Crystal Structures and Coordination Behavior of Aqua- and Cyano-Co(III) Tetradehydrocorrins in the Heme Pocket of Myoglobin

Diversifying the functions of heme proteins with non-porphyrin cofactors

Heme proteins perform diverse biochemical functions using a single iron porphyrin cofactor. This versatility makes them attractive platforms for the development of new functional proteins. While directed evolution and metal substitution have expanded the properties, reactivity, and applications of heme proteins, the incorporation of porphyrin analogs remains an underexplored approach. This review discusses the replacement of heme with non-porphyrin cofactors, such as porphycene, corrole, tetradehydrocorrin, phthalocyanine, and salophen, and the attendant properties of these conjugates. While structurally similar, each ligand exhibits distinct optical and redox properties, as well as unique chemical reactivity. These hybrids serve as model systems to elucidate the effects of the protein environment on the electronic structure, redox potentials, optical properties, or other features of the porphyrin analog. Protein encapsulation can confer distinct chemical reactivity or selectivity of artificial metalloenzymes that cannot be achieved with the small molecule catalyst alone. Additionally, these conjugates can interfere with heme acquisition and uptake in pathogenic bacteria, providing an inroad to innovative antibiotic strategies. Together, these examples illustrate the diverse functionality that can be achieved by cofactor substitution. The further expansion of this approach will access unexplored chemical space, enabling the development of superior catalysts and the creation of heme proteins with emergent properties.

Effect of Macrocycles on the Photochemical and Electrochemical Properties of Cobalt-Dehydrocorrin Complex: Formation and Investigation of Co(I) Species

Co(II)-pyrocobester (P-Co(II)), a dehydrocorrin complex, was semisynthesized from vitamin B12 (cyanocobalamin), and its photochemical and electrochemical properties were investigated and compared to those of the cobester (C-Co(II)), the cobalt-corrin complex. The UV-vis absorptions of P-Co(II) in CH2Cl2, ascribed to the π-π* transition, were red-shifted compared to those of C-Co(II) due to the π-expansion of the macrocycle in the pyrocobester. The reversible redox couple of P-Co(II) was observed at E1/2 = -0.30 V vs Ag/AgCl in CH3CN, which was assigned to the Co(II)/Co(I) redox couple by UV-vis, ESR, and molecular orbital analysis. This redox couple was positively shifted by 0.28 V compared to that of C-Co(II). This is caused by the high electronegativity of the dehydrocorrin macrocycle, which was estimated by DFT calculations for the free-base ligands. The reactivity of the Co(I)-pyrocobester (P-Co(I)) was evaluated by the reaction with methyl iodide in CV and UV-vis to form a photosensitive Co(III)-CH3 complex (P-Co(III)-CH3). The properties of the excited state of P-Co(I), *Co(I), were also investigated by femtosecond transient absorption (TA) spectroscopy. The lifetime of *Co(I) was estimated to be 29 ps from the kinetic trace at 587 nm. The lifetime of *Co(I) became shorter in the presence of Ar-X, such as iodobenzonitrile (1a), bromobenzonitrile (1b), and chlorobenzonitrile (1c), and the rate constants of electron transfer (ET) between the *Co(I) and Ar-X were determined to be 2.9 × 1011 M-1 s-1, 4.9 × 1010 M-1 s-1, and 1.0 × 1010 M-1 s-1 for 1a, 1b, and 1c, respectively.

Syntheses and Properties of Metalated Tetradehydrocorrins

The monoanionic tetrapyrrolic macrocycle B,C-tetradehydrocorrin (TDC) resides chemically between corroles and corrins. This chemical space remains largely unexplored due to a lack of reliable synthetic strategies. We now report the preparation and characterization of Co(II)- and Ni(II)-metalated TDC derivatives ([Co-TDC]⁺ and [Ni-TDC]⁺, respectively) with a combination of crystallographic, electrochemical, computational, and spectroscopic techniques. [Ni-TDC]⁺ was found to undergo primarily ligand-centered electrochemical reduction, leading to hydrogenation of the macrocycle under cathodic electrolysis in the presence of acid. Transient absorption (TA) spectroscopy reveals that [Ni-TDC]⁺ and the two-electron-reduced [Ni-TDC]^- possess long-lived excited states, whereas the excited state of singly reduced [Ni-TDC] exhibits picosecond dynamics. The Co(I) compound [Co-TDC] is air stable, highlighting the notable property of the TDC ligand to stabilize low-valent metal centers in contradistinction to other tetrapyrroles such as corroles, which typically stabilize metals in higher oxidation states.

