Metal-substituted protein MRI contrast agents engineered for enhanced relaxivity and ligand sensitivity

Directed evolution of Escherichia coli surface-displayed Vitreoscilla hemoglobin as an artificial metalloenzyme for the synthesis of 5-imino-1,2,4-thiadiazoles

Artificial metalloenzymes (ArMs) are constructed by anchoring organometallic catalysts to an evolvable protein scaffold. They present the advantages of both components and exhibit considerable potential for the in vivo catalysis of new-to-nature reactions. Herein, Escherichia coli surface-displayed Vitreoscilla hemoglobin (VHbSD-Co) that anchored the cobalt porphyrin cofactor instead of the original heme cofactor was used as an artificial thiourea oxidase (ATOase) to synthesize 5-imino-1,2,4-thiadiazoles. After two rounds of directed evolution using combinatorial active-site saturation test/iterative saturation mutagenesis (CAST/ISM) strategy, the evolved six-site mutation VHbSD-Co (6SM-VHbSD-Co) exhibited significant improvement in catalytic activity, with a broad substrate scope (31 examples) and high yields with whole cells. This study shows the potential of using VHb ArMs in new-to-nature reactions and demonstrates the applicability of E. coli surface-displayed methods to enhance catalytic properties through the substitution of porphyrin cofactors in hemoproteins in vivo.

Biogenic Imaging Contrast Agents

Imaging contrast agents are widely investigated in preclinical and clinical studies, among which biogenic imaging contrast agents (BICAs) are developing rapidly and playing an increasingly important role in biomedical research ranging from subcellular level to individual level. The unique properties of BICAs, including expression by cells as reporters and specific genetic modification, facilitate various in vitro and in vivo studies, such as quantification of gene expression, observation of protein interactions, visualization of cellular proliferation, monitoring of metabolism, and detection of dysfunctions. Furthermore, in human body, BICAs are remarkably helpful for disease diagnosis when the dysregulation of these agents occurs and can be detected through imaging techniques. There are various BICAs matched with a set of imaging techniques, including fluorescent proteins for fluorescence imaging, gas vesicles for ultrasound imaging, and ferritin for magnetic resonance imaging. In addition, bimodal and multimodal imaging can be realized through combining the functions of different BICAs, which helps overcome the limitations of monomodal imaging. In this review, the focus is on the properties, mechanisms, applications, and future directions of BICAs.

A facile and sensitive magnetic relaxation sensing strategy based on the conversion of Fe3+ ions to Prussian blue precipitates for the detection of alkaline phosphatase and ascorbic acid oxidase

Herein, a novel magnetic relaxation sensing strategy based on the change in Fe³⁺ content has been proposed by utilizing the conversion of Fe³⁺ ions to Prussian blue (PB) precipitates. Compared with the common detection approach based on the valence state change of Fe³⁺ ions, our strategy can cause a larger change in the relaxation time of water protons and higher detection sensitivity since PB precipitate can induce a larger change in the Fe³⁺ ion concentration and has a weaker effect on the relaxation process of water protons relative to Fe²⁺ ions. Then, we employ alkaline phosphatase (ALP) as a model target to verify the feasibility and detection performance of the as-proposed strategy. Actually, ascorbic acid (AA) generated from the ALP-catalyzed L-ascorbyl-2-phosphate hydrolysis reaction can reduce potassium ferricyanide into potassium ferrocyanide, and potassium ferrocyanide reacts with Fe³⁺ to form PB precipitates, leading to a higher relaxation time. Under optimum conditions, the method for ALP detection has a wide linear range from 5 to 230 mU/mL, and the detection limit is 0.28 mU/mL, sufficiently demonstrating the feasibility and satisfactory analysis performance of this strategy, which opens up a new path for the construction of magnetic relaxation sensors. Furthermore, this strategy has also been successfully applied to ascorbic acid oxidase detection, suggesting its expansibility in magnetic relaxation detection.

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.

Fabrication of magnetic nanoprobes for ultrahigh-field magnetic resonance imaging

Ultrahigh-field magnetic resonance imaging (UHF-MRI) has been attracting tremendous attention in biomedical imaging owing to its high signal-to-noise ratio, superior spatial resolution, and fast imaging speed. However, at UHF-MRI, there is a lack of proper imaging probes that can impart superior imaging sensitivity of disease lesions because conventional contrast agents generally produce pronounced susceptibility artifacts and induce very strong T₂ decay effects, thus hindering satisfactory imaging performance. This review focused on the recent development of high-performance nanoprobes that can improve the sensitivity and specificity of UHF-MRI. Firstly, the contrast enhancement mechanism of nanoprobes at UHF-MRI has been elucidated. In particular, the strategies for modulating nanoprobe performance, including size effects, metal alloying and magnetic-dopant effects, surface effects, and stimuli-response regulation, have been comprehensively discussed. Furthermore, we illustrate the remarkable advances in the design of UHF-MRI nanoprobes for medical diagnosis, such as early-stage primary tumor and metastasis imaging, angiography, and dynamic monitoring of biosignaling factors in vivo. Finally, we provide a summary and outlook on the development of cutting-edge UHF-MRI nanoprobes for advanced biomedical imaging.

