Duhme-Klair, A.K.; Wilson, K.S.

Nat. Catal. 2018, 1, 680-688, 10.1038/s41929-018-0124-3

Artificial metalloenzymes that contain protein-anchored synthetic catalysts are attracting increasing interest. An exciting, but still unrealized advantage of non-covalent anchoring is its potential for reversibility and thus component recycling. Here we present a siderophore–protein combination that enables strong but redox-reversible catalyst anchoring, as exemplified by an artificial transfer hydrogenase (ATHase). By linking the iron(iii)-binding siderophore azotochelin to an iridium-containing imine-reduction catalyst that produces racemic product in the absence of the protein CeuE, but a reproducible enantiomeric excess if protein bound, the assembly and reductively triggered disassembly of the ATHase was achieved. The crystal structure of the ATHase identified the residues involved in high-affinity binding and enantioselectivity. While in the presence of iron(iii), the azotochelin-based anchor binds CeuE with high affinity, and the reduction of the coordinated iron(iii) to iron(ii) triggers its dissociation from the protein. Thus, the assembly of the artificial enzyme can be controlled via the iron oxidation state.

Ghirlanda, G.

Biochim. Biophys. Acta, Bioenerg. 2016, 1857, 598-603, 10.1016/j.bbabio.2015.09.001

Bioinspired, protein-based molecular catalysts utilizing base metals at the active are emerging as a promising avenue to sustainable hydrogen production. The protein matrix modulates the intrinsic reactivity of organometallic active sites by tuning second-sphere and long-range interactions. Here, we show that swapping Co-Protoporphyrin IX for Fe-Protoporphyrin IX in cytochrome b562 results in an efficient catalyst for photoinduced proton reduction to molecular hydrogen. Further, the activity of wild type Co-cyt b562 can be modulated by a factor of 2.5 by exchanging the coordinating methionine with alanine or aspartic acid. The observed turnover numbers (TON) range between 125 and 305, and correlate well with the redox potential of the Co-cyt b562 mutants. The photosensitized system catalyzes proton reduction with high efficiency even under an aerobic atmosphere, implicating its use for biotechnological applications. This article is part of a Special Issue entitled Biodesign for Bioenergetics — the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

Kazlauskas, R.J.

ChemCatChem 2010, 2, 953-957, 10.1002/cctc.201000159

CA confidential: Replacing the active‐site zinc in carbonic anhydrase (CA) by rhodium forms a new enzymatic catalyst for cofactor‐free hydroformylation of styrene with syn gas. Unlike free rhodium, this rhodium–protein hybrid, [Rh]–CA, is regioselective (8.4:1) for linear over branched aldehyde product, which is a 40‐fold change in regioselectivity compared to free rhodium.

Mahy, J.-P.

J. Protein Chem. 2002, 21, 473-477, 10.1023/A:1021351120772

Catalytic antibodies with a metalloporphyrin cofactor represent a new generation of biocatalysts tailored for selective oxidations. Thus monoclonal antibodies, 3A3, were raised against microperoxidase 8 (MP8), and the corresponding 3A3-MP8 complexes were shown previously to have a high peroxidase activity. This paper shows that those complexes also catalyzed efficiently the nitration of phenol into 2- and 4-nitrophenol by NO2 − in the presence of H2O2. pH dependence studies suggested that no amino acid from the antibody protein participated in the heterolytic cleavage of the O-O bond of H2O2. The inhibition of the reaction by cyanide and radical scavengers suggested a MP8-mediated peroxidase-like mechanism, involving the reduction of high-valent iron-oxo species by NO2 − and phenol producing, respectively, NO2 · and phenoxy radicals, which then reacted to give nitrophenols. Finally, the antibody protein appears to have two major roles: (i) it protects MP8 toward oxidative degradations and (ii) it induces a regioselectivity of the reaction toward the formation of 2-nitrophenol.

Nelson, H.M.

