Artificial Metalloenzymes Constructed From Hierarchically‐Assembled Proteins

Abstract

The design of artificial metalloenzymes has become an important topic in biological chemistry and inorganic chemistry due to the potential applications of artificial metalloenzymes in nanoscience and biotechnology. One of the general methods used to produce artificially metalloenzymes involves the encapsulation of non-natural metal cofactors within protein scaffolds. This method has been used in the construction of small artificial metalloproteins with high activity and selectivity. However, the important roles of protein assemblies have not yet been systematically investigated in this field, even though natural enzymatic systems employ protein assemblies as molecular scaffolds for elaborate enzymatic reactions. In recent years, the above-mentioned general strategy has been applied to functionalize protein assemblies such as protein cages and protein crystals. These assembled structures form confined interior environments, which can be used to accommodate metal complex catalysts and to prepare metal nanoparticles. The development of artificial metalloenzymes with hierarchically-assembled proteins would enable us to provide powerful tools for industrial and biological applications. In this Focus Review, we discuss the most significant recent research in this field as well as future directions.

Expanding the Chemical Diversity in Artificial Imine Reductases Based on the Biotin–Streptavidin Technology

“…With the aim of rapidly generating chemical diversity, a novel strategy for the formation and evaluation of biotinylated complexes is disclosed. Tethering the biotin-anchor to the Cp* moiety leaves three free coordination sites on a d 6 Notably, the widely used amino sulfonamide ligand scaffold (ligand 9) performs only moderately in comparison to the best ligands under the experimental conditions adopted for this screening. With high-throughput screening in mind, no effort was made to improve the performance of individual metal-ligand combinations.…”

“…Considering the robustness of the MCp* moiety, [22,23] we reasoned that for d 6 transition metals tethering the biotin anchor to the Cp* unit would leave three free coordination sites around the metal for further functionalization. It is widely accepted that three-legged piano stool complex-catalyzed asymmetric transfer hydrogenation proceeds via an outer sphere hydride transfer mechanism, which requires only one free coordination site around the metal for the reaction to proceed.…”

A Photoactive Carbon‐Monoxide‐Releasing Protein Cage for Dose‐Regulated Delivery in Living Cells

“…This coordination is stabilized by ah ydrogen bond between W1 and Oe of Asp38 (2.62 ) and aw eak interaction with Sg of Cys48 with ad istance of 2.82 ( Figure 2c). Although there is no electron density assignable to CO ligands in the crystal structure,asdescribed above,t hree peaks arising from the CO-stretching frequencies of 1 indicate the presence of CO ligands with the facMn(CO) 3 coordination structure in 1 ( Figure S2a). Moreover, the CO coordination was confirmed by ATR-IR spectroscopy of 1 before and after crystallization in aprecipitation buffer in the dark ( Figure S2b).…”

“…[1][2][3] Assemblies,s uch as cages,t ubes,a nd frameworks show great potential for use as molecular scaffolds and for development of catalysts,o ptical materials,a nd magnetic materials when metal complexes or metal ions are accumulated in the unique environments by metal coordination and chemical modification of amino acids. [4][5][6][7] These composites,w hich have large numbers of metal compounds appropriately aligned on or within the scaffolds,s how ah igh reactivity when externally stimulated, for example,b yl ight irradiation.…”

“…[16,17] Delivery of carbon monoxide (CO) into living cells represents ac hallenging task [18] and could possibly be achieved by applying protein assemblies involving metal complexes.M etal carbonyl complexes have been used as carbon-monoxide-releasing molecules (CORMs) for both in vitro and in vivo delivery of CO for the purpose of investigating the function of CO as an intracellular signaling molecule. [19][20][21] [Ru(CO) 3 Cl 2 ] 2 (CORM-2) and Ru-(CO) 3 Cl(glycinate) (CORM-3), which are commonly used CORMs,p rovide cytoprotective and vasoactive functions in living cells. [18][19][20][21] Since CO is spontaneously released from the Ru-carbonyl complexes as ar esult of ligand exchange reactions with nucleophilic molecules in the cell, [22] it is difficult to determine the details of the mechanisms of COinduced cellular functions.T oa ddress this issue,s timulusresponsive CORMs have been developed as photo-, [23,24] enzyme-, [25] and pH-active [26] compounds.P hotoactive CORMs are expected to have ap otential to control the timing of release and the dose of released CO by an external light stimulus although they are only applicable to accessible surfaces of living cells and model animals.…”