Renaud, J.-L.; Ward, T.R.

Catal. Lett. 2016, 146, 564-569, 10.1007/s10562-015-1681-6

We report on the synthesis of biotinylated (cyclopentadienone)iron tricarbonyl complexes, the in situ generation of the corresponding streptavidin conjugates and their application in asymmetric hydrogenation of imines and ketones.

Marchi-Delapierre, C.

ACS Catal. 2020, 10, 5631-5645, 10.1021/acscatal.9b04904

Artificial enzymes represent an attractive alternative to design abiotic biocatalysis. EcNikA-Ru1, an artificial metalloenzyme developed by embedding a ruthenium-based catalyst into the cavity of the periplasmic nickel-binding protein NikA, was found to efficiently and selectively transform certain alkenes. The objective of this study was to provide a rationale on the enzymatic function and the unexpected substrate-dependent chemoselectivity of EcNikA-Ru1 thanks to a dual experimental/computational study. We observed that the de novo active site allows the formation of the terminal oxidant via the formation of a ruthenium aquo species that subsequently reacts with the hypervalent iodine of phenyl iodide diacetic acid. The oxidation process relies on a RuIV═O pathway via a two-step reaction with a radical intermediate, resulting in the formation of either a chlorohydrin or an epoxide. The results emphasize the impact of the protein scaffold on the kinetics of the reaction, through (i) the promotion of the starting oxidizing species via the exchange of a CO ligand with a water molecule; and (ii) the control of the substrate orientation on the intermediate structures, formed after the RuIV═O attack. When a Cα attack is preferred, chlorohydrins are formed while an attack on Cβ leads to an epoxide. This work provides evidence that artificial enzymes mimic the behavior of their natural counterparts.

Ward, T.R.

Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 4683-4687, 10.1073/pnas.0409684102

Most physiological and biotechnological processes rely on molecular recognition between chiral (handed) molecules. Manmade homogeneous catalysts and enzymes offer complementary means for producing enantiopure (single-handed) compounds. As the subtle details that govern chiral discrimination are difficult to predict, improving the performance of such catalysts often relies on trial-and-error procedures. Homogeneous catalysts are optimized by chemical modification of the chiral environment around the metal center. Enzymes can be improved by modification of gene encoding the protein. Incorporation of a biotinylated organometallic catalyst into a host protein (avidin or streptavidin) affords versatile artificial metalloenzymes for the reduction of ketones by transfer hydrogenation. The boric acid·formate mixture was identified as a hydrogen source compatible with these artificial metalloenzymes. A combined chemo-genetic procedure allows us to optimize the activity and selectivity of these hybrid catalysts: up to 94% (R) enantiomeric excess for the reduction of p-methylacetophenone. These artificial metalloenzymes display features reminiscent of both homogeneous catalysts and enzymes.

Ward, T.R.

J. Am. Chem. Soc. 2006, 128, 8320-8328, 10.1021/ja061580o

Incorporation of biotinylated racemic three-legged d6-piano stool complexes in streptavidin yields enantioselective transfer hydrogenation artificial metalloenzymes for the reduction of ketones. Having identified the most promising organometallic catalyst precursors in the presence of wild-type streptavidin, fine-tuning of the selectivity is achieved by saturation mutagenesis at position S112. This choice for the genetic optimization site is suggested by docking studies which reveal that this position lies closest to the biotinylated metal upon incorporation into streptavidin. For aromatic ketones, the reaction proceeds smoothly to afford the corresponding enantioenriched alcohols in up to 97% ee (R) or 70% (S). On the basis of these results, we suggest that the enantioselection is mostly dictated by CH/π interactions between the substrate and the η6-bound arene. However, these enantiodiscriminating interactions can be outweighed in the presence of cationic residues at position S112 to afford the opposite enantiomers of the product.

Ward, T.R.

Angew. Chem. Int. Ed. 2011, 50, 3026-3029, 10.1002/anie.201007820

Man‐made activity: Introduction of a biotinylated iridium piano stool complex within streptavidin affords an artificial imine reductase (see scheme). Saturation mutagenesis allowed optimization of the activity and the enantioselectivity of this metalloenzyme, and its X‐ray structure suggests that a nearby lysine residue acts as a proton source during the transfer hydrogenation.

Apfel, U.-P.; Happe, T.; Kurisu, G.

Chem. Sci. 2016, 7, 959-968, 10.1039/C5SC03397G

Crystal structures of semisynthetic [FeFe]-hydrogenases with variations in the [2Fe] cluster show little structural differences despite strong effects on activity.

Apfel, U.-P.; Happe, T.

Dalton Trans. 2017, 46, 16947-16958, 10.1039/C7DT03785F

Combination of biological and chemical methods allow for creation of [FeFe]-hydrogenases with an artificial synthetic cofactor.

Klein Gebbink, R.J.M.; van Koten, G.