Angew. Chem. Int. Ed. 2019, 58, 16400-16404, 10.1002/anie.201905333

Herein we report the discovery of a AuI–DNA hybrid catalyst that is compatible with biological media and whose reactivity can be regulated by small complementary nucleic acid sequences. The development of this catalytic system was enabled by the discovery of a novel AuI‐mediated base pair. We found that AuI binds DNA containing C‐T mismatches. In the AuI–DNA catalyst's latent state, the AuI ion is sequestered by the mismatch such that it is coordinatively saturated, rendering it catalytically inactive. Upon addition of an RNA or DNA strand that is complementary to the latent catalyst's oligonucleotide backbone, catalytic activity is induced, leading to a sevenfold increase in the formation of a fluorescent product, forged through a AuI‐catalyzed hydroamination reaction. Further development of this catalytic system will expand not only the chemical space available to synthetic biological systems but also allow for temporal and spatial control of transition‐metal catalysis through gene transcription.

Holland, P.L.

J. Inorg. Biochem. 2021, 219, 111430, 10.1016/j.jinorgbio.2021.111430

Artificial metalloenzymes (ArMs) consist of an unnatural metal or cofactor embedded in a protein scaffold, and are an excellent platform for applying the concepts of protein engineering to catalysis. In this Focused Review, we describe the application of ArMs as simple, tunable artificial models of the active sites of complex natural metalloenzymes for small-molecule activation. In this sense, ArMs expand the strategies of synthetic model chemistry to protein-based supporting ligands with potential for participation from the second coordination sphere. We focus specifically on ArMs that are structural, spectroscopic, and functional models of enzymes for activation of small molecules like CO, CO2, O2, N2, and NO, as well as production/consumption of H2. These ArMs give insight into the identities and roles of metalloenzyme structural features within and near the cofactor. We give examples of ArM work relevant to hydrogenases, acetyl-coenzyme A synthase, superoxide dismutase, heme oxygenases, nitric oxide reductase, methyl-coenzyme M reductase, copper-O2 enzymes, and nitrogenases.

ChemCatChem 2014, n/a-n/a, 10.1002/cctc.201301055

The incorporation of a Rh complex with a maleimide moiety into the cavity of the nitrobindin β‐barrel scaffold by a covalent linkage at the 96‐position (Cys) provides a hybrid biocatalyst that promotes the polymerization of phenylacetylene. The appropriate structural optimization of the cavity by mutagenesis enhances the stereoselectivity of the polymer with a trans content of 82 % at 25 °C and pH 8.0. The X‐ray crystal structure of one of the hybrid biocatalysts at a resolution of 2.0 Å reveals that the Rh complex is located in the β‐barrel cavity without any perturbation to the total protein structure. Crystal structure analysis and molecular modeling support the fact that the stereoselectivity is enhanced by the effective control of monomer access to the Rh complex within the limited space of the protein cavity.

Klein Gebbink, R.J.M.

Chem. - Eur. J. 2015, 21, 15676-15685, 10.1002/chem.201502381

A series of Grubbs‐type catalysts that contain lipase‐inhibiting phosphoester functionalities have been synthesized and reacted with the lipase cutinase, which leads to artificial metalloenzymes for olefin metathesis. The resulting hybrids comprise the organometallic fragment that is covalently bound to the active amino acid residue of the enzyme host in an orthogonal orientation. Differences in reactivity as well as accessibility of the active site by the functionalized inhibitor became evident through variation of the anchoring motif and substituents on the N‐heterocyclic carbene ligand. Such observations led to the design of a hybrid that is active in the ring‐closing metathesis and the cross‐metathesis of N,N‐diallyl‐p‐toluenesulfonamide and allylbenzene, respectively, the latter being the first example of its kind in the field of artificial metalloenzymes.

Arold, S.T.; Eppinger, J.; Groll, M.

