Enantiocomplementary Synthesis of γ-Nitroketones Using Designed and Evolved Carboligases

Design and Evolution of an Enzyme for the Asymmetric Michael Addition of Cyclic Ketones to Nitroolefins by Enamine Catalysis

Consistent introduction of novel enzymes is required for developing efficient biocatalysts for challenging biotransformations. Absorbing catalytic modes from organocatalysis may be fruitful for designing new-to-nature enzymes with novel functions. Herein we report a newly designed artificial enzyme harboring a catalytic pyrrolidine residue that catalyzes the asymmetric Michael addition of cyclic ketones to nitroolefins through enamine activation with high efficiency. Diverse chiral γ-nitro cyclic ketones with two stereocenters were efficiently prepared with excellent stereoselectivity (up to 97 % e.e., >20 : 1 d.r.) and good yield (up to 86 %). This work provides an efficient biocatalytic strategy for cyclic ketone functionalization, and highlights the usefulness of artificial enzymes for extending biocatalysis to further non-natural reactions.

A Type 1 Aldolase, NahE, Catalyzes a Stereoselective Nitro-Michael Reaction: Synthesis of β-Aryl-γ-nitrobutyric Acids

Michael addition reactions are highly useful in organic synthesis and are commonly accomplished using organocatalysts. However, the corresponding biocatalytic Michael additions are rare, typically lack synthetically useful substrate scope, and suffer from low stereoselectivity. Herein we report a biocatalytic nitro-Michael addition, catalyzed by NahE, that proceeds with low catalyst loading at room temperature in moderate to excellent enantioselectivity and high yields. A series of β-nitrostyrenes reacted with pyruvate in the presence of NahE to give, after oxidative decarboxylation, β-aryl-γ-nitrobutyric acids in up to 99 % yield without need for chromatography, providing a simple preparative-scale route to chiral GABA analogues. This reaction represents the first example of an aldolase displaying promiscuous Michaelase activity and opens the use of nitroalkenes in place of aldehydes as substrates for aldolases.

Stereoselective Three-Step One-Pot Cascade Combining Amino- and Biocatalysis to Access Chiral γ-Nitro Alcohols

The combination of small-molecule catalysis and enzyme catalysis represents an underexploited area of research with huge potential in asymmetric synthetic chemistry due to both compatibility of reaction conditions and complementary reactivity. Herein, we describe the telescopic synthesis of chiral nitro alcohols starting from commercially available benzaldehyde derivatives through the one-pot three-step chemoenzymatic cascade combination of a Wittig reaction, chiral-thiourea-catalysed asymmetric conjugate addition, and ketoreductase-mediated reduction to access the corresponding target compounds in moderate to excellent overall isolated yields (36-80 %) and high diastereomeric and enantiomeric ratios (up to >97 : 3). This represents the first example of the combination of an organocatalysed asymmetric conjugate addition via iminium ion activation and a bioreduction step catalysed by ketoreductases.

Biocatalytic One-Carbon Transfer – A Review

This review provides an overview of different C1 building blocks as substrates of enzymes, or part of their cofactors, and the resulting­ functionalized products. There is an emphasis on the broad range of possibilities of biocatalytic one-carbon extensions with C1 sources of different oxidation states. The identification of uncommon biosynthetic strategies, many of which might serve as templates for synthetic or biotechnological applications, towards one-carbon extensions is supported by recent genomic and metabolomic progress and hence we refer principally to literature spanning from 2014 to 2020.

