Cross-Linked Artificial Enzyme Crystals as Heterogeneous Catalysts for Oxidation Reactions

Selective inhibition of NikA mediated Ni(II) import in E. coli by the Indium(III)-EDTA complex

Nickel is a required nutrient for bacteria to produce [NiFe]-hydrogenase and urease enzymes. [NiFe]-hydrogenase catalyzes the reversible conversion of hydrogen into protons and electrons and urease catalyzes the hydrolysis of urea into carbon dioxide and ammonia-both key in bacterial pathogenesis. As such, nickel trafficking and homeostasis are interesting targets for potential antibacterial strategies. In E. coli, NikA binds a Ni(II)-(L-His)2 chelate in the periplasm and delivers this complex to the NikBCDE transporter. Blocking Ni(II) uptake by NikA would prevent the biosynthesis of active [NiFe]-hydrogenase. Fe(III)-EDTA is a potent ligand for NikA, however due to the potential for reduction of Fe(III) to Fe(II), it has limited utility. Using Fe(III)-EDTA as a starting point for inhibitor design, similar stable complexes of Bismuth(III), Lutetium(III) and Indium(III) were investigated. The In(III)-EDTA complex is a potent inhibitor of cellular [NiFe]-hydrogenase activity (IC50 of 600 μM ± 100 μM) while being nontoxic to bacterial growth. The mechanism of In(III)-EDTA hydrogenase inhibition was confirmed by the inhibition of Ni(II)-dependent processing of HycE (hydrogenase-3), which could be rescued with the addition of exogenous nickel. To elucidate the binding affinity of In(III)-EDTA to NikA, isothermal titration calorimetry (ITC) was carried out, revealing stoichiometric 1:1 binding with a Kd of 17.3 µM ± 3.0 µM. Indium concentrations determined by inductively coupled plasma mass spectrometry in E. coli cells in the presence or absence of NikA showed no discernable difference, further supporting the competitive inhibition of nickel uptake by blocking NikA.

Hyper-Expandable Cross-Linked Protein Crystals as Scaffolds for Catalytic Reactions

Scaffolding catalytic reactions within porous materials is a powerful strategy to enhance the reaction rates of multicatalytic systems. However, it remains challenging to develop materials with high porosity, high diversity of functional groups within the pores, and guest-adaptive tunability. Furthermore, it is challenging to capture large catalysts such as enzymes within porous materials. Protein-based materials are promising candidates to overcome these limitations, owing to their large pore sizes and potential for stimuli-responsive adaptability. In this work, hydrogel beads were generated from cross-linked lysozyme crystals. These swellable lysozyme cross-linked crystals (SLCCs) expand more than 10 mL per gram of crystal following a simple treatment in ethanol, followed by the addition of water. SLCCs are sensitive to the solution environment and change their extent of swelling from adjusting the concentration and identity of the ions in the solution, or by changing the flexibility of the protein backbone, such as adding dithiothreitol to reduce the protein disulfide bonds. SLCCs can adsorb a wide range of catalysts ranging from transition metal complexes to large biomacromolecules, such as the 160 kDa enzyme glucose oxidase (GOx). Transition metal catalysts and enzymes captured within SLCCs maintained their catalytic activity and exhibited minimal leaching. We performed a cascade reaction by adsorbing GOx and the transition metal catalyst Fe-TAML into SLCCs, resulting in enhanced activity compared to a free-floating reaction. SLCCs offer a promising combination of attributes as scaffolds for multicatalytic reactions, including gram-scale batch preparation, tunable expansion to greater than 20-fold in volume, guest-responsive adaptable behavior, and facile capture of a wide array of small molecule and enzyme-catalysts.

