Biocompatibility and therapeutic potential of glycosylated albumin artificial metalloenzymes

Inside a Shell-Organometallic Catalysis Inside Encapsulin Nanoreactors

Compartmentalization of chemical reactions inside cells are a fundamental requirement for life. Encapsulins are self-assembling protein-based nanocompartments from the prokaryotic repertoire that present a highly attractive platform for intracellular compartmentalization of chemical reactions by design. Using single-molecule Förster resonance energy transfer and 3D-MINFLUX analysis, we analyze fluorescently labeled encapsulins on a single-molecule basis. Furthermore, by equipping these capsules with a synthetic ruthenium catalyst via covalent attachment to a non-native host protein, we are able to perform in vitro catalysis and go on to show that engineered encapsulins can be used as hosts for transition metal catalysis inside living cells in confined space.

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.

In vivo metal-catalyzed SeCT therapy by a proapoptotic peptide

Selective cell tagging (SeCT) therapy is a strategy for labeling a targeted cell with certain chemical moieties via a catalytic chemical transformation in order to elicit a therapeutic effect. Herein, we report a cancer therapy based on targeted cell surface tagging with proapoptotic peptides (Ac-GGKLFG-X; X = reactive group) that induce apoptosis when attached to the cell surface. Using either Au-catalyzed amidation or Ru-catalyzed alkylation, these proapoptotic peptides showed excellent therapeutic effects both in vitro and in vivo. In particular, co-treatment with proapoptotic peptide and the carrier-Ru complex significantly and synergistically inhibited tumor growth and prolonged survival rate of tumor-bearing mice after only a single injection. This is the first report of Ru catalyst application in vivo, and this approach could be used in SeCT for cancer therapy.

In situ activation of therapeutics through bioorthogonal catalysis

Bioorthogonal chemistry refers to any chemical reactions that can occur inside of living systems without interfering with native biochemical processes, which has become a promising strategy for modulating biological processes. The development of synthetic metal-based catalysts to perform bioorthogonal reactions has significantly expanded the toolkit of bioorthogonal chemistry for medicinal chemistry and synthetic biology. A wide range of homogeneous and heterogeneous transition metal catalysts (TMCs) have been reported, mediating different transformations such as cycloaddition reactions, as well as bond forming and cleaving reactions. However, the direct application of 'naked' TMCs in complex biological media poses numerous challenges, including poor water solubility, toxicity and catalyst deactivation. Incorporating TMCs into nanomaterials to create bioorthogonal nanocatalysts can solubilize and stabilize catalyst molecules, with the decoration of the nanocatalysts used to provide spatiotemporal control of catalysis. This review presents an overview of the advances in the creation of bioorthogonal nanocatalysts, highlighting different choice of nano-scaffolds, and the therapeutic and diagnostic applications.

Bioorthogonal catalytic patch

Bioorthogonal catalysis mediated by transition metals has inspired a new subfield of artificial chemistry complementary to enzymatic reactions, enabling the selective labelling of biomolecules or in situ synthesis of bioactive agents via non-natural processes. However, the effective deployment of bioorthogonal catalysis in vivo remains challenging, mired by the safety concerns of metal toxicity or complicated procedures to administer catalysts. Here, we describe a bioorthogonal catalytic device comprising a microneedle array patch integrated with Pd nanoparticles deposited on TiO₂ nanosheets. This device is robust and removable, and can mediate the local conversion of caged substrates into their active states in high-level living systems. In particular, we show that such a patch can promote the activation of a prodrug at subcutaneous tumour sites, restoring its parent drug's therapeutic anticancer properties. This in situ applied device potentiates local treatment efficacy and eliminates off-target prodrug activation and dose-dependent side effects in healthy organs or distant tissues.

Engineering Dirhodium Artificial Metalloenzymes for Diazo Coupling Cascade Reactions*

Artificial metalloenzymes (ArMs) are commonly used to control the stereoselectivity of catalytic reactions, but controlling chemoselectivity remains challenging. In this study, we engineer a dirhodium ArM to catalyze diazo cross-coupling to form an alkene that, in a one-pot cascade reaction, is reduced to an alkane with high enantioselectivity (typically >99 % ee) by an alkene reductase. The numerous protein and small molecule components required for the cascade reaction had minimal effect on ArM catalysis. Directed evolution of the ArM led to improved yields and E/Z selectivities for a variety of substrates, which translated to cascade reaction yields. MD simulations of ArM variants were used to understand the structural role of the cofactor on ArM conformational dynamics. These results highlight the ability of ArMs to control both catalyst stereoselectivity and chemoselectivity to enable reactions in complex media that would otherwise lead to undesired side reactions.

