Diversifying Metal-Ligand Cooperative Catalysis in Semi-Synthetic [Mn]-Hydrogenases

Metal- and ligand-substitution-induced changes in the kinetics and thermodynamics of hydrogen activation and hydricity in a dinuclear metal complex

Catalytic function in organometallic complexes is achieved by carefully selecting their central metals and ligands. In this study, the effects of a metal and a ligand on the kinetics and thermodynamics of hydrogen activation, hydricity degree of the hydride complex, and susceptibility to electronic oxidation in bioinspired NiFe complexes, [NiIIX FeII(Cl)(CO)Y]+ ([NiFe(Cl)(CO)]+; X = N,N'-diethyl-3,7-diazanonane-1,9-dithiolato and Y = 1,2-bis(diphenylphosphino)ethane), were investigated. The density functional theory calculations revealed that the following order thermodynamically favored hydrogen activation: [NiFe(CO)]2+ > [NiRu(CO)]2+ > [NiFe(CNMe)]2+ ∼ [PdRu(CO)]2+ ∼ [PdFe(CO)]2+ ≫ [NiFe(NCS)]+. Moreover, the reverse order thermodynamically favored the hydricity degree.

Acyl and CO Ligands in the [Fe]-Hydrogenase Cofactor Scramble upon Photolysis

[Fe]-hydrogenase harbors the iron-guanylylpyridinol (FeGP) cofactor, in which the Fe(II) complex contains acyl-carbon, pyridinol-nitrogen, cysteine-thiolate and two CO as ligands. Irradiation with UV-A/blue light decomposes the FeGP cofactor to a 6-carboxymethyl-4-guanylyl-2-pyridone (GP) and other components. Previous in vitro biosynthesis experiments indicated that the acyl- and CO-ligands in the FeGP cofactor can scramble, but whether scrambling occurred during biosynthesis or photolysis was unclear. Here, we demonstrate that the [18 O1 -carboxy]-group of GP is incorporated into the FeGP cofactor by in vitro biosynthesis. MS/MS analysis of the 18 O-labeled FeGP cofactor revealed that the produced [18 O1 ]-acyl group is not exchanged with a CO ligand of the cofactor, indicating that the acyl and CO ligands are scrambled during photolysis rather than biosynthesis, which ruled out any biosynthesis mechanisms allowing acyl/CO ligands scrambling. Time-resolved infrared spectroscopy indicated that an acyl-Fe(CO)3 intermediate is formed during photolysis, in which scrambling of the CO and acyl ligands can occur. This finding also suggests that the light-excited FeGP cofactor has a higher affinity for external CO. These results contribute to our understanding of the biosynthesis and photosensitive properties of this unique H2 -activating natural complex.

Manganese Transfer Hydrogenases Based on the Biotin-Streptavidin Technology

Artificial (transfer) hydrogenases have been developed for organic synthesis, but they rely on precious metals. Native hydrogenases use Earth-abundant metals, but these cannot be applied for organic synthesis due, in part, to their substrate specificity. Herein, we report the design and development of manganese transfer hydrogenases based on the biotin-streptavidin technology. By incorporating bio-mimetic Mn(I) complexes into the binding cavity of streptavidin, and through chemo-genetic optimization, we have obtained artificial enzymes that hydrogenate ketones with nearly quantitative yield and up to 98 % enantiomeric excess (ee). These enzymes exhibit broad substrate scope and high functional-group tolerance. According to QM/MM calculations and X-ray crystallography, the S112Y mutation, combined with the appropriate chemical structure of the Mn cofactor plays a critical role in the reactivity and enantioselectivity of the artificial metalloenzyme (ArMs). Our work highlights the potential of ArMs incorporating base-meal cofactors for enantioselective organic synthesis.