EcNikA, a versatile tool in the field of artificial metalloenzymes

This review describes the multiple advantages of using of EcNikA, a nickel transport protein, in the design of artificial metalloenzymes as alternative catalysts for synthetic biology. The rationale behind the strategy of artificial enzyme design is discussed, with particular emphasis on de novo active site reconstitution. The impact of the protein scaffold on the artificial active site and thus the final catalytic properties is detailed, highlighting the considerable aptitude of hybrid systems to catalyze selective reactions, from alkene to thioether transformations (epoxidation, hydroxychlorination, sulfoxidation). The different catalytic approaches - from in vitro to in cristallo - are compared, revealing the considerable advantages of protein crystals in terms of stabilization and acceleration of reaction kinetics. The versatility of proteins, based on metal and ligand diversity and medium/physical conditions, are thus illustrated for oxidation catalysis.

Redox-reversible siderophore-based catalyst anchoring within cross-linked artificial metalloenzyme aggregates enables enantioselectivity switching

The immobilisation of artificial metalloenzymes (ArMs) holds promise for the implementation of new biocatalytic reactions. We present the synthesis of cross-linked artificial metalloenzyme aggregates (CLArMAs) with excellent recyclability, as an alternative to carrier-based immobilisation strategies. Furthermore, iron-siderophore supramolecular anchoring facilitates redox-triggered cofactor release, enabling CLArMAs to be recharged with alternative cofactors for diverse selectivity.

Review on sterilization techniques, and the application potential of phage lyase and lyase immobilization in fighting drug-resistant bacteria

Many human health problems and property losses caused by pathogenic contamination cannot be underestimated. Bactericidal techniques have been extensively studied to address this issue of public health and economy. Bacterial resistance develops as a result of the extensive use of single or multiple but persistent usage of sterilizing drugs, and the emergence of super-resistant bacteria brings new challenges. Therefore, it is crucial to control pathogen contamination by applying innovative and effective sterilization techniques. As organisms that exist in nature and can specifically kill bacteria, phages have become the focus as an alternative to antibacterial agents. Furthermore, phage-encoded lyases are proteins that play important roles in phage sterilization. The in vitro sterilization of phage lyase has been developed as a novel biosterilization technique to reduce bacterial resistance and is more environmentally friendly than conventional sterilization treatments. For the shortcomings of enzyme applications, this review discusses the enzyme immobilization methods and the application potential of immobilized lyases for sterilization. Although some techniques provide effective solutions, immobilized lyase sterilization technology has been proven to be a more effective innovation for efficient pathogen killing and reducing bacterial resistance. We hope that this review can provide new insights for the development of sterilization techniques.

An artificial metalloenzyme that can oxidize water photocatalytically: design, synthesis, and characterization

In nature, light-driven water oxidation (WO) catalysis is performed by photosystem II via the delicate interplay of different cofactors positioned in its protein scaffold. Artificial systems for homogeneous photocatalytic WO are based on small molecules that often have limited solubility in aqueous solutions. In this work, we alleviated this issue and present a cobalt-based WO-catalyst containing artificial metalloenzyme (ArM) that is active in light-driven, homogeneous WO catalysis in neutral-pH aqueous solutions. A haem-containing electron transfer protein, cytochrome B5 (CB5), served to host a first-row transition-metal-based WO catalyst, CoSalen (CoIISalen, where H2Salen = N,N'-bis(salicylidene)ethylenediamine), thus producing an ArM capable of driving photocatalytic WO. The CoSalen ArM formed a water-soluble pre-catalyst in the presence of [Ru(bpy)3](ClO4)2 as photosensitizer and Na2S2O8 as the sacrificial electron acceptor, with photocatalytic activity similar to that of free CoSalen. During photocatalysis, the CoSalen-protein interactions were destabilized, and the protein partially unfolded. Rather than forming tens of nanometer sized CoOx nanoparticles as free CoSalen does under photocatalytic WO conditions, the CB5 : CoSalen ArM showed limited protein cross-linking and remained soluble. We conclude that a weak, dynamic interaction between a soluble cobalt species and apoCB5 was formed, which generated a catalytically active adduct during photocatalysis. A detailed analysis was performed on protein stability and decomposition processes during the harsh oxidizing reaction conditions of WO, which will serve for the future design of WO ArMs with improved activity and stability.