ACS Catal. 2019, 9, 11371-11380, 10.1021/acscatal.9b02896

Artificial metalloenzymes (ArMs) have high potential in biotechnological applications as they combine the versatility of transition-metal catalysis with the substrate selectivity of enzymes. An ideal host protein should allow high-yield recombinant expression, display thermal and solvent stability to withstand harsh reaction conditions, lack nonspecific metal-binding residues, and contain a suitable cavity to accommodate the artificial metal site. Moreover, to allow its rational functionalization, the host should provide an intrinsic reporter for metal binding and structural changes, which should be readily amendable to high-resolution structural characterization. Herein, we present the design, characterization, and de novo functionalization of a fluorescent ArM scaffold, named mTFP*, that achieves these characteristics. Fluorescence measurements allowed direct assessment of the scaffold’s structural integrity. Protein X-ray structures and transition metal Förster resonance energy transfer (tmFRET) studies validated the engineered metal coordination sites and provided insights into metal binding dynamics at the atomic level. The implemented active metal centers resulted in ArMs with efficient Diels–Alderase and Friedel–Crafts alkylase activities.

Lu, Y.

Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 8581-8586, 10.1073/pnas.1000526107

A structural and functional model of bacterial nitric oxide reductase (NOR) has been designed by introducing two glutamates (Glu) and three histidines (His) in sperm whale myoglobin. X-ray structural data indicate that the three His and one Glu (V68E) residues bind iron, mimicking the putative FeB site in NOR, while the second Glu (I107E) interacts with a water molecule and forms a hydrogen bonding network in the designed protein. Unlike the first Glu (V68E), which lowered the heme reduction potential by ∼110 mV, the second Glu has little effect on the heme potential, suggesting that the negatively charged Glu has a different role in redox tuning. More importantly, introducing the second Glu resulted in a ∼100% increase in NOR activity, suggesting the importance of a hydrogen bonding network in facilitating proton delivery during NOR reactivity. In addition, EPR and X-ray structural studies indicate that the designed protein binds iron, copper, or zinc in the FeB site, each with different effects on the structures and NOR activities, suggesting that both redox activity and an intermediate five-coordinate heme-NO species are important for high NOR activity. The designed protein offers an excellent model for NOR and demonstrates the power of using designed proteins as a simpler and more well-defined system to address important chemical and biological issues.

Utschig, L.M.

Chem. Sci. 2016, 7, 7068-7078, 10.1039/c6sc03121h

A series of Ru–protein–Co biohybrids have been prepared using the electron transfer proteins ferredoxin (Fd) and flavodoxin (Fld) as scaffolds for photocatalytic hydrogen production. The light-generated charge separation within these hybrids has been monitored by transient optical and electron paramagnetic resonance spectroscopies. Two distinct electron transfer pathways are observed. The Ru–Fd–Co biohybrid produces up to 650 turnovers of H2 utilizing an oxidative quenching mechanism for Ru(II)* and a sequential electron transfer pathway via the native [2Fe–2S] cluster to generate a Ru(III)–Fd–Co(I) charge separated state that lasts for ∼6 ms. In contrast, a direct electron transfer pathway occurs for the Ru–ApoFld–Co biohybrid, which lacks an internal electron relay, generating Ru(I)–ApoFld–Co(I) charge separated state that persists for ∼800 μs and produces 85 turnovers of H2 by a reductive quenching mechanism for Ru(II)*. This work demonstrates the utility of protein architectures for linking donor and catalytic function via direct or sequential electron transfer pathways to enable stabilized charge separation which facilitates photocatalysis for solar fuel production.

Ward, T.R.

Adv. Synth. Catal. 2007, 349, 1923-1930, 10.1002/adsc.200700022

We report on our efforts to create efficient artificial metalloenzymes for the enantioselective hydrogenation of N‐protected dehydroamino acids using either avidin or streptavidin as host proteins. Introduction of chiral amino acid spacers – phenylalanine or proline – between the biotin anchor and the flexible aminodiphosphine moiety 1, combined with saturation mutagenesis at position S112X of streptavidin, affords second generation artificial hydrogenases displaying improved organic solvent tolerance, reaction rates (3‐fold) and (S)‐selectivities (up to 95 % ee for N‐acetamidoalanine and N‐acetamidophenylalanine). It is shown that these artificial metalloenzymes follow Michaelis–Menten kinetics with an increased affinity for the substrate and a higher kcat than the protein‐free catalyst (compare kcat 3.06 min−1 and KM 7.38 mM for [Rh(COD)Biot‐1]+ with kcat 12.30 min−1 and KM 4.36 mM for [Rh(COD)Biot‐(R)‐Pro‐1]+ ⊂ WT Sav). Finally, we present a straightforward protocol using Biotin‐Sepharose to immobilize artificial metalloenzymes (>92 % ee for N‐acetamidoalanine and N‐acetamidophenylalanine using [Rh(COD)Biot‐(R)‐Pro‐1]+ ⊂ Sav S112W).