1 Introduction

2 Methane, Methanol, and Methylamine

3 Glycine

4 Nitromethane

5 SAM and SAM Ylide

6 Other C1 Building Blocks

7 Formaldehyde and Glyoxylate as Formaldehyde Equivalents

8 Cyanide

9 Formic Acid

10 Formyl-CoA and Oxalyl-CoA

11 Carbon Monoxide

12 Carbon Dioxide

13 Conclusions

Unlocking New Reactivities in Enzymes by Iminium Catalysis

The application of biocatalysis in conquering challenging synthesis requires the constant input of new enzymes. Developing novel biocatalysts by absorbing catalysis modes from synthetic chemistry has yielded fruitful new-to-nature enzymes. Organocatalysis was originally bio-inspired and has become the third pillar of asymmetric catalysis. Transferring organocatalytic reactions back to enzyme platforms is a promising approach for biocatalyst creation. Herein, we summarize recent developments in the design of novel biocatalysts that adopt iminium catalysis, a fundamental branch in organocatalysis. By repurposing existing enzymes or constructing artificial enzymes, various biocatalysts for iminium catalysis have been created and optimized via protein engineering to promote valuable abiological transformations. Recent advances in iminium biocatalysis illustrate the power of combining chemomimetic biocatalyst design and directed evolution to generate useful new-to-nature enzymes.

Catalysts’ evolution in the asymmetric conjugate addition of nitroalkanes to electron-poor alkenes

This review provides a journey of the catalyst usage for the enantioselective conjugate addition of nitroalkanes to electron-poor olefins from the early attempts to the latest achievements. Selected applications are also reported.

Novel catalytic property of fructose-6-phosphate aldolase in directly conversion of two 1-hydroxyalkanones to diketones

Asymmetric CC bond formation catalyzed by aldolases requires the supplementation of nucleophiles and receptors in the reaction medium. However, aldol condensation using a single ketone as substrate has never been reported yet. In this work, we discovered that d-fructose-6-phosphate aldolase (FSA) could convert two 1-hydroxyalkanones, such as hydroxyacetone (HA) and 1-hydroxy-2-butanone, into two type of diketones. The initial product synthesis rate increased 3-fold and the yield reached to 56 %, when pure oxygen was directly inputted into the reaction medium. The results confirmed that oxygen participated in this reaction and hydrogen peroxide was generated. Metal ions Co²⁺ and Cu²⁺ remarkably increased the conversion yield compared with the control. For this reaction mechanism, we conjectured that HA may be oxidized to methylglyoxal by enzyme FSA in the presence of oxygen in the medium, and then FSA catalyzes the aldol addition between HA and its oxidative product MG to form diketone products. The obtained diketones could serve as important precursors for preparing furans and pyrroles.

Tryptophan Synthase: Biocatalyst Extraordinaire

Tryptophan synthase (TrpS) has emerged as a paragon of noncanonical amino acid (ncAA) synthesis and is an ideal biocatalyst for synthetic and biological applications. TrpS catalyzes an irreversible, C-C bond-forming reaction between indole and serine to make l-tryptophan; native TrpS complexes possess fairly broad specificity for indole analogues, but are difficult to engineer to extend substrate scope or to confer other useful properties due to allosteric constraints and their heterodimeric structure. Directed evolution freed the catalytically relevant TrpS β-subunit (TrpB) from allosteric regulation by its TrpA partner and has enabled dramatic expansion of the enzyme's substrate scope. This review examines the long and storied career of TrpS from the perspective of its application in ncAA synthesis and biocatalytic cascades.

Enhancing promiscuous chemistries of a Schiff-base forming enzyme by divergent evolution

Directed evolution has emerged as a powerful technique for the rapid tailoring of enzymes toward particular synthetic demands, spawning a number of enzymes capable of complex chemical transformations. During random mutagenesis of a protein, changes in fitness must be assayed in order to quantify and understand the relative effect a given mutation has, and the assay employed must be carefully chosen to report directly on the transformation of interest. Here, we describe a series of medium-throughput screening techniques that have been utilized for the evolution and engineering of an artificial carboligase, RA95.5-8, resulting in improvement of catalytic efficiency of a number of promiscuous chemistries. The methods make use of common analytical chemistry equipment and low-cost materials, and may help inspire development of novel screening workflows for related transformations.