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.

A Co(TAML)-based artificial metalloenzyme for asymmetric radical-type oxygen atom transfer catalysis

We show that the incorporation of a biotinylated Co(TAML) cofactor within streptavidin enables asymmetric radical-type oxygen atom transfer catalysis with improved activity and enantioselectivity.

Displaying a Protein Cage on a Protein Crystal by In-Cell Crystal Engineering

The development of solid biomaterials has rapidly progressed in recent years in applications in bionanotechnology. The immobilization of proteins, such as enzymes, within protein crystals is being used to develop solid catalysts and functionalized materials. However, an efficient method for encapsulating protein assemblies has not yet been established. This work presents a novel approach to displaying protein cages onto a crystalline protein scaffold using in-cell protein crystal engineering. The polyhedra crystal (PhC) scaffold, which displays a ferritin cage, was produced by coexpression of polyhedrin monomer (PhM) and H1-ferritin (H1-Fr) monomer in Escherichia coli. The H1-tag is derived from the H1-helix of PhM. Our technique represents a unique strategy for immobilizing protein assemblies onto in-cell protein crystals and is expected to contribute to various applications in bionanotechnology.

Spatially Patterned, Porous Protein Crystals as Multifunctional Materials

While the primary use of protein crystals has historically been in crystallographic structure determination, they have recently emerged as promising materials with many advantageous properties such as high porosity, biocompatibility, stability, structural and functional versatility, and genetic/chemical tailorability. Here, we report that the utility of protein crystals as functional materials can be further augmented through their spatial patterning and control of their morphologies. To this end, we took advantage of the chemically and kinetically controllable nature of ferritin self-assembly and constructed core-shell crystals with chemically distinct domains, tunable structural patterns, and morphologies. The spatial organization within ferritin crystals enabled the generation of patterned, multi-enzyme frameworks with cooperative catalytic behavior. We further exploited the differential growth kinetics of ferritin crystal facets to assemble Janus-type architectures with an anisotropic arrangement of chemically distinct domains. These examples represent a step toward using protein crystals as reaction vessels for complex multi-step reactions and broadening their utility as functional, solid-state materials. Our results demonstrate that morphology control and spatial patterning, which are key concepts in materials science and nanotechnology, can also be applied for engineering protein crystals.

Cross-Linked Crystals of Dirhodium Tetraacetate/RNase A Adduct Can Be Used as Heterogeneous Catalysts

Due to their unique coordination structure, dirhodium paddlewheel complexes are of interest in several research fields, like medicinal chemistry, catalysis, etc. Previously, these complexes were conjugated to proteins and peptides for developing artificial metalloenzymes as homogeneous catalysts. Fixation of dirhodium complexes into protein crystals is interesting to develop heterogeneous catalysts. Porous solvent channels present in protein crystals can benefit the activity by increasing the probability of substrate collisions at the catalytic Rh binding sites. Toward this goal, the present work describes the use of bovine pancreatic ribonuclease (RNase A) crystals with a pore size of 4 nm (P3₂21 space group) for fixing [Rh₂(OAc)₄] and developing a heterogeneous catalyst to perform reactions in an aqueous medium. The structure of the [Rh₂(OAc)₄]/RNase A adduct was investigated by X-ray crystallography: the metal complex structure remains unperturbed upon protein binding. Using a number of crystal structures, metal complex accumulation over time, within the RNase A crystals, and structures at variable temperatures were evaluated. We also report the large-scale preparation of microcrystals (∼10-20 μm) of the [Rh₂(OAc)₄]/RNase A adduct and cross-linking reaction with glutaraldehyde. The catalytic olefin cyclopropanation reaction and self-coupling of diazo compounds by these cross-linked [Rh₂(OAc)₄]/RNase A crystals were demonstrated. The results of this work reveal that these systems can be used as heterogeneous catalysts to promote reactions in aqueous solution. Overall, our findings demonstrate that the dirhodium paddlewheel complexes can be fixed in porous biomolecule crystals, like those of RNase A, to prepare biohybrid materials for catalytic applications.

