (bulky) metal catalysts. Among others, we covalently attached a Grubbs-Hoveyda-type catalyst for olefin metathesis to FhuA. This artificial metathease was then used in various chemical transformations, ranging from polymerizations (ring-opening metathesis polymerization) to enzymatic cascades involving cross-metathesis. Ultimately, we generated a catalytically active membrane by copolymerizing FhuA and pyrrole. The resulting biohybrid material was then equipped with the Grubbs-Hoveyda-type catalyst and used in ring-closing metathesis.The number of reports on FhuA and its various applications indicates that it is a versatile building block to generate hybrid catalysts and materials. We hope that our research will inspire future research efforts at the interface of biotechnology, catalysis, and material science in order to create biohybrid systems that offer smart solutions for current challenges in catalysis, material science, and medicine.

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.

Enantioselective conjugate addition of malonates to α,β-unsaturated aldehydes catalysed by 4-oxalocrotonate tautomerase

The enantioselective conjugate addition of malonates to α,β-unsaturated aldehydes catalysed by 4-oxalocrotonate tautomerase is described. High conversions, high enantioselectivities, and good isolation yields were achieved for a range of substrates. We further completed a four-step synthesis of the antidepressant (+)-femoxetine by utilizing this reaction and an enzymatic reductive amination reaction.

A Membrane-Embedded Macromolecular Catalyst with Substrate Selectivity in Live Cells

Substrate selectivity is one of the most attractive features of natural enzymes from their "bind-to-catalyze" working flow and is thus a goal for the development of synthetic enzyme mimics that mediate abiotic transformations. However, despite the recent success in the preparation of substrate-selective enzyme mimics based on single-chain nanoparticles, examples extending such selectivity into living systems have been absent. In this article, we report the in cellulo substrate selectivity of an enzyme-mimicking macromolecular catalyst based on a cationic dense-shell nanoparticle (DSNP) scaffold. With a systematic study on DSNP's structure-activity relationship, we demonstrate that the DSNP has excellent membrane affinity that is governed by several contributing factors, namely, charge density, type of charge, and particle size, and the best-performing phosphonium-rich DSNP can be used as a membrane-embedded catalyst (MEC) for efficient on-membrane synthesis. Importantly, the DSNP catalyst retains its selectivity toward lipophilic and anionic substrates when working as an MEC for on-membrane ligation. The usefulness of such substrate selectivity and on-membrane catalysis strategy was exemplified with several molecules of interest with low cell permeability and anionic nature, which were successfully transported into eukaryotic cells by after their formation directly on the cell membrane.

Progress, Challenges, and Opportunities with Artificial Metalloenzymes in Biosynthesis

In this Perspective, we present progress, outstanding challenges, and opportunities for the incorporation of artificial metalloenzymes (ArMs) into biosynthetic pathways. We first explain discoveries within the field of ArMs that led to the potential inclusion of these enzymes in biosynthesis. We then describe the specific barriers that our laboratory, in collaboration with the laboratories of Keasling and Mukhopadhyay, addressed to establish a biosynthetic pathway containing an ArM. This biosynthesis produced an unnatural cyclopropyl terpenoid by combining heterologous production of the terpene with modification of its terminal alkene by an ArM built from a cytochrome P450. Finally, we describe the remaining challenges and opportunities related to the application of ArMs in synthetic biology.

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.

NIR-II Light Leveraged Dual Drug Synthesis for Orthotopic Combination Therapy

Pd-catalyzed bioorthogonal bond cleavage reactions are widely used and frequently reported. It is circumscribed by low reaction efficiency, which may encumber the therapeutic outcome when applied to physiological environments. Herein, an NIR-II light promoted integrated catalyst (CuS@PDA/Pd) (PDA - polydopamine) is designed to accelerate the reaction efficiency and achieve a dual bioorthogonal reaction for combination therapy. As NIR-II light can penetrate deeply into tissue, the Pd-mediated cleavage reaction can be promoted both in vitro and in vivo by the photothermal properties of CuS, beneficial to orthotopic 4T1 tumor treatment. In addition, CuS also catalyzes the synthesis of active resveratrol analogs by the CuAAC reaction. These simultaneously produced anticancer agents result in enhanced antitumor cytotoxicity in comparison to the single treatments. This is a fascinating study to devise an integrated catalyst boosted by NIR-II light for dual bioorthogonal catalysis, which may provide the impetus for efficient bioorthogonal combination therapy in vivo.

MeDBA: the Metalloenzyme Data Bank and Analysis platform

Metalloenzymes are attractive research targets in fields of chemistry, biology, and medicine. Given that metalloenzymes can manifest conservation of metal-coordination and ligand binding modes, the excavation and expansion of metalloenzyme-specific knowledge is of interest in bridging metalloenzyme-related fields. Building on our previous metalloenzyme-ligand association database, MeLAD, we have expanded the scope of metalloenzyme-specific knowledge and services, by forming a versatile platform, termed the Metalloenzyme Data Bank and Analysis (MeDBA). The MeDBA provides: (i) manual curation of metalloenzymes into different categories, that this M-I, M-II and M-III; (ii) comprehensive information on metalloenzyme activities, expression profiles, family and disease links; (iii) structural information on metalloenzymes, in particular metal binding modes; (iv) metalloenzyme substrates and bioactive molecules acting on metalloenzymes; (v) excavated metal-binding pharmacophores and (vi) analysis tools for structure/metal active site comparison and metalloenzyme profiling. The MeDBA is freely available at https://medba.ddtmlab.org.

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.

Evaluation of acute toxicity of cancer-targeting albumin-based artificial metalloenzymes

Recently, the development of abiotic metal-mediated drug delivery has been significant growth in the fields of anticancer approach and biomedical application. However, the intrinsic toxicity of abiotic metal catalysts makes in vivo use difficult. Our group developed a system of cancer-targeting albumin-based artificial metalloenyzmes (ArMs) capable of performing localized drug synthesis and selective tagging therapy in vivo for cancer therapy. The toxicity of the system at higher concentrations was investigated in vitro and in vivo in the study to demonstrate its safety for potential application in clinical trials. In cell-based experiments, the study revealed that the cytotoxicity of metal catalysts anchored within the binding cavity of the cancer-targeting ArMs could be significantly reduced compared to free-in-solution metal catalysts. Moreover, the in vivo data demonstrated that the cancer-targeting ArMs did not cause considerable damage in organs or change in the hematological parameters in a single-dose (160 mg/Kg) toxicity study in rats. Therefore, the system is safe, highlighting that it could be used in clinical trials for cancer treatment.

Organometallic catalysis in aqueous and biological environments: harnessing the power of metal carbenes

Translating the power of transition metal catalysis to the native habitats of enzymes can significantly expand the possibilities of interrogating or manipulating natural biological systems, including living cells and organisms. This is especially relevant for organometallic reactions that have shown great potential in the field of organic synthesis, like the metal-catalyzed transfer of carbenes. While, at first sight, performing metal carbene chemistry in aqueous solvents, and especially in biologically relevant mixtures, does not seem obvious, in recent years there has been a growing number of reports demonstrating the feasibility of the task. Either using small molecule metal catalysts or artificial metalloenzymes, a number of carbene transfer reactions that tolerate aqueous and biorelevant media are being developed. This review intends to summarize the most relevant contributions, and establish the state of the art in this emerging research field.

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.