Ward, T.R.

C. R. Chim. 2007, 10, 678-683, 10.1016/j.crci.2007.02.020

We report on our efforts to create efficient artificial metalloenzymes for the enantioselective hydrogenation of N-protected dehydroamino acids using streptavidin as host protein. Introduction of an (R)-proline spacer between the biotin anchor and the diphosphine moiety affords a versatile ligand Biot-(R)-Pro-1 which displays good (S)-selectivities in the presence of streptavidin (91% ee). The resulting artificial metalloenzyme [Rh(Biot-(R)-Pro-1)(COD)]+ ⊂ WT-Sav displays increased stability against organic solvents.

Seelig, B.; Szostak, J.W.

Nature 2007, 448, 828-831, 10.1038/nature06032

Enzymes are exceptional catalysts that facilitate a wide variety of reactions under mild conditions, achieving high rate-enhancements with excellent chemo-, regio- and stereoselectivities. There is considerable interest in developing new enzymes for the synthesis of chemicals and pharmaceuticals1,2,3 and as tools for molecular biology. Methods have been developed for modifying and improving existing enzymes through screening, selection and directed evolution4,5. However, the design and evolution of truly novel enzymes has relied on extensive knowledge of the mechanism of the reaction6,7,8,9,10. Here we show that genuinely new enzymatic activities can be created de novo without the need for prior mechanistic information by selection from a naive protein library of very high diversity, with product formation as the sole selection criterion. We used messenger RNA display, in which proteins are covalently linked to their encoding mRNA11, to select for functional proteins from an in vitro translated protein library of >1012independent sequences without the constraints imposed by any in vivo step. This technique has been used to evolve new peptides and proteins that can bind a specific ligand12,13,14,15,16,17,18, from both random-sequence libraries12,14,15,16 and libraries based on a known protein fold17,18. We now describe the isolation of novel RNA ligases from a library that is based on a zinc finger scaffold18,19, followed by in vitro directed evolution to further optimize these enzymes. The resulting ligases exhibit multiple turnover with rate enhancements of more than two-million-fold.

Lewis, J.C.

Curr. Opin. Chem. Biol. 2017, 37, 48-55, 10.1016/j.cbpa.2016.12.027

Catalytic CH bond functionalization has become an important tool for organic synthesis. Metalloenzymes offer a solution to one of the foremost challenges in this field, site-selective CH functionalization, but they are only capable of catalyzing a subset of the CH functionalization reactions known to small molecule catalysts. To overcome this limitation, metalloenzymes have been repurposed by exploiting the reactivity of their native cofactors toward substrates not found in nature. Additionally, new reactivity has been accessed by incorporating synthetic metal cofactors into protein scaffolds to form artificial metalloenzymes. The selectivity and activity of these catalysts has been tuned using directed evolution. This review covers the recent progress in developing and optimizing both repurposed and artificial metalloenzymes as catalysts for selective CH bond functionalization.

Arnold, F.H.

Curr. Opin. Chem. Biol. 2019, 49, 67-75, 10.1016/j.cbpa.2018.10.004

C–H functionalization is an attractive strategy to construct and diversify molecules. Heme proteins, predominantly cytochromes P450, are responsible for an array of C–H oxidations in biology. Recent work has coupled concepts from synthetic chemistry, computation, and natural product biosynthesis to engineer heme protein systems to deliver products with tailored oxidation patterns. Heme protein catalysis has been shown to go well beyond these native reactions and now accesses new-to-nature C–H transformations, including C–N and C–C bond forming processes. Emerging work with these systems moves us along the ambitious path of building complexity from the ubiquitous C–H bond.