Bioorthogonal strategies for the in vivo synthesis or release of drugs

The site-specific delivery of antitumor agents is a rapidly developing field that relies on prodrug activation and uncaging strategies. For this purpose, a wide range of homogeneous and heterogeneous biocompatible activators/catalysts have been developed to convert caged drugs with low toxicity and high stability in physiological settings into active substances in a bioorthogonal manner. The current methods allow for the site-specific delivery of activators and prodrugs to organelles, target cells, or tumors in living organisms. Here, we present an overview of the latest advances in catalytic drugs, highlighting the expanding toolbox of bioorthogonal activation strategies made possible by transition metals acting as activators or catalysts.

Artificial Organelles: Towards Adding or Restoring Intracellular Activity

Compartmentalization is one of the main characteristics that define living systems. Creating a physically separated microenvironment allows nature a better control over biological processes, as is clearly specified by the role of organelles in living cells. Inspired by this phenomenon, researchers have developed a range of different approaches to create artificial organelles: compartments with catalytic activity that add new function to living cells. In this review we will discuss three complementary lines of investigation. First, orthogonal chemistry approaches are discussed, which are based on the incorporation of catalytically active transition metal-containing nanoparticles in living cells. The second approach involves the use of premade hybrid nanoreactors, which show transient function when taken up by living cells. The third approach utilizes mostly genetic engineering methods to create bio-based structures that can be ultimately integrated with the cell's genome to make them constitutively active. The current state of the art and the scope and limitations of the field will be highlighted with selected examples from the three approaches.

Engineering a Metathesis-Catalyzing Artificial Metalloenzyme Based on HaloTag

Artificial metalloenzymes (ArMs) are created by embedding a synthetic metal catalyst into a protein scaffold. ArMs have the potential to merge the catalytic advantages of natural enzymes with the reaction scope of synthetic catalysts. The choice of the protein scaffold is of utmost importance to tune the activity of the ArM. Herein, we show the repurposing of HaloTag, a self-labeling protein widely used in chemical biology, to create an ArM scaffold for metathesis. This monomeric protein scaffold allows for covalent attachment of metathesis cofactors, and the resulting ArMs are capable of catalyzing ring-closing metathesis. Both chemical and genetic engineering were explored to determine the evolvability of the resulting ArM. Additionally, exploration of the substrate scope revealed a reaction with promising turnover numbers (>48) and conversion rates (>96%).

A minimally-masked inactive prodrug of panobinostat that is bioorthogonally activated by gold chemistry

The recent incorporation of Au chemistry in the bioorthogonal toolbox has opened up new opportunities to deliver biologically independent reactions in living environments. Herein we report that the O-propargylation of the hydroxamate group of the potent HDAC inhibitor panobinostat leads to a vast reduction of its anticancer properties (>500-fold). We also show that this novel prodrug is converted back into panobinostat in the presence of Au catalysts in vitro and in cell culture.

Controlled Ligand Exchange Between Ruthenium Organometallic Cofactor Precursors and a Naïve Protein Scaffold Generates Artificial Metalloenzymes Catalysing Transfer Hydrogenation

Many natural metalloenzymes assemble from proteins and biosynthesised complexes, generating potent catalysts by changing metal coordination. Here we adopt the same strategy to generate artificial metalloenzymes (ArMs) using ligand exchange to unmask catalytic activity. By systematically testing RuII (η6 -arene)(bipyridine) complexes designed to facilitate the displacement of functionalised bipyridines, we develop a fast and robust procedure for generating new enzymes via ligand exchange in a protein that has not evolved to bind such a complex. The resulting metal cofactors form peptidic coordination bonds but also retain a non-biological ligand. Tandem mass spectrometry and 19 F NMR spectroscopy were used to characterise the organometallic cofactors and identify the protein-derived ligands. By introduction of ruthenium cofactors into a 4-helical bundle, transfer hydrogenation catalysts were generated that displayed a 35-fold rate increase when compared to the respective small molecule reaction in solution.