Tetrahedron Lett. 2011, 52, 1601-1604, 10.1016/j.tetlet.2011.01.106

The preparation of a heterogeneous bifunctional catalytic system, combining the catalytic properties of an organometallic catalyst (racemization) with those of an enzyme (enantioselective acylation) is described. A novel ruthenium phosphonate inhibitor was synthesized and covalently anchored to a lipase immobilized on a solid support (CALB, Novozym® 435). The immobilized bifunctional catalytic system showed activity in both racemization of (S)-1-phenylethanol and selective acylation of 1-phenylethanol.

Hu, X.; Shima, S.

Angew. Chem. Int. Ed. 2021, 60, 13350-13357, 10.1002/anie.202100443

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.

Ueno, T.

Chem. Commun. 2011, 47, 2074, 10.1039/C0CC03015E

We have constructed a robust β-helical nanotube from the component proteins of bacteriophage T4 and modified this nanotube with RuII(bpy)3 and ReI(bpy)(CO)3Cl complexes. The photocatalytic system arranged on the tube catalyzes the reduction of CO2 with higher reactivity than that of the mixture of the monomeric forms.

Berggren, G.; Lindblad, P.

Energy Environ. Sci. 2018, 11, 3163-3167, 10.1039/C8EE01975D

[FeFe]-Hydrogenases are hydrogen producing metalloenzymes with excellent catalytic capacities, highly relevant in the context of a future hydrogen economy. Here we demonstrate the synthetic activation of a heterologously expressed [FeFe]-hydrogenase in living cells of Synechocystis PCC 6803, a photoautotrophic microbial chassis with high potential for biotechnological energy applications. H2-Evolution assays clearly show that the non-native, semi-synthetic enzyme links to the native metabolism in living cells.

Lubitz, W.; Reijerse, E.

Biochemistry 2015, 54, 1474-1483, 10.1021/bi501391d

[FeFe]-hydrogenases are to date the only enzymes for which it has been demonstrated that the native inorganic binuclear cofactor of the active site Fe2(adt)(CO)3(CN)2 (adt = azadithiolate = [S-CH2-NH-CH2-S]2–) can be synthesized on the laboratory bench and subsequently inserted into the unmaturated enzyme to yield fully functional holo-enzyme (Berggren, G. et al. (2013) Nature 499, 66–70; Esselborn, J. et al. (2013) Nat. Chem. Biol. 9, 607–610). In the current study, we exploit this procedure to introduce non-native cofactors into the enzyme. Mimics of the binuclear subcluster with a modified bridging dithiolate ligand (thiodithiolate, N-methylazadithiolate, dimethyl-azadithiolate) and three variants containing only one CN– ligand were inserted into the active site of the enzyme. We investigated the activity of these variants for hydrogen oxidation as well as proton reduction and their structural accommodation within the active site was analyzed using Fourier transform infrared spectroscopy. Interestingly, the monocyanide variant with the azadithiolate bridge showed ∼50% of the native enzyme activity. This would suggest that the CN– ligands are not essential for catalytic activity, but rather serve to anchor the binuclear subsite inside the protein pocket through hydrogen bonding. The inserted artificial cofactors with a propanedithiolate and an N-methylazadithiolate bridge as well as their monocyanide variants also showed residual activity. However, these activities were less than 1% of the native enzyme. Our findings indicate that even small changes in the dithiolate bridge of the binuclear subsite lead to a rather strong decrease of the catalytic activity. We conclude that both the Brønsted base function and the conformational flexibility of the native azadithiolate amine moiety are essential for the high catalytic activity of the native enzyme.

Ward, T.R.

Inorg. Chim. Acta 2010, 363, 601-604, 10.1016/j.ica.2009.02.001

Artificial metalloenzymes based on the incorporation of biotinylated ruthenium piano–stool complexes within streptavidin can be readily optimized by chemical or genetic means. We performed genetic modifications of such artificial metalloenzymes for the transfer hydrogenation of aromatic ketones, by combining targeted point mutations of the host protein. Upon using the P64G-L124V double mutant of streptavidin in combination with the [η6-(p-cymene)Ru(Biot-p-L)Cl] complex, the enantioselectivity can be increased up to 98% ee (R) for the reduction of p-methylacetophenone, which is the highest selectivity obtained up to date with an artificial transfer hydrogenase.

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.

Reetz, M.T.

Chimia 2002, 56, 721-723, 10.2533/000942902777679920

The first step in applying the recently proposed concept concerning the application of directed evolution to the creation of selective hybrid catalysts is described, specifically the covalent attachment of Mn-salen moieties and of Cu-, Pd-, and Rh-complexes of dipyridine derivatives as well as the implantation of a diphosphine moiety in a protein, future steps being cycles of mutagenesis/screening.

Ward, T.R.

Angew. Chem. Int. Ed. 2008, 47, 1400-1404, 10.1002/anie.200704865

A structure is worth a thousand words: Guided by the X‐ray structure of an S‐selective artificial transfer hydrogenase, designed evolution was used to optimize the selectivity of hybrid catalysts. Fine‐tuning of the second coordination sphere of the ruthenium center (see picture, orange sphere) by introduction of two point mutations allowed the identification of selective artificial transfer hydrogenases for the reduction of dialkyl ketones.