Mahy, J.-P.

Org. Biomol. Chem. 2009, 7, 3208, 10.1039/b907534h

Two new artificial hemoproteins or “hemozymes”, obtained by non covalent insertion of Fe(III)-meso-tetra-p-carboxy- and -p-sulfonato-phenylporphyrin into xylanase A from Streptomyces lividans, were characterized by UV-visible spectroscopy and molecular modeling studies, and were found to catalyze the chemo- and stereoselective oxidation of thioanisole into the S sulfoxide, the best yield (85 ± 4%) and enantiomeric excess (40% ± 3%) being obtained with Fe(III)-meso-tetra-p-carboxyphenylporphyrin-Xln10A as catalyst in the presence of imidazole as co-catalyst.

Korendovych, I.V.

Angew. Chem. Int. Ed. 2020, 59, 8108-8112, 10.1002/anie.201916712

The self-assembly of short peptides gives rise to versatile nanoassemblies capable of promoting efficient catalysis. We have semi-rationally designed a series of seven-residue peptides that form hemin-binding catalytic amyloids to facilitate enantioselective cyclopropanation with efficiencies that rival those of engineered hemin proteins. These results demonstrate that: 1) Catalytic amyloids can bind complex metallocofactors to promote practically important multisubstrate transformations. 2) Even essentially flat surfaces of amyloid assemblies can impart a substantial degree of enantioselectivity without the need for extensive optimization. 3) The ease of peptide preparation allows for straightforward incorporation of unnatural amino acids and the preparation of peptides made from d-amino acids with complete reversal of enantioselectivity.

Ueno, T.

Dalton Trans. 2012, 41, 11424, 10.1039/C2DT31030A

We have succeeded in preparing semi-synthesized proteins bound to Sc3+ ion which can promote an epoxide ring-opening reaction. The Sc3+ binding site was created on the surface of [(gp5βf)3]2 (N. Yokoi et al., Small, 2010, 6, 1873) by introducing a cysteine residue for conjugation of a bpy moiety using a thiol–maleimide coupling reaction. Three cysteine mutants [(gp5βf_X)3]2 (X = G18C, L47C, N51C) were prepared to introduce a bpy in different positions because it had been reported that Sc3+ ion can serve as a Lewis-acid catalyst for an epoxide ring-opening reaction upon binding of epoxide to bpy and two –ROH groups. G18C_bpy with Sc3+ can accelerate the rate of catalysis of the epoxide ring-opening reaction and has the highest rate of conversion among the three mutants. The value is more than 20 times higher than that of the mixtures of [(gp5βf)3]2/2,2′-bipyridine and L-threonine/2,2′-bipyridine. The elevated activity was obtained by the cooperative effect of stabilizing the Sc3+ coordination and accumulation of substrates on the protein surface. Thus, we expect that the semi-synthetic approach can provide insights into new rational design of artificial metalloenzymes.

Gray, H.B.; Imperiali, B.

J. Am. Chem. Soc. 1993, 115, 8455-8456, 10.1021/ja00071a068

n/a

Bren, K.L.

Inorg. Chem. 2016, 55, 467-477, 10.1021/acs.inorgchem.5b02054

There has been great interest in the development of stable, inexpensive, efficient catalysts capable of reducing aqueous protons to hydrogen (H2), an alternative to fossil fuels. While synthetic H2 evolution catalysts have been in development for decades, recently there has been great progress in engineering biomolecular catalysts and assemblies of synthetic catalysts and biomolecules. In this Forum Article, progress in engineering proteins to catalyze H2 evolution from water is discussed. The artificial enzymes described include assemblies of synthetic catalysts and photosynthetic proteins, proteins with cofactors replaced with synthetic catalysts, and derivatives of electron-transfer proteins. In addition, a new catalyst consisting of a thermophilic cobalt-substituted cytochrome c is reported. As an electrocatalyst, the cobalt cytochrome shows nearly quantitative Faradaic efficiency and excellent longevity with a turnover number of >270000.

Berggren, G.