Unique Mechanical Properties of Gel-Incorporating Protein Crystals

Crystalline materials that are grown in gel media exhibit reinforced mechanical characteristics. Studies on the mechanical properties of protein crystals are limited in numbers because of the difficulty in growing high-quality large crystals. This study shows the demonstration of the unique macroscopic mechanical properties by compression tests of large protein crystals grown in both solution and agarose gel. Particularly, the gel-incorporating protein crystals exhibit larger elastic limits and a higher fracture stress compared with the native protein crystals without gel. Conversely, the change in the Young's modulus corresponding to if the crystals incorporate the gel network is negligible. This suggests that gel networks affect only the fracture phenomenon. Thus, reinforced mechanical characteristics that cannot be obtained by the gel or the protein crystal alone can be developed. By combining the gel media and protein crystals, the gel-incorporating protein crystals show the potential to toughen without sacrificing other mechanical properties.

The Synthetic Approaches to 1,2-Chlorohydrins

This short review highlights the hitherto realised synthetic approaches towards organic 1,2-chlorohydrins by functionalisation of alkenes (i.e., 1,2-chlorohydroxylation), which is the most prominent access route to this class of compounds. Also, some other synthetic approaches involving the reduction of α-chloroketones, the epoxide opening ring by chloride anions and the utilisation of Grignard reagents for the synthesis of these compounds and chlorination of allylic alcohols are highlighted. Finally, enzymatic reactions for the formation of chlorohydrins are briefly summarised followed by a short view on natural products containing this moiety.

1 Introduction

2 Applications for the Synthesis of 1,2-Chlorohydrins

2.1 Chlorohydroxylation of Alkenes

2.2 Reduction of Chloroketones

2.3 Metalorganic Reagents

2.4 Epoxide Ring Opening

2.5 Chlorination of Allylic Alcohols

2.6 Biochemical Methods

2.7 Selected Applications in Natural Product Total Synthesis

3 Conclusion

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

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

Artificial Metalloproteins: At the Interface between Biology and Chemistry

Artificial metalloproteins (ArMs) have recently gained significant interest due to their potential to address issues in a broad scope of applications, including biocatalysis, biotechnology, protein assembly, and model chemistry. ArMs are assembled by the incorporation of a non-native metallocofactor into a protein scaffold. This can be achieved by a number of methods that apply tools of chemical biology, computational de novo design, and synthetic chemistry. In this Perspective, we highlight select systems in the hope of demonstrating the breadth of ArM design strategies and applications and emphasize how these systems address problems that are otherwise difficult to do so with strictly biochemical or synthetic approaches.

Multilayered Ordered Protein Arrays Self-Assembled from a Mixed Population of Virus-like Particles

Biology shows many examples of spatially controlled assembly of cells and biomacromolecules into hierarchically organized structures, to which many of the complex biological functions are attributed. While such biological structures have inspired the design of synthetic materials, it is still a great challenge to control the spatial arrangement of individual building blocks when assembling multiple types of components into bulk materials. Here, we report self-assembly of multilayered, ordered protein arrays from mixed populations of virus-like particles (VLPs). We systematically tuned the magnitude of the surface charge of the VLPs via mutagenesis to prepare four different types of VLPs for mixing. A mixture of up to four types of VLPs selectively assembled into higher-order structures in the presence of oppositely charged dendrimers during a gradual lowering of the ionic strength of the solution. The assembly resulted in the formation of three-dimensional ordered VLP arrays with up to four distinct layers including a central core, with each layer comprising a single type of VLP. A coarse-grained computational model was developed and simulated using molecular dynamics to probe the formation of the multilayered, core-shell structure. Our findings establish a simple and versatile bottom-up strategy to synthesize multilayered, ordered materials by controlling the spatial arrangement of multiple types of nanoscale building blocks in a one-pot fabrication.