The metal dependence of single-metal mediated phosphodiester bond cleavage: a QM/MM study of a multifaceted human enzyme

Nucleases catalyze the cleavage of phosphodiester bonds in nucleic acids using a range of metal cofactors. Although it is well accepted that many nucleases rely on two metal ions, the one-metal mediated pathway is debated. Furthermore, one-metal mediated nucleases maintain activity in the presence of many different metals, but the underlying reasons for this broad metal specificity are unknown. The human apurinic/apyrimidinic endonuclease (APE1), which plays a key role in DNA repair, transcription regulation, and gene expression, is a prototypical example of a one-metal dependent nuclease. Although Mg²⁺ is the native metal cofactor, APE1 remains catalytically active in the presence of several metals, with the rate decreasing as Mg²⁺ > Mn²⁺ > Ni²⁺ > Zn²⁺, while Ca²⁺ completely abolished the activity. The present work uses quantum mechanics-molecular mechanics techniques to map APE1-facilitated phosphodiester bond hydrolysis in the presence of these metals. The structural differences in stationary points along the reaction pathway shed light on the interplay between several factors that allow APE1 to remain catalytically active for various metals, with the trend in the barrier heights correlating with the experimentally reported APE1 catalytic activity. In contrast, Ca²⁺ significantly changes the metal coordination and active site geometry, and thus completely inhibits catalysis. Our work thereby provides support for the controversial single-metal mediated phosphodiester bond cleavage and clarifies uncertainties regarding the role of the metal and metal identity in this important reaction. This information is key for future medicinal and biotechnological applications including disease diagnosis and treatment, and protein engineering.

Emerging nanobiotechnology-encoded relaxation tuning establishes new MRI modes to localize, monitor and predict diseases

Magnetic resonance imaging (MRI) is one of the most important techniques in the diagnosis of many diseases including cancers, where contrast agents (CAs) are usually necessary to improve its precision and sensitivity. Previous MRI CAs are confined to the signal-to-noise ratio (SNR) elevation of lesions for precisely localizing lesions. As nanobiotechnology advances, some new MRI CAs or nanobiotechnology-enabled MRI modes have been established to vary the longitudinal or transverse relaxation of CAs, which are harnessed to detect lesion targets, monitor disease evolution, predict or evaluate curative effect, etc. These distinct cases provide unexpected insights into the correlation of the design principles of these nanobiotechnologies and corresponding MRI CAs with their potential applications. In this review, first, we briefly present the principles, classifications and applications of conventional MRI CAs, and then elucidate the recent advances in relaxation tuning via the development of various nanobiotechnologies with emphasis on the design strategies of nanobiotechnology and the corresponding MRI CAs to target the tumor microenvironment (TME) and biological targets or activities in tumors or other diseases. In addition, we exemplified the advantages of these strategies in disease theranostics and explored their potential application fields. Finally, we analyzed the present limitations, potential solutions and future development direction of MRI after its combination with nanobiotechnology.

Longitudinal manganese-enhanced magnetic resonance imaging of neural projections and activity

Manganese-enhanced magnetic resonance imaging (MEMRI) holds exceptional promise for preclinical studies of brain-wide physiology in awake-behaving animals. The objectives of this review are to update the current information regarding MEMRI and to inform new investigators as to its potential. Mn(II) is a powerful contrast agent for two main reasons: (1) high signal intensity at low doses; and (2) biological interactions, such as projection tracing and neural activity mapping via entry into electrically active neurons in the living brain. High-spin Mn(II) reduces the relaxation time of water protons: at Mn(II) concentrations typically encountered in MEMRI, robust hyperintensity is obtained without adverse effects. By selectively entering neurons through voltage-gated calcium channels, Mn(II) highlights active neurons. Safe doses may be repeated over weeks to allow for longitudinal imaging of brain-wide dynamics in the same individual across time. When delivered by stereotactic intracerebral injection, Mn(II) enters active neurons at the injection site and then travels inside axons for long distances, tracing neuronal projection anatomy. Rates of axonal transport within the brain were measured for the first time in "time-lapse" MEMRI. When delivered systemically, Mn(II) enters active neurons throughout the brain via voltage-sensitive calcium channels and clears slowly. Thus behavior can be monitored during Mn(II) uptake and hyperintense signals due to Mn(II) uptake captured retrospectively, allowing pairing of behavior with neural activity maps for the first time. Here we review critical information gained from MEMRI projection mapping about human neuropsychological disorders. We then discuss results from neural activity mapping from systemic Mn(II) imaged longitudinally that have illuminated development of the tonotopic map in the inferior colliculus as well as brain-wide responses to acute threat and how it evolves over time. MEMRI posed specific challenges for image data analysis that have recently been transcended. We predict a bright future for longitudinal MEMRI in pursuit of solutions to the brain-behavior mystery.

