Hemoproteins Reconstituted with Artificial Metal Complexes as Biohybrid Catalysts

Abstract

In nature, heme cofactor-containing proteins participate not only in electron transfer and O 2 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 H 2 O 2 and O 2 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(sp 3 )−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 stereoselec...

Myoglobin Reconstituted with Ni Tetradehydrocorrin as a Methane‐Generating Model of Methyl‐coenzyme M Reductase

“…Furthermore, we have focused on the protein matrix effect on physicochemical properties and reactivities of the metal‐cofactor models. For example, myoglobin (Mb), an oxygen‐binding hemoprotein, is found to provide a simple and useful scaffold for a hydrophobic cofactor‐binding cavity . The present study demonstrates methane generation by the Ni(TDHC) complex within a Mb matrix using a mild reductant (Figure b).…”

Rational design of heme enzymes for biodegradation of pollutants toward a green future

“…Meanwhile, their catalytic efficiencies were not high enough, which hindered general applications for bioremediation [14][15][16]. To overcome the limitations, rational design of artificial enzymes provides a chance to create more functional enzymes, such as for artificial oxidases and reductases [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. To date, impressive progress has also been made in the design of artificial enzymes for biodegradation [33][34][35][36][37][38][39].…”

Molecular Dynamics Simulation and Kinetic Study of Fluoride Binding to V21C/V66C Myoglobin with a Cytoglobin-like Disulfide Bond

“…Protein design is able to not only reveal the structure-function relationship of native proteins, but also create artificial proteins with advanced functions [1][2][3][4][5][6][7][8][9][10][11][12]. This is especially the case for heme protein design, which has received much attention in the last few decades, and various approaches have been established for rational design, such as the introduction of non-heme metal ions and unnatural amino acids, and the use of heme mimics to act as an active site [1][2][3][4][5][6][7][8][9][10][11][12]. Importantly, computer modeling and molecular dynamics (MD) simulation play key roles in guiding the protein design [13][14][15][16][17][18].…”