Tetraphenylporphyrin Enters the Ring: First Example of a Complex Between Highly Bulky Porphyrins and a Protein

Tetraphenylporphyrin (TPP) is a symmetrically substituted synthetic porphyrin whose properties can be readily modified, providing it with significant advantages over naturally occurring porphyrins. Herein, we report the first example of a stable complex between a native biomolecule, the haemoprotein HasA, and TPP as well as its derivatives. The X-ray crystal structures of nine different HasA-TPP complexes were solved at high resolutions. HasA capturing TPP derivatives was also demonstrated to inhibit growth of the opportunistic pathogen Pseudomonas aeruginosa . Mutant variants of HasA binding FeTPP were shown to possess a different mode of coordination, permitting the cyclopropanation of styrene.

Engineering of protein crystals for use as solid biomaterials

Protein crystals have attracted a great deal of attention as solid biomaterials because they have porous structures created by regular assemblies of proteins. The lattice structures of protein crystals are controlled by designing molecular interfacial interactions via covalent bonds and non-covalent bonds. Protein crystals have been functionalized as templates to immobilize foreign molecules such as metal nanoparticles, metal complexes, and proteins. These hybrid crystals are used as functional materials for catalytic reactions and structural analysis. Furthermore, in-cell protein crystals have been studied extensively, providing progress in rapid protein crystallization and crystallography. This review highlights recent advances in crystal engineering for protein crystallization and generation of solid functional materials both in vitro and within cells.

mRNA delivery via non-viral carriers for biomedical applications

As an emerging new class of nucleic acid drugs, messenger RNA (mRNA) has huge potential in immunotherapy, regenerative medicine, vaccine, and gene editing. Comparing with siRNA and pDNA, mRNA is more vulnerable to nucleases in vivo. However, the lack of effective and safe delivery methods impedes the broad application of mRNA-based therapeutics. Up to now, the delivery of mRNA remains largely unexplored, and therefore, is a hot topic in the field of gene therapy. In this review, we will summarize the ongoing challenges in mRNA-based therapeutics and unmet requirements for delivery vehicles in terms of the unique structure of mRNA. We then highlight the advancement in mRNA delivery in both fundamental research and clinical applications. Finally, a prospective will be proposed upon reviewing the current progress in mRNA delivery.

LEAFY protein crystals with a honeycomb structure as a platform for selective preparation of outstanding stable bio-hybrid materials

Well-organized protein assemblies offer many properties that justify their use for the design of innovative bionanomaterials. Herein, crystals of the oligomerization domain of the LEAFY protein from Ginkgo biloba, organized in a honeycomb architecture, were used as a modular platform for the selective grafting of a ruthenium-based complex. The resulting bio-hybrid crystalline material was fully characterized by UV-visible and Raman spectroscopy and by mass spectrometry and LC-MS analysis after selective enzymatic digestion. Interestingly, insertion of complexes within the tubular structure affords an impressive increase in stability of the crystals, eluding the use of stabilizing cross-linking strategies.

Repurposing metalloproteins as mimics of natural metalloenzymes for small-molecule activation

Artificial metalloenzymes (ArMs) consist of an unnatural metal or cofactor embedded in a protein scaffold, and are an excellent platform for applying the concepts of protein engineering to catalysis. In this Focused Review, we describe the application of ArMs as simple, tunable artificial models of the active sites of complex natural metalloenzymes for small-molecule activation. In this sense, ArMs expand the strategies of synthetic model chemistry to protein-based supporting ligands with potential for participation from the second coordination sphere. We focus specifically on ArMs that are structural, spectroscopic, and functional models of enzymes for activation of small molecules like CO, CO₂, O₂, N₂, and NO, as well as production/consumption of H₂. These ArMs give insight into the identities and roles of metalloenzyme structural features within and near the cofactor. We give examples of ArM work relevant to hydrogenases, acetyl-coenzyme A synthase, superoxide dismutase, heme oxygenases, nitric oxide reductase, methyl-coenzyme M reductase, copper-O₂ enzymes, and nitrogenases.