Directed Evolution of Artificial Metalloenzymes in Whole Cells

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

Assembly and Evolution of Artificial Metalloenzymes within E. coli Nissle 1917 for Enantioselective and Site-Selective Functionalization of C─H and C═C Bonds

The potential applications afforded by the generation and reactivity of artificial metalloenzymes (ArMs) in microorganisms are vast. We show that a non-pathogenic E. coli strain, Nissle 1917 (EcN), is a suitable host for the creation of ArMs from cytochrome P450s and artificial heme cofactors. An outer-membrane receptor in EcN transports an iridium porphyrin into the cell, and the Ir-CYP119 (CYP119 containing iridium porphyrin) assembled in vivo catalyzes carbene insertions into benzylic C-H bonds enantioselectively and site-selectively. The application of EcN as a whole-cell screening platform eliminates the need for laborious processing procedures, drastically increases the ease and throughput of screening, and accelerates the development of Ir-CYP119 with improved catalytic properties. Studies to identify the transport machinery suggest that a transporter different from the previously assumed ChuA receptor serves to usher the iridium porphyrin into the cytoplasm.

Corrole–protein interactions in H-NOX and HasA

Mutagenesis was utilised to reveal corrole–protein interactions in H-NOX and HasA. The key interaction is a hydrogen bond between the PO unit of the corrole and a protonated histidine residue.

Designer Heme Proteins: Achieving Novel Function with Abiological Heme Analogues

Heme proteins have proven to be a convenient platform for the development of designer proteins with novel functionalities. This is achieved by substituting the native iron porphyrin cofactor with a heme analogue that possesses the desired properties. Replacing the iron center of the porphyrin with another metal provides one inroad to novel protein function. A less explored approach is substitution of the porphyrin cofactor with an alternative tetrapyrrole macrocycle or a related ligand. In general, these ligands exhibit chemical properties and reactivity that are distinct from those of porphyrins. While these techniques have most prominently been utilized to develop artificial metalloenzymes, there are many other applications of this methodology to problems in biochemistry, health, and medicine. Incorporation of synthetic cofactors into protein environments represents a facile way to impart water solubility and biocompatibility. It circumvents the laborious synthesis of water-soluble cofactors, which often introduces substantial charge that leads to undesired bioaccumulation. To this end, the incorporation of unnatural cofactors in heme proteins has enabled the development of designer proteins as optical oxygen sensors, MRI contrast agents, spectroscopic probes, tools to interrogate protein function, antibiotics, and fluorescent proteins.Incorporation of an artificial cofactor is frequently accomplished by denaturing the holoprotein with removal of the heme; the refolded apoprotein is then reconstituted with the artificial cofactor. This process often results in substantial protein loss and does not necessarily guarantee that the refolded protein adopts the native structure. To circumvent these issues, our laboratory has pioneered the use of the RP523 strain of E. coli to incorporate artificial cofactors into heme proteins using expression-based methods. This strain lacks the ability to biosynthesize heme, and the bacterial cell wall is permeable to heme and related molecules. In this way, heme analogues supplemented in the growth medium are incorporated into heme proteins. This approach can also be leveraged for the direct expression of the apoprotein for subsequent reconstitution.These methodologies have been exploited to incorporate non-native cofactors into heme proteins that are resistant to harsh environmental conditions: the heme nitric oxide/oxygen binding protein (H-NOX) from Caldanaerobacter subterraneus (Cs) and the heme acquisition system protein A (HasA) from Pseudomonas aeruginosa (Pa). The exceptional stability of these proteins makes them ideal scaffolds for biomedical applications. Optical oxygen sensing has been accomplished using a phosphorescent ruthenium porphyrin as the artificial heme cofactor. Paramagnetic manganese and gadolinium porphyrins yield high-relaxivity, protein-based MRI contrast agents. A fluorescent phosphorus corrole serves as a heme analogue to produce fluorescent proteins. Iron complexes of nonporphyrin cofactors bound to HasA inhibit the growth of pathogenic bacteria. Moreover, HasA can deliver a gallium phthalocyanine into the bacterial cytosol to serve as a sensitizer for photochemical sterilization. Together, these examples illustrate the potential for designer heme proteins to address burgeoning problems in the areas of health and medicine. The concepts and methodologies presented in this Account can be extended to the development of next-generation biomedical sensing and imaging agents to identify and quantify clinically relevant metabolites and other key disease biomarkers.

A Heme-Acquisition Protein Reconstructed with a Cobalt 5-Oxaporphyrinium Cation and Its Growth-Inhibition Activity Toward Multidrug-Resistant Pseudomonas aeruginosa

Using artificial hemes for the reconstruction of natural heme proteins represents a fascinating approach to enhance the bioactivity of the latter. Here, we report the synthesis of various metal 5-oxaporphyrinium cations as cofactors, and a cobalt 5-oxaporphyrinium cation was successfully incorporated into the heme-acquisition protein (HasA) secreted by Pseudomonas aeruginosa. We hypothesize that the oxaporphyrinium cation strongly bound to the HasA-specific outer membrane receptor (HasR) due to its cationic charge, which prevents the subsequent acquisition of heme. In fact, the reconstructed HasA inhibited the growth of Pseudomonas aeruginosa and even of multidrug-resistant P. aeruginosa.

