Ward, T.R.

Top. Organomet. Chem. 2009, 10.1007/3418_2008_3

Artificial metalloenzymes can be created by incorporating an active metal catalyst precursor in a macromolecular host. When considering such artificial metalloenzymes, the first point to address is how to localize the active metal moiety within the protein scaffold. Although a covalent anchoring strategy may seem most attractive at first, supramolecular anchoring strategy has proven most successful thus far. In this context and inspired by Whitesides’ seminal paper, we have exploited the biotin–avidin technology to anchor a biotinylated active metal catalyst precursor within either avidin or streptavidin. A combined chemical and genetic strategy allows a rapid (chemogenetic) optimization of both the activity and the selectivity of the resulting artificial metalloenzymes. The chiral environment, provided by second coordination sphere interactions between the metal and the host protein, can be varied by introduction of a spacer between the biotin anchor and the metal moiety or by variation of the ligand scaffold. Alternatively, mutagenesis of the host protein allows a fine tuning of the activity and the selectivity. With this protocol, we have been able to produce artificial metalloenzymes based on the biotin–avidin technology for the enantioselective hydrogenation of N-protected dehydroaminoacids, the transfer hydrogenation of prochiral ketones as well as the allylic alkylation of symmetric substrates. In all cases selectivities >90% were achieved. Most recently, guided by an X-ray structure of an artificial metalloenzyme, we have extended the chemogenetic optimization to a designed evolution scheme. Designed evolution combines rational design with combinatorial screening. In this chapter, we emphasize the similarities and the differences between artificial metalloenzymes and their homogeneous or enzymatic counterparts.

Watanabe, Y.

Top. Organomet. Chem. 2009, 10.1007/3418_2008_4

Molecular design of artificial metalloproteins is one of the most attractive subjects in bioinorganic chemistry. Protein vacant space has been utilized to prepare metalloproteins because it provides a unique chemical environment for application to catalysts and to biomaterials bearing electronic, magnetic, and medical properties. Recently, X-ray crystal structural analysis has increased in this research area because it is a powerful tool for understanding the interactions of metal complexes and protein scaffolds, and for providing rational design of these composites. This chapter reviews the recent studies on the preparation methods and X-ray crystal structural analyses of metal/protein composites, and their functions as catalysts, metal-drugs, etc.

Reetz, M.T.

Top. Organomet. Chem. 2009, 10.1007/3418_2008_12

Whereas the directed evolution of stereoselective enzymes provides a useful tool in asymmetric catalysis, generality cannot be claimed because enzymes as catalysts are restricted to a limited set of reaction types. Therefore, a new concept has been proposed, namely directed evolution of hybrid catalysts in which proteins serve as hosts for anchoring ligand/transition metal entities. Accordingly, appropriate genetic mutagenesis methods are applied to the gene of a given protein host, providing after expression a library of mutant proteins. These are purified and a ligand/transition metal anchored site-specifically. Following en masse ee-screening, the best hit is identified, and the corresponding mutant gene is used as a template for another round of mutagenesis, expression, purification, bioconjugation, and screening. This allows for a Darwinian optimization of transition metal catalysts.

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.

Kazlauskas, R.J.

Top. Organomet. Chem. 2009, 10.1007/3418_2008_1

Carbonic anhydrase binds a zinc ion in a hydrophobic active site using the imidazole groups of three histidine residues. The natural role of carbonic anhydrase is to catalyze the reversible hydration of carbon dioxide to bicarbonate, but it also catalyzes hydrolysis of esters with moderate enantioselectivity. Replacing the active-site zinc with manganese yielded manganese-substituted carbonic anhydrase (CA[Mn]), which shows peroxidase activity with a bicarbonate-dependent mechanism. In the presence of bicarbonate and hydrogen peroxide, CA[Mn] catalyzed the efficient oxidation of o-dianisidine with k cat /K M = 1.4 × 106 M−1s−1, which is comparable to that for horseradish peroxidase, k cat /K M = 57 × 106 M−1s−1. CA[Mn] also catalyzed the moderately enantioselective epoxidation of olefins to epoxides (E = 5 for p-chlorostyrene). This enantioselectivity is similar to that for natural heme-based peroxidases, but has the advantage that CA[Mn] avoids formation of aldehyde side products. CA[Mn] degrades during the epoxidation, limiting the yield of the epoxidations to <12%. Replacement of active-site residues Asn62, His64, Asn67, Gln92, or Thr200 with alanine by site-directed mutagenesis decreased the enantioselectivity showing that the active site controls enantioselectivity of the epoxidation.