Production of Cross-Linked Lipase Crystals at a Preparative Scale

The autoimmobilization of enzymes via cross-linked enzyme crystals (CLECs) has regained interest in recent years, boosted by the extensive knowledge gained in protein crystallization, the decrease of cost and laboriousness of the process, and the development of potential applications. In this work, we present the crystallization and preparative-scale production of reinforced cross-linked lipase crystals (RCLLCs) using a commercial detergent additive as a raw material. Bulk crystallization was carried out in 500 mL of agarose media using the batch technique. Agarose facilitates the homogeneous production of crystals, their cross-linking treatment, and their extraction. RCLLCs were active in an aqueous solution and in hexane, as shown by the hydrolysis of p-nitrophenol butyrate and α-methylbenzyl acetate, respectively. RCLLCs presented both high thermal and robust operational stability, allowing the preparation of a packed-bed chromatographic column to work in a continuous flow. Finally, we determined the three-dimensional (3D) models of this commercial lipase crystallized with and without phosphate at 2.0 and 1.7 Å resolutions, respectively.

Facile Fabrication of Protein-Macrocycle Frameworks

Precisely defined protein aggregates, as exemplified by crystals, have applications in functional materials. Consequently, engineered protein assembly is a rapidly growing field. Anionic calix[n]arenes are useful scaffolds that can mold to cationic proteins and induce oligomerization and assembly. Here, we describe protein-calixarene composites obtained via cocrystallization of commercially available sulfonato-calix[8]arene (sclx₈) with the symmetric and “neutral” protein RSL. Cocrystallization occurred across a wide range of conditions and protein charge states, from pH 2.2–9.5, resulting in three crystal forms. Cationization of the protein surface at pH ∼ 4 drives calixarene complexation and yielded two types of porous frameworks with pore diameters >3 nm. Both types of framework provide evidence of protein encapsulation by the calixarene. Calixarene-masked proteins act as nodes within the frameworks, displaying octahedral-type coordination in one case. The other framework formed millimeter-scale crystals within hours, without the need for precipitants or specialized equipment. NMR experiments revealed macrocycle-modulated side chain pK_a values and suggested a mechanism for pH-triggered assembly. The same low pH framework was generated at high pH with a permanently cationic arginine-enriched RSL variant. Finally, in addition to protein framework fabrication, sclx₈ enables de novo structure determination.

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.

Incorporation of a Cp*Rh(III)-dithiophosphate Cofactor with Latent Activity into a Protein Scaffold Generates a Biohybrid Catalyst Promoting C(sp2)-H Bond Functionalization

A Cp*Rh(III)-dithiophosphate cofactor with "latent" catalytic activity was developed to construct an artificial metalloenzyme representing a new type of biohybrid catalyst which is capable of promoting C(sp²)-H bond functionalization within the β-barrel structure of nitrobindin (NB). To covalently conjugate the Cp*Rh(III) cofactor into a specific position of the hydrophobic cavity of NB via a maleimide-Cys linkage, strong chelation of the dithiophosphate ligand is employed to protect the rhodium metal center against attack by nucleophilic amino acid residues in the protein. It is found that subsequent addition of the Ag⁺ ion induces dissociation of the dithiophosphate ligands, thereby activating the catalytic activity of the Cp*Rh(III) cofactor. The resulting "active" biohybrid catalyst promotes cycloaddition of acetophenone oxime with diphenylacetylene via C(sp²)-H bond activation. This catalytic activity is enhanced 2.3-fold with the introduction of two glutamate residues (A100E/L125E) adjacent to the Cp*Rh(III) cofactor. The Cp*Rh(III) cofactor with switchable activity from a "latent" form to an "active" form provides a new strategy for generating biohybrid catalysts incorporating a variety of highly reactive transition metal complexes specifically within its protein scaffolds.