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

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

De novo biosynthesis of a nonnatural cobalt porphyrin cofactor in E. coli and incorporation into hemoproteins

Enzymes that bear a nonnative or artificially introduced metal center can engender novel reactivity and enable new spectroscopic and structural studies. In the case of metal-organic cofactors, such as metalloporphyrins, no general methods exist to build and incorporate new-to-nature cofactor analogs in vivo. We report here that a common laboratory strain, Escherichia coli BL21(DE3), biosynthesizes cobalt protoporphyrin IX (CoPPIX) under iron-limited, cobalt-rich growth conditions. In supplemented minimal media containing CoCl₂, the metabolically produced CoPPIX is directly incorporated into multiple hemoproteins in place of native heme b (FePPIX). Five cobalt-substituted proteins were successfully expressed with this new-to-nature cobalt porphyrin cofactor: myoglobin H64V V68A, dye decolorizing peroxidase, aldoxime dehydratase, cytochrome P450 119, and catalase. We show conclusively that these proteins incorporate CoPPIX, with the CoPPIX making up at least 95% of the total porphyrin content. In cases in which the native metal ligand is a sulfur or nitrogen, spectroscopic parameters are consistent with retention of native metal ligands. This method is an improvement on previous approaches with respect to both yield and ease-of-implementation. Significantly, this method overcomes a long-standing challenge to incorporate nonnatural cofactors through de novo biosynthesis. By utilizing a ubiquitous laboratory strain, this process will facilitate spectroscopic studies and the development of enzymes for CoPPIX-mediated biocatalysis.

A Bioinspired Nanoprobe with Multilevel Responsive T1 -Weighted MR Signal-Amplification Illuminates Ultrasmall Metastases

Metastasis remains the major cause of death in cancer patients. Thus, there is a need to sensitively detect tumor metastasis, especially ultrasmall metastasis, for early diagnosis and precise treatment of cancer. Herein, an ultrasensitive T₁ -weighted magnetic resonance imaging (MRI) contrast agent, UMFNP-CREKA is reported. By conjugating the ultrasmall manganese ferrite nanoparticles (UMFNPs) with a tumor-targeting penta-peptide CREKA (Cys-Arg-Glu-Lys-Ala), ultrasmall breast cancer metastases are accurately detected. With a behavior similar to neutrophils' immunosurveillance process for eliminating foreign pathogens, UMFNP-CREKA exhibits a chemotactic "targeting-activation" capacity. UMFNP-CREKA is recruited to the margin of tumor metastases by the binding of CREKA with fibrin-fibronectin complexes, which are abundant around tumors, and then release of manganese ions (Mn²⁺ ) to the metastasis in response to pathological parameters (mild acidity and elevated H₂ O₂ ). The localized release of Mn²⁺ and its interaction with proteins affects a marked amplification of T₁ -weighted magnetic resonance (MR) signals. In vivo T₁ -weighted MRI experiments reveal that UMFNP-CREKA can detect metastases at an unprecedented minimum detection limit of 0.39 mm, which has significantly extended the detection limit of previously reported MRI probe.

Mn-Porphyrin-Based Metal-Organic Framework with High Longitudinal Relaxivity for Magnetic Resonance Imaging Guidance and Oxygen Self-Supplementing Photodynamic Therapy

A nanoplatform for magnetic resonance imaging guidance and oxygen self-supplementing photodynamic therapy (PDT) was constructed on the basis of a porous metal-organic framework (PCN-222(Mn)), which was built by simple Mn-porphyrin ligands and biocompatible Zr⁴⁺ ions. Because of the good dispersibility of Mn³⁺ in the open framework and the high water affinity of the channel, PCN-222(Mn) exhibits a high longitudinal relaxivity of ∼35.3 mM^-1 s^-1 (1.0 T). In addition, it shows good catalytic activity for the conversion of endogenous hydrogen peroxide into oxygen, thereby improving tumor hypoxia during photodynamic therapy. The intravenous injection of PCN-222(Mn) into tumor-bearing mice mode provided good T₁-weighted contrast of the tumor site and effectively inhibited tumor growth upon a single-laser irradiation. The findings provide insights for the development of multifunctional theranostic nanoplatforms based on simple components.

Background Signal-Free Magnetic Bioassay for Food-Borne Pathogen and Residue of Veterinary Drug via Mn(VII)/Mn(II) Interconversion

Paramagnetic ion-mediated sensors can greatly simplify current magnetic sensors for biochemical assays, but it remains challenging because of the limited sensitivity. Herein, we report a magnetic immunosensor relying on Mn(VII)/Mn(II) interconversion and the corresponding change in the low-field nuclear magnetic resonance (LF-NMR) of the transverse relaxation rate (R₂). The fact that the NMR R₂ of the water protons detected in Mn(II) aqueous solution is much stronger than Mn(VII) aqueous solution enables the modulation of the LF-NMR signal intensity of R₂. By employing immunomagnetic separation and enzyme-catalyzed reaction, this Mn(VII)/Mn(II) interconversion allows the development of a background signal-free magnetic immunosensor with a high signal-to-background ratio that enables detection of ractopamine and Salmonella with high sensitivity (the limits of detection for ractopamine and Salmonella are 8.1 pg/mL and 20 cfu/mL, respectively). This Mn-mediated magnetic immunosensor not only retains the good stability but also greatly improves the sensitivity of conventional paramagnetic ion-mediated magnetic sensors, offering a promising platform for sensitive, stable, and convenient bioanalysis.

