Artificial metalloenzyme

An artificial metalloenzyme (26) was designed by Breslow et al. 24). It was the first example of a complete artificial enzyme, having a substrate binding cyclodextrin cavity and a Ni2+ ion-chelated nucleophilic group for catalysis. Metalloenzyme (26) behaves a real catalyst, exhibiting turnover, and enhances the rate of hydrolysis of p-nitrophenyl acetate more than 103 fold. The catalytic group of 26 is a -Ni2+ complex which itself is active toward the substrate 1, but not toward such a substrate having no metal ion affinity at a low catalyst concentration. It is appearent that the metal ion in 26 activates the oximate anion by chelation, but not the substrate directly as believed in carboxypeptidase. 

Cieus M, Ward TR. Designed evolution of artificial metalloenzymes protein catalysts made to order. Org Biomol Chem 2007 5 1835-1844. 

Figure .17 Artificial metalloenzymes (a) Strategyto incorporate a catalytically active metal fragment within a host protein.

FIGURE 28 Artificial metalloenzymes (A) strategy for incorporating a catalytically active metal fragment into a host protein (Wilson and Whitesides (97)) (B) hydrogenation of alkenes via biotin-(strep)avidin methodology (Wilson and Whitesides (97) and Skander et al. (9S)). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

Letondor C, Ward TR. Artificial metalloenzymes for enantiose-lective catalysis recent advances. ChemBioChem. 2006 7 1845-1852. 

Abe S, Ueno T, Reddy PAN, Okazaki S, Hikage T, Suzuki A, Yamane T, Nakajima H, Watanabe Y. Design and structure analysis of artificial metalloproteins selective coordination of His64 to copper complexes with square-planar structure in the apo-myoglobin scaffold. Inorg. Chem. 2007 46 5137-5139. Ohashi M, Koshiyama T, Ueno T, Yanase M, Fuji H, Watanabe Y. Preparation of artificial metalloenzymes by insertion of chromium Schiff base complexes into apomyoglobin mutants. Angew. Chem Int. Ed. 2003 42 1005-1008. 

Collot J, Gradinaru J, Humbert N, Skander M, Zocchi A, Ward TR. Artificial metalloenzymes for enantioselective catalysis based on biotin-avidin. J. Am. Chem. Soc. 2003 125 9030-9031. Letondor C, Humbert N, Ward TR. Artificial metalloenzymes based on biotin-avidin technology for the enantioselective reduction of ketones by transfer hydrogenation. I roc. Natl. Acad. Sci. U.S.A. 2005 102 4683-4687. 

Thomas CM, Letondor C, Humbert N, Ward TR. Aqueous oxidation of alcohols catalyzed by artificial metalloenzymes based on the biotin-avidin technology. J. Organomet. Chem. 2005 690 4488 491. 

Material scientists have exploited a range of ferritin superfamily proteins as supramolecular templates to encapsulate nanoparticles and/or as well-defined building blocks for fabrication of higher order assembly. For example, the organometallic Rh(nbd) (nbd = norbomadiene) can be immobilised at specific sites within the apoferritin molecule where it can catalyse the polymerisation of phenylacetylene within the protein shell (Figure 19.10). This is but one example of the quest to develop highly effective artificial metalloenzymes by rational design of metal coordination sites within the ferritin molecule. 

An effective multinuclear artificial metalloenzyme would be obtained if an artificial active site comprising of two or more proximal metal centers is designed. A trinuclear artificial metallopeptidase was prepared by using BH... 

Fig. 2 Artificial metalloenzymes and asymmetric reactions catalyzed by them. Metal cofactors are introduced by a covalent modification of biotin, b double anchoring to myoglobin, and c non-covalent insertion to serum albumin...

Artificial Metalloenzymes for Enantioselective Catalysis Based on the Biotin-Avidin Technology... 

Abstract 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. 

Key words Allylic alkylation, Artificial metalloenzyme, Biotin-avidin technology, Chemogenetic optimization, Designed evolution, Enantioselective catalysis. Hybrid catalyst. Hydrogenation, Streptavidin, Transfer hydrogenation. 

Artificial metalloenzymes, as reviewed here, are hybrid catalysts resulting from the introduction of a metal complex with catalytic activity into a macromolecular host, avidin or streptavidin (referred to as (strept)avidin hereafter). This provides a well-defined enantiopure second coordination sphere, thus potentially inducing selectivity in the catalyzed reaction [2-16], The incorporation of an organometallic moiety within a protein may combine the advantages of both catalytic strategies, which are, in many regards, complementary (Table 1). 

We reasoned that one could mimic Nature by incorporating cofactors and metal ions to broaden the scope of accessible reactions catalyzed by protein scaffolds. Different approaches for the generation of artificial metalloenzymes have recently been reviewed [2-16]. Herein, we present the developments in the field of artificial metalloenzymes for enantioselective catalysis based on the biotin-avidin technology. The discussion includes a short introduction on the biotin-avidin technology followed by several examples of chemogenetic optimization of the performance of artificial metalloenzymes based on this technology. 

Fig. 2 Biotin-avidin technology Artificial metalloenzymes [M(L )(biotin-ligand)]c(strept)avidin for enantioselective catalysis are based on the anchoring of a catalyticaUy active metal fragment within a host protein via a hgand, a spacer, and biotin. Chemical optimization can be achieved either by varying the spacer or the metal chelate moiety ML ). Saturation mutagenesis at a position close to the metal moiety ( ) can be used for genetic optimization...

Inspired by the visionary paper of Whitesides, we adapted and extended the concept of artificial metalloenzymes (or hybrid catalysts) based on the biotin-avidin technology to enantioselective hydrogenation, transfer hydrogenation, and aUyhc alkylation reactions, which are summarized herein. 

Scheme 1 Artificial metalloenzymes based on the biotin-avidin technology for the hydrogenation of alkenes. Operating conditions used by a Whitesides [34] and b Chan [35]...

In order to favor the alkylation over the hydrolysis of the acetate-bearing substrate in aqueous media, the addition of a cationic surfactant proved beneficial (Table 4, entries 1-3). Having identified suitable reaction conditions for the AAA catalyzed by artificial metalloenzymes, we proceeded to screen the diversity matrix provided by combining 22 (strept)avidin isoforms with 13 [Pd(ii -allyl) (Biot-spacer-l)]+ type complexes. The results are summarized as a fingerprint in Fig. 6 and selected results are listed in Table 4 (entries 4-8). 

In contrast to other reactions implemented so far with artificial metalloenzymes there is, to the best of our knowledge, no enzyme known that catalyzes such C-C bond-forming allylic alkylations. The final section of this chapter focuses on carbonyl reduction via a transfer hydrogenation mechanism. 

Table 5 Selected results for the transfer hydrogenation of procMral ketones by artificial metalloenzymes [59]...

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