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 acid⅐formate 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.second coordination sphere ͉ asymmetric catalysis ͉ chemzymes T he asymmetric reduction of CAO and CAN bonds is one of the most fundamental transformations in organic chemistry (1-3). Although enzymatic and organometallic catalysis have evolved along very different paths, both methodologies can achieve high levels of enantioselection for this transformation.Oxidoreductases such as alcohol dehydrogenases can perform this task very efficiently and selectively (4-7). To achieve this, however, these enzymes rely on precious cofactors NAD(P)H, which need to be regenerated (8). Alternatively, whole cells can be used. These contain multiple dehydrogenases, all of the necessary cofactors, and the metabolic pathways for their regeneration (5).Asymmetric transfer hydrogenation (Meerwein-PonndorfVerley reduction) based on d 6 piano-stool complexes has proven to be versatile for the asymmetric reduction of ketones and imines (2, 9, 10). Regeneration of the organometallic hydride is achieved by a ␤-H abstraction between the catalyst precursor and a sacrificial hydrogen donor (isopropanol or formate). These catalysts nicely complement other organometallic systems that rely on dihydrogen (3).It is interesting to note that theoretical studies suggest that the transfer hydrogenation catalyzed by d 6 piano-stool complexes proceeds without coordination of the substrate to the metal, as illustrated in transition-state structure 1 (11-14) (Fig. 1). The chiral recognition pattern for this organometallic transformation is thus reminiscent of enzymatic catalysis. Indeed, the second coordination sphere provided by a protein is optimized to steer the enantiodiscrimination step without necessarily requiring covalent (or dative) binding of the substrate to the enzyme.In recent years, chemo-enzymatic catalysis has attracted increasing attention. In such systems, an ...

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.

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/pi interactions between the substrate and the eta6-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.

Creating new catalytic function in proteins. Anchoring an organometallic moiety within a protein affords artificial metalloenzymes for enantioselective catalysis. Both chemical and genetic tools can be applied in the optimization of such systems, which lie at the interface between homogeneous and enzymatic catalysis. This minireview presents the latest developments in the field of artificial metalloenzymes.