Highly malleable haem-binding site of the haemoprotein HasA permits stable accommodation of bulky tetraphenylporphycenes

Iron(iii)- and cobalt(iii)-9,10,19,20-tetraphenylporphycenes, which possess bulky phenyl groups at the four meso positions of porphycene, were successfully incorporated into the haem acquisition protein HasA secreted by Pseudomonas aeruginosa. Crystal structure analysis revealed that loops surrounding the haem-binding site are highly flexible, remodelling themselves to accommodate bulky metal complexes with significantly different structures from the native haem cofactor.

Noble-Metal Substitution in Hemoproteins: An Emerging Strategy for Abiological Catalysis

Enzymes have evolved to catalyze a range of biochemical transformations with high efficiencies and unparalleled selectivities, including stereoselectivities, regioselectivities, chemoselectivities, and substrate selectivities, while typically operating under mild aqueous conditions. These properties have motivated extensive research to identify or create enzymes with reactivity that complements or even surpasses the reactivity of small-molecule catalysts for chemical reactions. One of the limitations preventing the wider use of enzymes in chemical synthesis, however, is the narrow range of bond constructions catalyzed by native enzymes. One strategy to overcome this limitation is to create artificial metalloenzymes (ArMs) that combine the molecular recognition of nature with the reactivity discovered by chemists. This Account describes a new approach for generating ArMs by the formal replacement of the natural iron found in the porphyrin IX (PIX) of hemoproteins with noble metals. Analytical techniques coupled with studies of chemical reactivity have demonstrated that expression of apomyoglobins and apocytochrome P450s (for which "apo-" denotes the cofactor-free protein) followed by reconstitution with metal-PIX cofactors in vitro creates proteins with little perturbation of the native structure, suggesting that the cofactors likely reside within the native active site. By means of this metal substitution strategy, a large number of ArMs have been constructed that contain varying metalloporphyrins and mutations of the protein. The studies discussed in this Account encompass the use of ArMs containing noble metals to catalyze a range of abiological transformations with high chemoselectivity, enantioselectivity, diastereoselectivity, and regioselectivity. These transformations include intramolecular and intermolecular insertion of carbenes into C-H, N-H, and S-H bonds, cyclopropanation of vinylarenes and of internal and nonconjugated alkenes, and intramolecular insertions of nitrenes into C-H bonds. The rates of intramolecular insertions into C-H bonds catalyzed by thermophilic P450 enzymes reconstituted with an Ir(Me)-PIX cofactor are now comparable to the rates of reactions catalyzed by native enzymes and, to date, 1000 times greater than those of any previously reported ArM. This reactivity also encompasses the selective intermolecular insertion of the carbene from ethyl diazoacetate into C-H bonds over dimerization of the carbene to form alkenes, a class of carbene insertion or selectivity not reported to occur with small-molecule catalysts. These combined results highlight the potential of well-designed ArMs to catalyze abiological transformations that have been challenging to achieve with any type of catalyst. The metal substitution strategy described herein should complement the reactivity of native enzymes and expand the scope of enzyme-catalyzed reactions.

Endogenous insertion of non-native metalloporphyrins into human membrane cytochrome P450 enzymes

Human cytochrome P450 enzymes are membrane-bound heme-containing monooxygenases. As is the case for many heme-containing enzymes, substitution of the metal in the center of the heme can be useful for mechanistic and structural studies of P450 enzymes. For many heme proteins, the iron protoporphyrin prosthetic group can be extracted and replaced with protoporphyrin containing another metal, but human membrane P450 enzymes are not stable enough for this approach. The method reported herein was developed to endogenously produce human membrane P450 proteins with a nonnative metal in the heme. This approach involved coexpression of the P450 of interest, a heme uptake system, and a chaperone in Escherichia coli growing in iron-depleted minimal medium supplemented with the desired trans-metallated protoporphyrin. Using the steroidogenic P450 enzymes CYP17A1 and CYP21A2 and the drug-metabolizing CYP3A4, we demonstrate that this approach can be used with several human P450 enzymes and several different metals, resulting in fully folded proteins appropriate for mechanistic, functional, and structural studies including solution NMR.