Recent Advances in Biocatalysis with Chemical Modification and Expanded Amino Acid Alphabet

The two main strategies for enzyme engineering, directed evolution and rational design, have found widespread applications in improving the intrinsic activities of proteins. Although numerous advances have been achieved using these ground-breaking methods, the limited chemical diversity of the biopolymers, restricted to the 20 canonical amino acids, hampers creation of novel enzymes that Nature has never made thus far. To address this, much research has been devoted to expanding the protein sequence space via chemical modifications and/or incorporation of noncanonical amino acids (ncAAs). This review provides a balanced discussion and critical evaluation of the applications, recent advances, and technical breakthroughs in biocatalysis for three approaches: (i) chemical modification of cAAs, (ii) incorporation of ncAAs, and (iii) chemical modification of incorporated ncAAs. Furthermore, the applications of these approaches and the result on the functional properties and mechanistic study of the enzymes are extensively reviewed. We also discuss the design of artificial enzymes and directed evolution strategies for enzymes with ncAAs incorporated. Finally, we discuss the current challenges and future perspectives for biocatalysis using the expanded amino acid alphabet.

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.

Disrupting tumor onset and growth via selective cell tagging (SeCT) therapy

This study presents the early framework of selective cell tagging (SeCT) therapy, which is the concept of preferentially labeling specific cells in vivo with chemical moieties that can elicit a therapeutic response. Using glycosylated artificial metalloenzyme (GArM)-based protein labeling, this study reports two separate functional strategies. In one approach, early tumor onset can be suppressed by tagging cancer cells in living mice with an integrin-blocking cyclic-Arg-Gly-Asp (cRGD) moiety, thereby disrupting cell adhesion onto the extracellular matrix. In another approach, tumor growth in mice can be reduced by tagging with a cytotoxic doxorubicin moiety. Subsequent cell death occurs following internalization and drug release. Overall, experiments have shown that mouse populations receiving the mixture of SeCT labeling reagents exhibited a significant delay/reduction in tumor onset and growth compared with controls. Highlighting its adaptability, this work represents a foundational step for further development of SeCT therapy and its potential therapeutic applications.

Functionalization of Ruthenium Olefin-Metathesis Catalysts for Interdisciplinary Studies in Chemistry and Biology

Hoveyda–Grubbs-type complexes, ruthenium catalysts for olefin metathesis, have gained increased interest as a research target in the interdisciplinary research fields of chemistry and biology because of their high functional group selectivity in olefin metathesis reactions and stabilities in aqueous media. This review article introduces the application of designed Hoveyda–Grubbs-type complexes for bio-relevant studies including the construction of hybrid olefin metathesis biocatalysts and the development of in-vivo olefin metathesis reactions. As a noticeable issue in the employment of Hoveyda–Grubbs-type complexes in aqueous media, the influence of water on the catalytic activities of the complexes and strategies to overcome the problems resulting from the water effects are also discussed. In connection to the structural effects of protein structures on the reactivities of Hoveyda–Grubbs-type complexes included in the protein, the regulation of metathesis activities through second-coordination sphere effect is presented, demonstrating that the reactivities of Hoveyda–Grubbs-type complexes are controllable by the structural modification of the complexes at outer-sphere parts. Finally, as a new-type reaction based on the ruthenium-olefin specific interaction, a recent finding on the ruthenium complex transfer reaction between Hoveyda–Grubbs-type complexes and biomolecules is introduced.

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.

Recent Advances in the Chemical Biology of N-Glycans

Asparagine-linked N-glycans on proteins have diverse structures, and their functions vary according to their structures. In recent years, it has become possible to obtain high quantities of N-glycans via isolation and chemical/enzymatic/chemoenzymatic synthesis. This has allowed for progress in the elucidation of N-glycan functions at the molecular level. Interaction analyses with lectins by glycan arrays or nuclear magnetic resonance (NMR) using various N-glycans have revealed the molecular basis for the recognition of complex structures of N-glycans. Preparation of proteins modified with homogeneous N-glycans revealed the influence of N-glycan modifications on protein functions. Furthermore, N-glycans have potential applications in drug development. This review discusses recent advances in the chemical biology of N-glycans.

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.

Palladium in biological media: Can the synthetic chemist's most versatile transition metal become a powerful biological tool?