Curr. Opin. Green Sustain. Chem. 2021, 32, 100521, 10.1016/j.cogsc.2021.100521

Hydrogenases are gas processing redox enzymes central in hydrogen metabolism. The interdisciplinary nature of hydrogenase research is underscored by the development of “artificial maturation”, enabling the preparation of semi-synthetic hydrogenases through the incorporation of synthetic cofactors into a range of apo-hydrogenase hosts under in vitro and in vivo conditions. Herein, we discuss how the preparation of such semi-synthetic [FeFe]-hydrogenases has elucidated structural elements of the cofactor critical for catalysis and reactivity towards known inhibitors. It has also provided a convenient method for exploring the biodiversity of this enzyme family and thereby facilitated investigation of the role of the outer-coordination sphere in tuning the reactivity of the H-cluster. In parallel, hijacking the assembly line of the [FeFe]-hydrogenase through incorporation of synthetic precursors has provided detailed insight into the biosynthesis of the H-cluster. Moreover, it has allowed the preparation of Mn analogs of [Fe] hydrogenase.

Lerner, R.A.

Science 1989, 243, 1184-1188, 10.1126/science.2922606

Monoclonal antibodies have been induced that are capable of catalyzing specific hydrolysis of the Gly-Phe bond of peptide substrates at neutral pH with a metal complex cofactor. The antibodies were produced by immunizing with a Co(III) triethylenetetramine (trien)-peptide hapten. These antibodies as a group are capable of binding trien complexes of not only Co(III) but also of numerous other metals. Six peptides were examined as possible substrates with the antibodies and various metal complexes. Two of these peptides were cleaved by several of the antibodies. One antibody was studied in detail, and cleavage was observed for the substrates with the trien complexes of Zn(II), Ga(III), Fe(III), In(III), Cu(II), Ni(II), Lu(III), Mg(II), or Mn(II) as cofactors. A turnover number of 6 x 10(-4) per second was observed for these substrates. These results demonstrate the feasibility of the use of cofactor-assisted catalysis in an antibody binding site to accomplish difficult chemical transformations.

Lu, Y.; Wang, J.

Chem. Sci. 2015, 6, 3881-3885, 10.1039/C5SC01126D

While nature employs various covalent and non-covalent strategies to modulate tyrosine (Y) redox potential and pKa in order to optimize enzyme activities, such approaches have not been systematically applied for the design of functional metalloproteins. Through the genetic incorporation of 3-methoxytyrosine (OMeY) into myoglobin, we replicated important features of cytochrome c oxidase (CcO) in this small soluble protein, which exhibits selective O2 reduction activity while generating a small amount of reactive oxygen species (ROS). These results demonstrate that the electron donating ability of a tyrosine residue in the active site is important for CcO function. Moreover, we elucidated the structural basis for the genetic incorporation of OMeY into proteins by solving the X-ray structure of OMeY specific aminoacyl-tRNA synthetase complexed with OMeY.

Lu, Y.; Wang, J.

Angew. Chem. Int. Ed. 2012, 51, 4312-4316, 10.1002/anie.201108756

Top model: Heme–copper oxidase (HCO) contains a histidine–tyrosine cross‐link in its heme a3/CuB oxygen reduction center. A functional model of HCO was obtained through the genetic incorporation of the unnatural amino acid imiTyr, which mimics the Tyr–His cross‐link, and of the CuB site into myoglobin (see picture). Like HCO, this small soluble protein exhibits selective O2‐reduction activity while generating little reactive oxygen species.

Hartwig, J.F.

Angew. Chem. Int. Ed. 2019, 58, 13954-13960, 10.1002/anie.201907460

The selective functionalization of one C-H bond over others in nearly identical steric and electronic environments can facilitate the construction of complex molecules. We report site-selective functionalizations of C-H bonds, differentiated solely by remote substituents, catalyzed by artificial metalloenzymes (ArMs) that are generated from the combination of an evolvable P450 scaffold and an iridium-porphyrin cofactor. The generated systems catalyze the insertion of carbenes into the C-H bonds of arange of phthalan derivatives containing substituents that render the two methylene positions in each phthalan inequivalent. These reactions occur with site-selectivity ratios of up to 17.8:1 and, in most cases, with pairs of enzyme mutants that preferentially form each of the two constitutional isomers. This study demonstrates the potential of abiotic reactions catalyzed by metalloenzymes to functionalize C-H bonds with site selectivity that is difficult to achieve with small-molecule catalysts.