Insulin Hexamer-Caged Gadolinium Ion as MRI Contrast-o-phore

High-relaxivity protein-complexes of Gd^III are being pursued as MRI contrast agents in hope that they can be used at much lower doses that would minimize toxic-side effects of Gd^III release from traditional contrast agents. We construct here a new type of protein-based MRI contrast agent, a proteinaceous cage based on a stable insulin hexamer in which Gd^III is captured inside a water filled cavity. The macromolecular structure and the large number of "free" Gd^III coordination sites available for water binding lead to exceptionally high relaxivities per one Gd^III ion. The Gd^III slowly diffuses out of this cage, but this diffusion can be prevented by addition of ligands that bind to the hexamer. The ligands that trigger structural changes in the hexamer, SCN^- , Cl^- and phenols, modulate relaxivities through an outside-in signaling that is allosterically transduced through the protein cage. Contrast-o-phores based on protein-caged metal ions have potential to become clinical contrast agents with environmentally-sensitive properties.

Strategies for sensing neurotransmitters with responsive MRI contrast agents

A great deal of research involving multidisciplinary approaches is currently dedicated to the understanding of brain function. The complexity of physiological processes that underlie neural activity is the greatest hurdle to faster advances. Among imaging techniques, MRI has great potential to enable mapping of neural events with excellent specificity, spatiotemporal resolution and unlimited tissue penetration depth. To this end, molecular imaging approaches using neurotransmitter-sensitive MRI agents have appeared recently to study neuronal activity, along with the first successful in vivo MRI studies. Here, we review the pioneering steps in the development of molecular MRI methods that could allow functional imaging of the brain by sensing the neurotransmitter activity directly. We provide a brief overview of other imaging and analytical methods to detect neurotransmitter activity, and describe the approaches to sense neurotransmitters by means of molecular MRI agents. Based on these initial steps, further progress in probe chemistry and the emergence of innovative imaging methods to directly monitor neurotransmitters can be envisaged.

T1-Mediated Nanosensor for Immunoassay Based on an Activatable MnO2 Nanoassembly

Current magnetic relaxation switching (MRS) sensors for detection of trace targets in complex samples still suffer from limitations in terms of relatively low sensitivity and poor stability. To meet this challenge, we develop a longitudinal relaxation time (T₁)-based nanosensor by using Mn²⁺ released from the reduction of a MnO₂ nanoassembly that can induce the change of T₁, and thus can greatly improve the sensitivity and overcome the "hook effect" of conventional MRS. Through the specific interaction between antigen and the antibody-functionalized MnO₂ nanoassembly, the T₁ signal of Mn²⁺ released from the nanoassembly is quantitatively determined by the antigen, which allows for highly sensitive and straightforward detection of targets. This approach broadens the applicability of magnetic biosensors and has great potential for applications in early diagnosis of disease biomarkers.

Reconstitution of full-length P450BM3 with an artificial metal complex by utilising the transpeptidase Sortase A

Mn-substituted full-length P450BM3 was constructed by transpeptidase Sortase A, showing catalytic hydroxylation of aliphatic C–H bonds with molecular oxygen.

A Distal Loop Controls Product Release and Chemo- and Regioselectivity in Cytochrome P450 Decarboxylases

Cytochrome P450 OleT utilizes hydrogen peroxide (H₂O₂) to catalyze the decarboxylation or hydroxylation of fatty acid (FA) substrates. Both reactions are initiated through the abstraction of a substrate hydrogen atom by the high-valent iron-oxo intermediate known as Compound I. Here, we specifically probe the influence of substrate coordination on OleT reaction partitioning through the combined use of fluorescent and electron paramagnetic resonance (EPR)-active FA probes and mutagenesis of a structurally disordered F-G loop that is distal from the heme-iron active site. Both probes are efficiently metabolized by OleT and efficiently trigger the formation of Compound I. Transient fluorescence and EPR reveal a slow product release step, mediated by the F-G loop, that limits OleT turnover. A single-amino acid change or excision of the loop reveals that this region establishes critical interactions to anchor FA substrates in place. The stabilization afforded by the F-G loop is essential for regulating regiospecific C-H abstraction and allowing for efficient decarboxylation to occur. These results highlight a regulatory strategy whereby the fate of activated oxygen species can be controlled at distances far removed from the site of chemistry.

Orthogonal Expression of an Artificial Metalloenzyme for Abiotic Catalysis

A cytochrome P450 was engineered to selectively incorporate Ir(Me)-deuteroporphyrin IX (Ir(Me)-DPIX), in lieu of heme, in bacterial cells. Cofactor selectivity was altered by introducing mutations within the heme-binding pocket to discriminate the deuteroporphyrin macrocycle, in combination with mutations to the P450 axial cysteine to accommodate a pendant methyl group on the Ir(Me) center. This artificial metalloenzyme was investigated for activity in non-native metallocarbenoid-mediated olefin cyclopropanation reactions and showed enhanced activity for aliphatic and electron-deficient olefins when compared to the native heme enzyme. This work provides a general strategy to augment the chemical functionality of heme enzymes in cells with application towards abiotic catalysis.