In Situ Absorption and Fluorescence Microspectroscopy Investigation of the Molecular Incorporation Process into Single Nanoporous Protein Crystals

Protein crystals exhibit distinct three-dimensional structures, which contain well-ordered nanoporous solvent channels, providing a chemically heterogeneous environment. In this paper, the incorporation of various molecules into the solvent channels of native hen egg-white lysozyme crystals was demonstrated using fluorescent dyes, including acridine yellow G, rhodamine 6G, and eosin Y. The process was evaluated on the basis of absorption and fluorescence microspectroscopy at a single-crystal level. The molecular loading process was clearly visualized as a function of time, and it was determined that the protein crystals could act as nanoporous materials. It was found that the incorporation process is strongly dependent on the molecular charge, leading to heterogeneous molecular aggregation, which suggests host-guest interaction of protein crystals from the viewpoint of nanoporous materials.

High mobility of lattice molecules and defects during the early stage of protein crystallization

Protein crystals are expected to be useful not only for their molecular structure analysis but also as functional materials due to their unique properties. Although the generation and the propagation of defects during crystallization play critical roles in the final properties of protein crystals, the dynamics of these processes are poorly understood. By time-resolved liquid-cell transmission electron microscopy, we observed that nanosized crystal defects are surprisingly mobile during the early stages of the crystallization of a lysozyme as a model protein. This highly dynamic behavior of defects reveals that the lattice molecules are mobile throughout the crystal structure. Moreover, the disappearance of the defects indicated that intermolecular bonds can break and reform rapidly with little energetic cost, as reported in theoretical studies. All these findings are in marked contrast to the generally accepted notion that crystal lattices are rigid with very limited mobility of individual lattice molecules.

Engineered assembly of a protein–cucurbituril biohybrid

Additional Q7 binding sites drive protein aggregation in solution and statistical disorder in the crystalline biohybrid suggest new possibilities for protein-based materials.

Protein encapsulation in the hollow space of hemocyanin crystals containing a covalently conjugated ligand

Encapsulation of guest molecules into the vacant space of biomacromolecular crystals has been utilized for various purposes including functioning as a protein container to protect against physical stress and structural determination of the guest. Todarodes pacificus hemocyanin (TpHc) is a hollow cylindrical decameric protein complex with an inner space 110 Å in diameter and 160 Å in height. In the crystal, TpHc forms a straw-like bundle and contains one reactive Cys (Cys3246) in the inner domain of each protomer. Here, we conjugated biotin onto Cys3246 of TpHc followed by incubation with streptavidin. The streptavidin was immobilized into the inner space of TpHc due to its interaction with biotin. Moreover, the complex containing TpHc and streptavidin was crystallized under the same conditions used for unmodified TpHc. In order to expand this methodology for a variety of proteins, we conjugated the ligand nitrilotriacetic acid (NTA) chelated to a Ni²⁺ ion (Ni²⁺-NTA) to TpHc. We found that His-tagged green fluorescent protein (GFP) was encapsulated into the Ni²⁺-NTA-conjugated TpHc via the interaction between the His-tag and the Ni²⁺-NTA group. X-ray crystallography demonstrated that the crystal packing of the complex containing TpHc and GFP was identical to that of the unmodified TpHc. Our guest immobilization method is distinct from previous approaches that are dependent on diffusion of the guest into the host crystal. Thus, our findings may accelerate the development of proteinaceous crystal engineering.

Titanium(iv)-folded single-chain polymeric nanoparticles as artificial metalloenzyme for asymmetric sulfoxidation in water

Intrachain TiIV-oxazoline complexation together with hydrophobic interaction triggered the self-folding of an oxazoline-containing single polymeric chain in water. The formed TiIV-folded single-chain polymeric nanoparticles (SCPNs) acted as metalloenzyme-mimetic catalysts in asymmetric sulfoxidation in water owing to their organized, compartmentalized structure, effective site isolation, and also secondary coordination sphere provided by a copolymer backbone. In addition, they also could be facilely recovered for reuse by simple thermo-controlled separation.

Crystalline protein scaffolds as a defined environment for the synthesis of bioinorganic materials

We discuss synthetic strategies and applications of highly ordered bioinorganic materials based on crystalline protein scaffolds.