Engineered Artificial Carboligases Facilitate Regioselective Preparation of Enantioenriched Aldol Adducts

Controlling regio- and stereoselectivity of aldol additions is generally challenging. Here we show that an artificial aldolase with high specificity for acetone as the aldol donor can be reengineered via single active site mutations to accept linear and cyclic aliphatic ketones with notable efficiency, regioselectivity, and stereocontrol. Biochemical and crystallographic data show how the mutated residues modulate the binding and activation of specific aldol donors, as well as their subsequent reaction with diverse aldehyde acceptors. Broadening the substrate scope of this evolutionarily naïve catalyst proved much easier than previous attempts to redesign natural aldolases, suggesting that such proteins may be excellent starting points for the development of customized biocatalysts for diverse practical applications.

Enabling protein-hosted organocatalytic transformations

In this review, the development of organocatalytic artificial enzymes will be discussed. This area of protein engineering research has underlying importance, as it enhances the biocompatibility of organocatalysis for applications in chemical and synthetic biology research whilst expanding the catalytic repertoire of enzymes. The approaches towards the preparation of organocatalytic artificial enzymes, techniques used to improve their performance (selectivity and reactivity) as well as examples of their applications are presented. Challenges and opportunities are also discussed.

Copper Catalyzed Diastereo- and Enantioselective 1,4-Addition Michael Reaction of 2,3-Dioxopyrrolidines with Nitroalkanes in Aqueous Media

A good diastereo- and enantioselective 1,4-addition Michael reaction catalyzed by a chiral copper complex was developed in aqueous media. A series of nitro-containing pyrrolidones could be gained in high yields with excellent diastereoselectivities and good ee values by virtue of this developed method. It affords a facile access to construct carbon-carbon bonds with water and air tolerance. Furthermore, the gram scale synthesis was conducted successfully to give rise to the corresponding products.

Nitroalkanes as Versatile Nucleophiles for Enzymatic Synthesis of Noncanonical Amino Acids

C-C bond-forming reactions often require nucleophilic carbon species rarely compatible with aqueous reaction media, thus restricting their appearance in biocatalysis. Here we report the use of nitroalkanes as a structurally versatile class of nucleophilic substrates for C-C bond formation catalyzed by variants of the β-subunit of tryptophan synthase (TrpB). The enzymes accept a wide range of nitroalkanes to form noncanonical amino acids, here the nitro group can serve as a handle for further modification. Using nitroalkane nucleophiles greatly expands the scope of compounds made by TrpB variants and establishes nitroalkanes as a valuable substrate class for biocatalytic C-C bond formation.

Biocatalytic Asymmetric Michael Additions of Nitromethane to α,β-Unsaturated Aldehydes via Enzyme-bound Iminium Ion Intermediates

The enzyme 4-oxalocrotonate tautomerase (4-OT) exploits an N-terminal proline as main catalytic residue to facilitate several promiscuous C-C bond-forming reactions via enzyme-bound enamine intermediates. Here we show that the active site of this enzyme can give rise to further synthetically useful catalytic promiscuity. Specifically, the F50A mutant of 4-OT was found to efficiently promote asymmetric Michael additions of nitromethane to various α,β-unsaturated aldehydes to give γ-nitroaldehydes, important precursors to biologically active γ-aminobutyric acids. High conversions, high enantiocontrol, and good isolated product yields were achieved. The reactions likely proceed via iminium ion intermediates formed between the catalytic Pro-1 residue and the α,β-unsaturated aldehydes. In addition, a cascade of three 4-OT(F50A)-catalyzed reactions followed by an enzymatic oxidation step enables assembly of γ-nitrocarboxylic acids from three simple building blocks in one pot. Our results bridge organo- and biocatalysis, and they emphasize the potential of enzyme promiscuity for the preparation of important chiral synthons.