Biomolecular MRI reporters: Evolution of new mechanisms

Magnetic resonance imaging (MRI) is a powerful technique for observing the function of specific cells and molecules inside living organisms. However, compared to optical microscopy, in which fluorescent protein reporters are available to visualize hundreds of cellular functions ranging from gene expression and chemical signaling to biomechanics, to date relatively few such reporters are available for MRI. Efforts to develop MRI-detectable biomolecules have mainly focused on proteins transporting paramagnetic metals for T1 and T2 relaxation enhancement or containing large numbers of exchangeable protons for chemical exchange saturation transfer. While these pioneering developments established several key uses of biomolecular MRI, such as imaging of gene expression and functional biosensing, they also revealed that low molecular sensitivity poses a major challenge for broader adoption in biology and medicine. Recently, new classes of biomolecular reporters have been developed based on alternative contrast mechanisms, including enhancement of spin diffusivity, interactions with hyperpolarized nuclei, and modulation of blood flow. These novel reporters promise to improve sensitivity and enable new forms of multiplexed and functional imaging.

Artificial Metalloenzymes on the Verge of New-to-Nature Metabolism

Residing at the interface of chemistry and biotechnology, artificial metalloenzymes (ArMs) offer an attractive technology to combine the versatile reaction repertoire of transition metal catalysts with the exquisite catalytic features of enzymes. While earlier efforts in this field predominantly comprised studies in well-defined test-tube environments, a trend towards exploiting ArMs in more complex environments has recently emerged. Integration of these artificial biocatalysts in enzymatic cascades and using them in whole-cell biotransformations and in vivo opens up entirely novel prospects for both preparative chemistry and synthetic biology. We highlight selected recent developments with a particular focus on challenges and opportunities in the in vivo application of ArMs.

Metal Substitution Modulates the Reactivity and Extends the Reaction Scope of Myoglobin Carbene Transfer Catalysts

Engineered myoglobins have recently emerged as promising scaffolds for catalyzing carbene-mediated transformations. In this work, we investigated the effect of altering the metal center and its first-sphere coordination environment on the carbene transfer reactivity of myoglobin. To this end, we first established an efficient protocol for the recombinant expression of myoglobin variants incorporating metalloporphyrins with non-native metals, including second- and third-row transition metals (ruthenium, rhodium, iridium). Characterization of the cofactor-substituted myoglobin variants across three different carbene transfer reactions (cyclopropanation, N-H insertion, S-H insertion) revealed a major influence of the nature of metal center, its oxidation state and first-sphere coordination environment on the catalytic activity, stereoselectivity, and/or oxygen tolerance of these artificial metalloenzymes. In addition, myoglobin variants incorporating manganese- or cobalt-porphyrins were found capable of catalyzing an intermolecular carbene C-H insertion reaction involving phthalan and ethyl α-diazoacetate, a reaction not supported by iron-based myoglobins and previously accessed only using iridium-based (bio)catalysts. These studies demonstrate how modification of the metalloporphyrin cofactor environment provides a viable and promising strategy to enhance the catalytic properties and extend the reaction scope of myoglobin-based carbene transfer catalysts.

Molecular imaging of the tumor microenvironment

The tumor microenvironment plays a critical role in tumor initiation, progression, metastasis, and resistance to therapy. It is different from normal tissue in the extracellular matrix, vascular and lymphatic networks, as well as physiologic conditions. Molecular imaging of the tumor microenvironment provides a better understanding of its function in cancer biology, and thus allowing for the design of new diagnostics and therapeutics for early cancer diagnosis and treatment. The clinical translation of cancer molecular imaging is often hampered by the high cost of commercialization of targeted imaging agents as well as the limited clinical applications and small market size of some of the agents. Because many different cancer types share similar tumor microenvironment features, the ability to target these biomarkers has the potential to provide clinically translatable molecular imaging technologies for a spectrum of cancers and broad clinical applications. There has been significant progress in targeting the tumor microenvironment for cancer molecular imaging. In this review, we summarize the principles and strategies of recent advances made in molecular imaging of the tumor microenvironment, using various imaging modalities for early detection and diagnosis of cancer.

Improving artificial metalloenzymes' activity by optimizing electron transfer

This feature article discusses the strategies to optimize electron transfer efficiency, towards enhancing the activity of artificial metalloenzymes.

An Evolved Orthogonal Enzyme/Cofactor Pair

We introduce a strategy that expands the functionality of hemoproteins through orthogonal enzyme/heme pairs. By exploiting the ability of a natural heme transport protein, ChuA, to promiscuously import heme derivatives, we have evolved a cytochrome P450 (P450BM3) that selectively incorporates a nonproteinogenic cofactor, iron deuteroporphyrin IX (Fe-DPIX), even in the presence of endogenous heme. Crystal structures show that selectivity gains are due to mutations that introduce steric clash with the heme vinyl groups while providing a complementary binding surface for the smaller Fe-DPIX cofactor. Furthermore, the evolved orthogonal enzyme/cofactor pair is active in non-natural carbenoid-mediated olefin cyclopropanation. This methodology for the generation of orthogonal enzyme/cofactor pairs promises to expand cofactor diversity in artificial metalloenzymes.