Myoglobins engineered with artificial cofactors serve as artificial metalloenzymes and models of natural enzymes

Metalloenzymes naturally achieve various reactivities by assembling limited types of cofactors with endogenous amino acid residues. Enzymes containing metal porphyrinoid cofactors such as heme, cobalamin and F430 exert precise control over the reactivities of the cofactors with protein matrices. This perspective article focuses on our recent efforts to assemble metal complexes of non-natural porphyrinoids within the protein matrix of myoglobin, an oxygen storage hemoprotein. Engineered myoglobins with suitable metal complexes as artificial cofactors demonstrate unique reactivities toward C-H bond hydroxylation, olefin cyclopropanation, methyl group transfer and methane generation. In these cases, the protein matrix enhances the catalytic activities of the cofactors and allows us to monitor the active intermediates. The present findings indicate that placing artificial cofactors in protein matrices provides a useful strategy for creating artificial metalloenzymes that catalyse otherwise unfavourable reactions and providing enzyme models for elucidating the complicated reaction mechanisms of natural enzymes.

Hemoproteins Reconstituted with Artificial Metal Complexes as Biohybrid Catalysts

In nature, heme cofactor-containing proteins participate not only in electron transfer and O₂ storage and transport but also in biosynthesis and degradation. The simplest and representative cofactor, heme b, is bound within the heme pocket via noncovalent interaction in many hemoproteins, suggesting that the cofactor is removable from the protein, leaving a unique cavity. Since the cavity functions as a coordination sphere for heme, it is of particular interest to investigate replacement of native heme with an artificial metal complex, because the substituted metal complex will be stabilized in the heme pocket while providing alternative chemical properties. Thus, cofactor substitution has great potential for engineering of hemoproteins with alternative functions. For these studies, myoglobin has been a focus of our investigations, because it is a well-known oxygen storage hemoprotein. However, the heme pocket of myoglobin has been only arranged for stabilizing the heme-bound dioxygen, so the structure is not suitable for activation of small molecules such as H₂O₂ and O₂ as well as for binding an external substrate. Thus, the conversion of myoglobin to an enzyme-like biocatalyst has presented significant challenges. The results of our investigations have provided useful information for chemists and biologists. Our own efforts to develop functionalized myoglobin have focused on the incorporation of a chemically modified cofactor into apomyoglobin in order to (1) construct an artificial substrate-binding site near the heme pocket, (2) increase cofactor reactivity, or (3) promote a new reaction that has never before been catalyzed by a native heme enzyme. In pursuing these objectives, we first found that myoglobin reconstituted with heme having a chemically modified heme-propionate side chain at the exit of the heme pocket has peroxidase activity with respect to oxidation of phenol derivatives. Our recent investigations have succeeded in enhancing oxidation and oxygenation activities of myoglobin as well as promoting new reactions by reconstitution of myoglobin with new porphyrinoid metal complexes. Incorporation of suitable metal porphyrinoids into the heme pocket has produced artificial enzymes capable of efficiently generating reactive high valent metal-oxo and metallocarbene intermediates to achieve the catalytic hydroxylation of C(sp³)-H bonds and cyclopropanation of olefin molecules, respectively. In other efforts, we have focused on nitrobindin, an NO-binding hemoprotein, because aponitrobindin includes a β-barrel cavity, which provides a robust structure highly similar to that of the native holoprotein. It was expected that the aponitrobindin would be suitable for development as a protein scaffold for a metal complex. Recently, it was confirmed that several organometallic complexes can bind to this scaffold and function as catalysts promoting hydrogen evolution or C-C bond formation. The hydrophobic β-barrel structure plays a significant role in substrate binding as well as controlling the stereoselectivity of the reactions. Furthermore, these catalytic activities and stereoselectivities are remarkably improved by mutation-dependent modifications of the cavity structure for the artificial cofactor. This Account demonstrates how apoproteins of hemoproteins can provide useful protein scaffolds for metal complexes. Further development of these concepts will provide a useful strategy for generation of robust and useful artificial metalloenzymes.