[Fe]-Hydrogenase, Cofactor Biosynthesis and Engineering

[Fe]-hydrogenase catalyzes the heterolytic cleavage of H2 and reversible hydride transfer to methenyl-tetrahydromethanopterin. The iron-guanylylpyridinol (FeGP) cofactor is the prosthetic group of this enzyme, in which mononuclear Fe(II) is ligated with a pyridinol and two CO ligands. The pyridinol ligand fixes the iron by an acyl carbon and a pyridinol nitrogen. Biosynthetic proteins for this cofactor are encoded in the hmd co-occurring (hcg) genes. The function of HcgB, HcgC, HcgD, HcgE, and HcgF was studied by using structure-to-function analysis, which is based on the crystal structure of the proteins and subsequent enzyme assays. Recently, we reported the catalytic properties of HcgA and HcgG, novel radical S-adenosyl methionine enzymes, by using an in vitro biosynthesis assay. Here, we review the properties of [Fe]-hydrogenase and the FeGP cofactor, and the biosynthesis of the FeGP cofactor. Finally, we discuss the expected engineering of [Fe]-hydrogenase and the FeGP cofactor.

Artificial Small Molecules as Cofactors and Biomacromolecular Building Blocks in Synthetic Biology: Design, Synthesis, Applications, and Challenges

Enzymes are essential catalysts for various chemical reactions in biological systems and often rely on metal ions or cofactors to stabilize their structure or perform functions. Improving enzyme performance has always been an important direction of protein engineering. In recent years, various artificial small molecules have been successfully used in enzyme engineering. The types of enzymatic reactions and metabolic pathways in cells can be expanded by the incorporation of these artificial small molecules either as cofactors or as building blocks of proteins and nucleic acids, which greatly promotes the development and application of biotechnology. In this review, we summarized research on artificial small molecules including biological metal cluster mimics, coenzyme analogs (mNADs), designer cofactors, non-natural nucleotides (XNAs), and non-natural amino acids (nnAAs), focusing on their design, synthesis, and applications as well as the current challenges in synthetic biology.

Emerging artificial metalloenzymes for asymmetric hydrogenation reactions

Artificial metalloenzymes (ArMs) utilize the best properties of homogenous transition metal catalysts and naturally occurring proteins. While synthetic metal complexes offer high tunability and broad-scope reactivity with a variety of substrates, enzymes further endow these complexes with enhanced aqueous stability and stereoselectivity. For these reasons, dozens of ArMs have been designed to perform catalytic asymmetric hydrogenation reactions, and hydrogenase ArMs are, in fact, the oldest class of ArMs. Herein, we report recent advances in the design of hydrogenase ArMs, including (i) the modification of natural [Fe]-hydrogenase by insertion of artificial metallocofactors, (ii) design of a novel ArM system from the tractable and inexpensive protein β-lactoglobulin to afford a high-performing transfer hydrogenase, and (iii) the design of chimeric streptavidin scaffolds that drastically alter the secondary coordination sphere of previously reported streptavidin/biotin transfer hydrogenase ArMs.

Diversifying Metal-Ligand Cooperative Catalysis in Semi-Synthetic [Mn]-Hydrogenases

The reconstitution of [Mn]-hydrogenases using a series of MnI complexes is described. These complexes are designed to have an internal base or pro-base that may participate in metal-ligand cooperative catalysis or have no internal base or pro-base. Only MnI complexes with an internal base or pro-base are active for H2 activation; only [Mn]-hydrogenases incorporating such complexes are active for hydrogenase reactions. These results confirm the essential role of metal-ligand cooperation for H2 activation by the MnI complexes alone and by [Mn]-hydrogenases. Owing to the nature and position of the internal base or pro-base, the mode of metal-ligand cooperation in two active [Mn]-hydrogenases is different from that of the native [Fe]-hydrogenase. One [Mn]-hydrogenase has the highest specific activity of semi-synthetic [Mn]- and [Fe]-hydrogenases. This work demonstrates reconstitution of active artificial hydrogenases using synthetic complexes differing greatly from the native active site.