Directed Evolution of Stereoselective Hybrid Catalysts

Biocatalytic Strategy for Highly Diastereo- and Enantioselective Synthesis of 2,3-Dihydrobenzofuran-Based Tricyclic Scaffolds

2,3-Dihydrobenzofurans are key pharmacophores in many natural and synthetic bioactive molecules. A biocatalytic strategy is reported here for the highly diastereo- and enantioselective construction of stereochemically rich 2,3-dihydrobenzofurans in high enantiopurity (>99.9% de and ee), high yields, and on a preparative scale via benzofuran cyclopropanation with engineered myoglobins. Computational and structure-reactivity studies provide insights into the mechanism of this reaction, enabling the elaboration of a stereochemical model that can rationalize the high stereoselectivity of the biocatalyst. This information was leveraged to implement a highly stereoselective route to a drug molecule and a tricyclic scaffold featuring five stereogenic centers via a single-enzyme transformation. This work expands the biocatalytic toolbox for asymmetric C-C bond transformations and should prove useful for further development of metalloprotein catalysts for abiotic carbene transfer reactions.

Artificial Metalloenzymes Based on the Biotin-Streptavidin Technology: Challenges and Opportunities

The biotin-streptavidin technology offers an attractive means to engineer artificial metalloenzymes (ArMs). Initiated over 50 years ago by Bayer and Wilchek, the biotin-(strept)avidin techonology relies on the exquisite supramolecular affinity of either avidin or streptavidin for biotin. This versatile tool, commonly referred to as "molecular velcro", allows nearly irreversible anchoring of biotinylated probes within a (strept)avidin host protein. Building upon a visionary publication by Whitesides from 1978, several groups have been exploiting this technology to create artificial metalloenzymes. For this purpose, a biotinylated organometallic catalyst is introduced within (strept)avidin to afford a hybrid catalyst that combines features reminiscent of both enzymes and organometallic catalysts. Importantly, ArMs can be optimized by chemogenetic means. Combining a small collection of biotinylated organometallic catalysts with streptavidin mutants allows generation of significant diversity, thus allowing optimization of the catalytic performance of ArMs. Pursuing this strategy, the following reactions have been implemented: hydrogenation, alcohol oxidation, sulfoxidation, dihydroxylation, allylic alkylation, transfer hydrogenation, Suzuki cross-coupling, C-H activation, and metathesis. In this Account, we summarize our efforts in the latter four reactions. X-ray analysis of various ArMs based on the biotin-streptavidin technology reveals the versatility and commensurability of the biotin-binding vestibule to accommodate and interact with transition states of the scrutinized organometallic transformations. In particular, streptavidin residues at positions 112 and 121 recurrently lie in close proximity to the biotinylated metal cofactor. This observation led us to develop a streamlined 24-well plate streptavidin production and screening platform to optimize the performance of ArMs. To date, most of the efforts in the field of ArMs have focused on the use of purified protein samples. This seriously limits the throughput of the optimization process. With the ultimate goal of complementing natural enzymes in the context of synthetic and chemical biology, we outline the milestones required to ultimately implement ArMs within a cellular environment. Indeed, we believe that ArMs may allow signficant expansion of the natural enzymes' toolbox to access new-to-nature reactivities in vivo. With this ambitious goal in mind, we report on our efforts to (i) activate the biotinylated catalyst precursor upon incorporation within streptavidin, (ii) minimize the effect of the cellular environment on the ArM's performance, and (iii) demonstrate the compatibility of ArMs with natural enzymes in cascade reactions.

A general method for artificial metalloenzyme formation through strain-promoted azide-alkyne cycloaddition

Strain-promoted azide-alkyne cycloaddition (SPAAC) can be used to generate artificial metalloenzymes (ArMs) from scaffold proteins containing a p-azido-L-phenylalanine (Az) residue and catalytically active bicyclononyne-substituted metal complexes. The high efficiency of this reaction allows rapid ArM formation when using Az residues within the scaffold protein in the presence of cysteine residues or various reactive components of cellular lysate. In general, cofactor-based ArM formation allows the use of any desired metal complex to build unique inorganic protein materials. SPAAC covalent linkage further decouples the native function of the scaffold from the installation process because it is not affected by native amino acid residues; as long as an Az residue can be incorporated, an ArM can be generated. We have demonstrated the scope of this method with respect to both the scaffold and cofactor components and established that the dirhodium ArMs generated can catalyze the decomposition of diazo compounds and both Si-H and olefin insertion reactions involving these carbene precursors.