Palladium catalysed reactions are ubiquitous in synthetic organic chemistry in both organic solvents and aqueous buffers. The broad reactivity of palladium catalysis has drawn interest as a means to conduct orthogonal transformations in biological settings. Successful examples have been shown for protein modification, in vivo drug decaging and as palladium-protein biohybrid catalysts for selective catalysis. Biological media represents a challenging environment for palladium chemistry due to the presence of a multitude of chelators, catalyst poisons and a requirement for milder reaction conditions e.g. lower temperatures. This review looks to identify successful examples of palladium-catalysed reactions in the presence of proteins or cells and analyse solutions to help to overcome the challenges of working in biological systems.

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.

Transition metal catalysts for the bioorthogonal synthesis of bioactive agents

The incorporation of abiotic transition metal catalysis into the chemical biology space has significantly expanded the tool kit of bioorthogonal chemistries accessible for cell culture and in vivo applications. A rich variety of homogeneous and heterogeneous catalysts has shown functional compatibility with physiological conditions and biostability in complex environs, enabling their exploitation as extracellular or intracellular factories of bioactive agents. Current trends in the field are focusing on investigating new metals and sophisticated catalytic devices and toward more applied activities, such as the integration of subcellular, cell- and site-targeting capabilities or the exploration of novel biomedical applications. We present herein an overview of the latest advances in the field, highlighting the increasing role of transition metals for the controlled release of therapeutics.

Unlocking the Full Evolutionary Potential of Artificial Metalloenzymes Through Direct Metal-Protein Coordination : A review of recent advances for catalyst development

Generation of artificial metalloenzymes (ArMs) has gained much inspiration from the general understanding of natural metalloenzymes. Over the last decade, a multitude of methods generating transition metal-protein hybrids have been developed and many of these new-to-nature constructs catalyse reactions previously reserved for the realm of synthetic chemistry. This perspective will focus on ArMs incorporating 4d and 5d transition metals. It aims to summarise the significant advances made to date and asks whether there are chemical strategies, used in nature to optimise metal catalysts, that have yet to be fully recognised in the synthetic enzyme world, particularly whether artificial enzymes produced to date fully take advantage of the structural and energetic context provided by the protein. Further, the argument is put forward that, based on precedence, in the majority of naturally evolved metalloenzymes the direct coordination bonding between the metal and the protein scaffold is integral to catalysis. Therefore, the protein can attenuate metal activity by positioning ligand atoms in the form of amino acids, as well as making non-covalent contributions to catalysis, through intermolecular interactions that pre-organise substrates and stabilise transition states. This highlights the often neglected but crucial element of natural systems that is the energetic contribution towards activating metal centres through protein fold energy. Finally, general principles needed for a different approach to the formation of ArMs are set out, utilising direct coordination inspired by the activation of an organometallic cofactor upon protein binding. This methodology, observed in nature, delivers true interdependence between metal and protein. When combined with the ability to efficiently evolve enzymes, new problems in catalysis could be addressed in a faster and more specific manner than with simpler small molecule catalysts.

Nitroreductase-Mediated Release of Inhibitors of Lysine-Specific Demethylase 1 (LSD1) from Prodrugs in Transfected Acute Myeloid Leukaemia Cells

Lysine-specific demethylase 1 (LSD1) has evolved as a promising therapeutic target for cancer treatment, especially in acute myeloid leukaemia (AML). To approach the challenge of site-specific LSD1 inhibition, we developed an enzyme-prodrug system with the bacterial nitroreductase NfsB (NTR) that was expressed in the virally transfected AML cell line THP1-NTR+ . The cellular activity of the NTR was proven with a new luminescent NTR probe. We synthesised a diverse set of nitroaromatic prodrugs that by design do not affect LSD1 and are reduced by the NTR to release an active LSD1 inhibitor. The emerging side products were differentially analysed using negative controls, thereby revealing cytotoxic effects. The 2-nitroimidazolyl prodrug of a potent LSD1 inhibitor emerged as one of the best prodrug candidates with a pronounced selectivity window between wild-type and transfected THP1 cells. Our prodrugs are selectively activated and release the LSD1 inhibitor locally, proving their suitability for future targeting approaches.

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.