Modified vitamin B12 derivatives with a peptide backbone for biomimetic studies and medicinal applications

This short review highlights the author’s group research on modified vitamin B[Formula: see text] derivatives with a peptide backbone as (1) inhibitors of B[Formula: see text]-dependent enzymes and as (2) models of cofactor B[Formula: see text]-protein complexes.

Stabilizing intramolecular cobalt–imidazole coordination with a remote methyl group in the backbone of a cofactor B12–protein model

This communication describes the stabilizing effect of a remote methyl group in the backbone of a cobalamin–protein mimic on intramolecular imidazole–cobalt coordination.

Prediction of the interaction of metallic moieties with proteins: An update for protein-ligand docking techniques

In this article, we present a new approach to expand the range of application of protein-ligand docking methods in the prediction of the interaction of coordination complexes (i.e., metallodrugs, natural and artificial cofactors, etc.) with proteins. To do so, we assume that, from a pure computational point of view, hydrogen bond functions could be an adequate model for the coordination bonds as both share directionality and polarity aspects. In this model, docking of metalloligands can be performed without using any geometrical constraints or energy restraints. The hard work consists in generating the convenient atom types and scoring functions. To test this approach, we applied our model to 39 high-quality X-ray structures with transition and main group metal complexes bound via a unique coordination bond to a protein. This concept was implemented in the protein-ligand docking program GOLD. The results are in very good agreement with the experimental structures: the percentage for which the RMSD of the simulated pose is smaller than the X-ray spectra resolution is 92.3% and the mean value of RMSD is < 1.0 Å. Such results also show the viability of the method to predict metal complexes-proteins interactions when the X-ray structure is not available. This work could be the first step for novel applicability of docking techniques in medicinal and bioinorganic chemistry and appears generalizable enough to be implemented in most protein-ligand docking programs nowadays available. © 2017 Wiley Periodicals, Inc.

Impact of the corrin framework of vitamin B12 on the electrochemical carbon-skeleton rearrangement in comparison to an imine/oxime planar ligand; tuning selectivity in 1,2-migration of a functional group by controlling electrolysis potential

Among the coenzyme B₁₂-dependent enzymes, methylmalonyl-CoA mutase (MMCM) catalyzes the carbon-skeleton rearrangement reaction between R-methylmalonyl-CoA and succinyl-CoA. Diethyl 2-bromomethyl-2-phenylmalonate, an alkyl bromide substrate having two different migrating groups (phenyl and carboxylic ester groups) on the β-carbon, was applied to the electrolysis mediated by a hydrophobic vitamin B₁₂ model complex, heptamethyl cobyrinate perchlorate in this study. The electrolysis of the substrate at -1.0V vs. Ag-AgCl by light irradiation afforded the simple reduced product (diethyl 2-methyl-2-phenylmalonate) and the phenyl migrated product (diethyl 2-benzyl-2-phenylmalonate), as well as the electrolysis of the substrate at -1.5V vs. Ag-AgCl in the dark. The electrolysis of the substrate at -2.0V vs. Ag-AgCl afforded the carboxylic ester migrated product (diethyl phenylsuccinate) as the major product. The selectivity for the migrating group was successfully tuned by controlling the electrolysis potential. We clarified that the cathodic chemistry of the Co(III) alkylated heptamethyl cobyrinate is critical for the selectivity of the migrating group through mechanistic investigations and comparisons to the simple vitamin B₁₂ model complex, an imine/oxime-type cobalt complex.