Molecular fMRI

Comprehensive analysis of brain function depends on understanding the dynamics of diverse neural signaling processes over large tissue volumes in intact animals and humans. Most existing approaches to measuring brain signaling suffer from limited tissue penetration, poor resolution, or lack of specificity for well-defined neural events. Here we discuss a new brain activity mapping method that overcomes some of these problems by combining MRI with contrast agents sensitive to neural signaling. The goal of this "molecular fMRI" approach is to permit noninvasive whole-brain neuroimaging with specificity and resolution approaching current optical neuroimaging methods. In this article, we describe the context and need for molecular fMRI as well as the state of the technology today. We explain how major types of MRI probes work and how they can be sensitized to neurobiological processes, such as neurotransmitter release, calcium signaling, and gene expression changes. We comment both on past work in the field and on challenges and promising avenues for future development.

SIGNIFICANCE STATEMENT

Brain researchers currently have a choice between measuring neural activity using cellular-level recording techniques, such as electrophysiology and optical imaging, or whole-brain imaging methods, such as fMRI. Cellular level methods are precise but only address a small portion of mammalian brains; on the other hand, whole-brain neuroimaging techniques provide very little specificity for neural pathways or signaling components of interest. The molecular fMRI techniques we discuss have particular potential to combine the specificity of cellular-level measurements with the noninvasive whole-brain coverage of fMRI. On the other hand, molecular fMRI is only just getting off the ground. This article aims to offer a snapshot of the status and future prospects for development of molecular fMRI techniques.

Conformational control of cofactors in nature - the influence of protein-induced macrocycle distortion on the biological function of tetrapyrroles

Tetrapyrrole-containing proteins are one of the most fundamental classes of enzymes in nature and it remains an open question to give a chemical rationale for the multitude of biological reactions that can be catalyzed by these pigment-protein complexes. There are many fundamental processes where the same (i.e., chemically identical) porphyrin cofactor is involved in chemically quite distinct reactions. For example, heme is the active cofactor for oxygen transport and storage (hemoglobin, myoglobin) and for the incorporation of molecular oxygen in organic substrates (cytochrome P450). It is involved in the terminal oxidation (cytochrome c oxidase) and the metabolism of H2O2 (catalases and peroxidases) and catalyzes various electron transfer reactions in cytochromes. Likewise, in photosynthesis the same chlorophyll cofactor may function as a reaction center pigment (charge separation) or as an accessory pigment (exciton transfer) in light harvesting complexes (e.g., chlorophyll a). Whilst differences in the apoprotein sequences alone cannot explain the often drastic differences in physicochemical properties encountered for the same cofactor in diverse protein complexes, a critical factor for all biological functions must be the close structural interplay between bound cofactors and the respective apoprotein in addition to factors such as hydrogen bonding or electronic effects. Here, we explore how nature can use the same chemical molecule as a cofactor for chemically distinct reactions using the concept of conformational flexibility of tetrapyrroles. The multifaceted roles of tetrapyrroles are discussed in the context of the current knowledge on distorted porphyrins. Contemporary analytical methods now allow a more quantitative look at cofactors in protein complexes and the development of the field is illustrated by case studies on hemeproteins and photosynthetic complexes. Specific tetrapyrrole conformations are now used to prepare bioengineered designer proteins with specific catalytic or photochemical properties.

Abiological catalysis by artificial haem proteins containing noble metals in place of iron

Enzymes that contain metal ions--that is, metalloenzymes--possess the reactivity of a transition metal centre and the potential of molecular evolution to modulate the reactivity and substrate-selectivity of the system. By exploiting substrate promiscuity and protein engineering, the scope of reactions catalysed by native metalloenzymes has been expanded recently to include abiological transformations. However, this strategy is limited by the inherent reactivity of metal centres in native metalloenzymes. To overcome this limitation, artificial metalloproteins have been created by incorporating complete, noble-metal complexes within proteins lacking native metal sites. The interactions of the substrate with the protein in these systems are, however, distinct from those with the native protein because the metal complex occupies the substrate binding site. At the intersection of these approaches lies a third strategy, in which the native metal of a metalloenzyme is replaced with an abiological metal with reactivity different from that of the metal in a native protein. This strategy could create artificial enzymes for abiological catalysis within the natural substrate binding site of an enzyme that can be subjected to directed evolution. Here we report the formal replacement of iron in Fe-porphyrin IX (Fe-PIX) proteins with abiological, noble metals to create enzymes that catalyse reactions not catalysed by native Fe-enzymes or other metalloenzymes. In particular, we prepared modified myoglobins containing an Ir(Me) site that catalyse the functionalization of C-H bonds to form C-C bonds by carbene insertion and add carbenes to both β-substituted vinylarenes and unactivated aliphatic α-olefins. We conducted directed evolution of the Ir(Me)-myoglobin and generated mutants that form either enantiomer of the products of C-H insertion and catalyse the enantio- and diastereoselective cyclopropanation of unactivated olefins. The presented method of preparing artificial haem proteins containing abiological metal porphyrins sets the stage for the generation of artificial enzymes from innumerable combinations of PIX-protein scaffolds and unnatural metal cofactors to catalyse a wide range of abiological transformations.