Artificial Glycoproteins as a Scaffold for Targeted Drug Therapy

Akin to a cellular "fingerprint," the glycocalyx is a glycan-enriched cellular coating that plays a crucial role in mediating cell-to-cell interactions. To gain a better understanding of the factors that govern in vivo recognition, artificial glycoproteins were initially created to probe changes made to the accumulation and biodistribution of specific glycan assemblies through biomimicry. As a result, the organ-specific accumulation for a variety of glycoproteins decorated with simple and/or complex glycans was identified. Additionally, binding trends with regard to cancer cell selectivity were also investigated. To exploit the knowledge gained from these studies, numerous groups thus became engaged in developing targeted drug methodologies based on the use of artificial glycoproteins. This has either been done through adopting the glycoprotein scaffold as a drug carrier, or to directly glycosylate therapeutic proteins/enzymes to localize their biological activity. The principle aim of this Review is to present the foundational research that has driven artificial glycoprotein-based targeting and subsequent adaptations with potential therapeutic applications.

Artificial Metalloenzymes: From Selective Chemical Transformations to Biochemical Applications

Artificial metalloenzymes (ArMs) comprise a synthetic metal complex in a protein scaffold. ArMs display performances combining those of both homogeneous catalysts and biocatalysts. Specifically, ArMs selectively catalyze non-natural reactions and reactions inspired by nature in water under mild conditions. In the past few years, the construction of ArMs that possess a genetically incorporated unnatural amino acid and the directed evolution of ArMs have become of great interest in the field. Additionally, biochemical applications of ArMs have steadily increased, owing to the fact that compartmentalization within a protein scaffold allows the synthetic metal complex to remain functional in a sea of inactivating biomolecules. In this review, we present updates on: 1) the newly reported ArMs, according to their type of reaction, and 2) the unique biochemical applications of ArMs, including chemoenzymatic cascades and intracellular/in vivo catalysis. We believe that ArMs have great potential as catalysts for organic synthesis and as chemical biology tools for pharmaceutical applications.

Enabling protein-hosted organocatalytic transformations

In this review, the development of organocatalytic artificial enzymes will be discussed. This area of protein engineering research has underlying importance, as it enhances the biocompatibility of organocatalysis for applications in chemical and synthetic biology research whilst expanding the catalytic repertoire of enzymes. The approaches towards the preparation of organocatalytic artificial enzymes, techniques used to improve their performance (selectivity and reactivity) as well as examples of their applications are presented. Challenges and opportunities are also discussed.

Proteins as diverse, efficient, and evolvable scaffolds for artificial metalloenzymes

We have extracted and categorized the desirable properties of proteins that are adapted as the scaffolds for artificial metalloenzymes.

Unlocking the therapeutic potential of artificial metalloenzymes

In order to harness the functionality of metals, nature has evolved over billions of years to utilize metalloproteins as key components in numerous cellular processes. Despite this, transition metals such as ruthenium, palladium, iridium, and gold are largely absent from naturally occurring metalloproteins, likely due to their scarcity as precious metals. To mimic the evolutionary process of nature, the field of artificial metalloenzymes (ArMs) was born as a way to benefit from the unique chemoselectivity and orthogonality of transition metals in a biological setting. In its current state, numerous examples have successfully incorporated transition metals into a variety of protein scaffolds. Using these ArMs, many examples of new-to-nature reactions have been carried out, some of which have shown substantial biocompatibility. Given the rapid rate at which this field is growing, this review aims to highlight some important studies that have begun to take the next step within this field; namely the development of ArM-centered drug therapies or biotechnological tools.

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.

Cancer-derived exosomes loaded with ultrathin palladium nanosheets for targeted bioorthogonal catalysis

The transformational impact of bioorthogonal chemistries has inspired new strategies for the in vivo synthesis of bioactive agents through non-natural means. Among these, palladium (Pd) catalysts have played a prominent role in the growing subfield of bioorthogonal catalysis by producing xenobiotics and uncaging biomolecules in living systems. However, delivering catalysts selectively to specific cell types still lags behind catalyst development. Here we have developed a bio-artificial device consisting of cancer-derived exosomes loaded with Pd catalysts by a method that enables the controlled assembly of Pd nanosheets directly inside the vesicles. This hybrid system mediates Pd-triggered dealkylation reactions in vitro and inside cells and displays preferential tropism for their progenitor cells. The use of Trojan exosomes to deliver abiotic catalysts into designated cancer cells creates the opportunity for a new targeted therapy modality: exosome-directed catalyst prodrug therapy, whose first steps are presented herein with the cell-specific release of the anticancer drug panobinostat.