Happe, T.

Nat. Chem. Biol. 2013, 9, 607-609, 10.1038/Nchembio.1311

Hydrogenases catalyze the formation of hydrogen. The cofactor ('H-cluster') of [FeFe]-hydrogenases consists of a [4Fe-4S] cluster bridged to a unique [2Fe] subcluster whose biosynthesis in vivo requires hydrogenase-specific maturases. Here we show that a chemical mimic of the [2Fe] subcluster can reconstitute apo-hydrogenase to full activity, independent of helper proteins. The assembled H-cluster is virtually indistinguishable from the native cofactor. This procedure will be a powerful tool for developing new artificial H2-producing catalysts.

Kazlauskas, R.J.

Chem. - Eur. J. 2009, 15, 1370-1376, 10.1002/chem.200801673

One useful synthetic reaction missing from nature's toolbox is the direct hydrogenation of substrates using hydrogen. Instead nature uses cofactors like NADH to reduce organic substrates, which adds complexity and cost to these reductions. To create an enzyme that can directly reduce organic substrates with hydrogen, researchers have combined metal hydrogenation catalysts with proteins. One approach is an indirect link where a ligand is linked to a protein and the metal binds to the ligand. Another approach is direct linking of the metal to protein, but nonspecific binding of the metal limits this approach. Herein, we report a direct hydrogenation of olefins catalyzed by rhodium(I) bound to carbonic anhydrase (CA‐[Rh]). We minimized nonspecific binding of rhodium by replacing histidine residues on the protein surface using site‐directed mutagenesis or by chemically modifying the histidine residues. Hydrogenation catalyzed by CA‐[Rh] is slightly slower than for uncomplexed rhodium(I), but the protein environment induces stereoselectivity favoring cis‐ over trans‐stilbene by about 20:1. This enzyme is the first cofactor‐independent reductase that reduces organic molecules using hydrogen. This catalyst is a good starting point to create variants with tailored reactivity and selectivity. This strategy to insert transition metals in the active site of metalloenzymes opens opportunities to a wider range of enzyme‐catalyzed reactions.

Bian, H.-D.; Huang, F.-P.

Tetrahedron: Asymmetry 2017, 28, 1700-1707, 10.1016/j.tetasy.2017.10.021

Four coordination complexes ML derived from an achiral Schiff base ligand (H2L = 2,2′-[(1,2-ethanediyl)bis(nitrilopropylidyne)]bisphenol) have been synthesized and characterized. A method is described for the enantioselective oxidation of a series of aryl alkyl sulfides using the coordination complexes in the presence of serum albumins (SAs) in an aqueous medium at ambient temperature. The mixture of metal complexes with serum albumins is useful for inducing asymmetric catalysis. The complex, albumin source and substrate influence stereoselective sulfoxidation. At optimal pH with the appropriate oxidant, some of ML/SA systems are identified as very efficient catalysts, giving the corresponding sulfoxides in excellent chemical yield (up to 100%) and good enantioselectivity (up to 94% ee) in certain cases. UV–visible spectroscopic data provide evidence that stronger binding between the complex and serum albumin lead to higher enantioselectivity.

Fontecilla-Camps, J.C.

Eur. J. Inorg. Chem. 2013, 2013, 3596-3600, 10.1002/ejic.201300592

The crystal structure of bovine β‐lactoglobulin bound to a complex consisting of a (η5‐Cp*)Rh(2,2′‐dipyridylamine) head and a lauric acid derived hydrophobic tail has been solved at 1.85 Å resolution. Previous work has shown that this hybrid catalyzes the transfer hydrogenation of an aryl ketone in neat water with formate as hydrogen donor with enantiomeric excess (ee) of about 26 %. Calculations using the X‐ray model indicate that the complex head can adopt discrete conformations, which may explain the ee observed.