Biocatalysis in organic chemistry and biotechnology: past, present, and future

Enzymes as catalysts in synthetic organic chemistry gained importance in the latter half of the 20th century, but nevertheless suffered from two major limitations. First, many enzymes were not accessible in large enough quantities for practical applications. The advent of recombinant DNA technology changed this dramatically in the late 1970s. Second, many enzymes showed a narrow substrate scope, often poor stereo- and/or regioselectivity and/or insufficient stability under operating conditions. With the development of directed evolution beginning in the 1990s and continuing to the present day, all of these problems can be addressed and generally solved. The present Perspective focuses on these and other developments which have popularized enzymes as part of the toolkit of synthetic organic chemists and biotechnologists. Included is a discussion of the scope and limitation of cascade reactions using enzyme mixtures in vitro and of metabolic engineering of pathways in cells as factories for the production of simple compounds such as biofuels and complex natural products. Future trends and problems are also highlighted, as is the discussion concerning biocatalysis versus nonbiological catalysis in synthetic organic chemistry. This Perspective does not constitute a comprehensive review, and therefore the author apologizes to those researchers whose work is not specifically treated here.

A hybrid ring-opening metathesis polymerization catalyst based on an engineered variant of the β-barrel protein FhuA

A β-barrel protein hybrid catalyst was prepared by covalently anchoring a Grubbs-Hoveyda type olefin metathesis catalyst at a single accessible cysteine amino acid in the barrel interior of a variant of β-barrel transmembrane protein ferric hydroxamate uptake protein component A (FhuA). Activity of this hybrid catalyst type was demonstrated by ring-opening metathesis polymerization of a 7-oxanorbornene derivative in aqueous solution.

Synthesis and Catalytic Activity of Amino Acids and Metallopeptides with Catalytically Active Metallocyclic Side Chains

Two approaches to prepare amino acids with catalytically active organometallic side chains are presented. These methods are notable in that they provide access either free or N-protected compounds that are structurally analogous to naturally occurring amino acids. The N-protected organo-metallic amino acids are compatible with standard peptide coupling conditions and can be used to prepare catalytically active metallopeptides.

Click reactions in protein chemistry: from the preparation of semisynthetic enzymes to new click enzymes

Click-chemistry is an approach based on cycloaddition reactions which has been successfully used as a chemical approach for complex organic molecules and which has recently starred in a boom in the world of protein chemistry. The advantage of the use of this technique in protein chemistry is based on a very high and efficient chemoselectivity, which usually requires simple or no purification and is extremely rate-accelerated in aqueous media. The perspective discusses some of the most recent advances in the application of this reaction in selective enzyme surface modification for the creation of new semisynthetic enzymes (fluorescence labeled enzymes, peptide-enzyme conjugates, glycosylated enzymes), and interestingly, the recent design and creation of "click" enzymes.

Artificial metalloenzymes based on the biotin-avidin technology: enantioselective catalysis and beyond

Artificial metalloenzymes are created by incorporating an organometallic catalyst within a host protein. The resulting hybrid can thus provide access to the best features of two distinct, and often complementary, systems: homogeneous and enzymatic catalysts. The coenzyme may be positioned with covalent, dative, or supramolecular anchoring strategies. Although initial reports date to the late 1970s, artificial metalloenzymes for enantioselective catalysis have gained significant momentum only in the past decade, with the aim of complementing homogeneous, enzymatic, heterogeneous, and organic catalysts. Inspired by a visionary report by Wilson and Whitesides in 1978, we have exploited the potential of biotin-avidin technology in creating artificial metalloenzymes. Owing to the remarkable affinity of biotin for either avidin or streptavidin, covalent linking of a biotin anchor to a catalyst precursor ensures that, upon stoichiometric addition of (strept)avidin, the metal moiety is quantitatively incorporated within the host protein. In this Account, we review our progress in preparing and optimizing these artificial metalloenzymes, beginning with catalytic hydrogenation as a model and expanding from there. These artificial metalloenzymes can be optimized by both chemical (variation of the biotin-spacer-ligand moiety) and genetic (mutation of avidin or streptavidin) means. Such chemogenetic optimization schemes were applied to various enantioselective transformations. The reactions implemented thus far include the following: (i) The rhodium-diphosphine catalyzed hydrogenation of N-protected dehydroaminoacids (ee up to 95%); (ii) the palladium-diphosphine catalyzed allylic alkylation of 1,3-diphenylallylacetate (ee up to 95%); (iii) the ruthenium pianostool-catalyzed transfer hydrogenation of prochiral ketones (ee up to 97% for aryl-alkyl ketones and ee up to 90% for dialkyl ketones); (iv) the vanadyl-catalyzed oxidation of prochiral sulfides (ee up to 93%). A number of noteworthy features are reminiscent of homogeneous catalysis, including straightforward access to both enantiomers of the product, the broad substrate scope, organic solvent tolerance, and an accessible range of reactions that are typical of homogeneous catalysts. Enzyme-like features include access to genetic optimization, an aqueous medium as the preferred solvent, Michaelis-Menten behavior, and single-substrate derivatization. The X-ray characterization of artificial metalloenzymes provides fascinating insight into possible enantioselection mechanisms involving a well-defined second coordination sphere environment. Thus, such artificial metalloenzymes combine attractive features of both homogeneous and enzymatic kingdoms. In the spirit of surface borrowing, that is, modulating ligand affinity by harnessing existing protein surfaces, this strategy can be extended to selectively binding streptavidin-incorporated biotinylated ruthenium pianostool complexes to telomeric DNA. This application paves the way for chemical biology applications of artificial metalloenzymes.