Kemp Eliminases of the AlleyCat Family Possess High Substrate Promiscuity

Minimalist enzymes designed to catalyze model reactions provide useful starting points for creating catalysts for practically important chemical transformations. We have shown that Kemp eliminases of the AlleyCat family facilitate conversion of leflunomide (an immunosupressor pro-drug) to its active form teriflunomide with outstanding rate enhancement (nearly four orders of magnitude) and catalytic proficiency (more than seven orders of magnitude) without any additional optimization. This remarkable activity is achieved by properly positioning the substrate in close proximity to the catalytic glutamate with very high pKa.

Enantioselective Synthesis of Pharmaceutically Active γ-Aminobutyric Acids Using a Tailor-Made Artificial Michaelase in One-Pot Cascade Reactions

Chiral γ-aminobutyric acid (GABA) analogues represent abundantly prescribed drugs, which are broadly applied as anticonvulsants, as antidepressants, and for the treatment of neuropathic pain. Here we report a one-pot two-step biocatalytic cascade route for synthesis of the pharmaceutically relevant enantiomers of γ-nitrobutyric acids, starting from simple precursors (acetaldehyde and nitroalkenes), using a tailor-made highly enantioselective artificial “Michaelase” (4-oxalocrotonate tautomerase mutant L8Y/M45Y/F50A), an aldehyde dehydrogenase with a broad non-natural substrate scope, and a cofactor recycling system. We also report a three-step chemoenzymatic cascade route for the efficient chemical reduction of enzymatically prepared γ-nitrobutyric acids into GABA analogues in one pot, achieving high enantiopurity (e.r. up to 99:1) and high overall yields (up to 70%). This chemoenzymatic methodology offers a step-economic alternative route to important pharmaceutically active GABA analogues, and highlights the exciting opportunities available for combining chemocatalysts, natural enzymes, and designed artificial biocatalysts in multistep syntheses.

Asymmetric Reductive Carbocyclization Using Engineered Ene Reductases

Ene reductases from the Old Yellow Enzyme (OYE) family reduce the C=C double bond in α,β-unsaturated compounds bearing an electron-withdrawing group, for example, a carbonyl group. This asymmetric reduction has been exploited for biocatalysis. Going beyond its canonical function, we show that members of this enzyme family can also catalyze the formation of C-C bonds. α,β-Unsaturated aldehydes and ketones containing an additional electrophilic group undergo reductive cyclization. Mechanistically, the two-electron-reduced enzyme cofactor FMN delivers a hydride to generate an enolate intermediate, which reacts with the internal electrophile. Single-site replacement of a crucial Tyr residue with a non-protic Phe or Trp favored the cyclization over the natural reduction reaction. The new transformation enabled the enantioselective synthesis of chiral cyclopropanes in up to >99 % ee.

Expanding the boundary of biocatalysis: design and optimization of in vitro tandem catalytic reactions for biochemical production

Biocatalysts have been increasingly used in the synthesis of fine chemicals and medicinal compounds due to significant advances in enzyme discovery and engineering. To mimic the synergistic effects of cascade reactions catalyzed by multiple enzymes in nature, researchers have been developing artificial tandem enzymatic reactions in vivo by harnessing synthetic biology and metabolic engineering tools. There is also growing interest in the development of one-pot tandem enzymatic or chemo-enzymatic processes in vitro due to their neat and concise catalytic systems and product purification procedures. In this review, we will briefly summarize the strategies of designing and optimizing in vitro tandem catalytic reactions, highlight a few representative examples, and discuss the future trend in this field.

Speeding up enzyme discovery and engineering with ultrahigh-throughput methods

Exploring the sequence space of enzyme catalysts is ultimately a numbers game. Ultrahigh-throughput screening methods for rapid analysis of millions of variants are therefore increasingly important for investigating sequence-function relationships, searching large metagenomic libraries for interesting activities, and accelerating enzyme evolution in the laboratory. Recent applications of such technologies are reviewed here, with a particular focus on the practical benefits of droplet-based microfluidics for the directed evolution of natural and artificial enzymes. Broader implementation of such rapid, cost-effective screening technologies is likely to redefine the way enzymes are studied and engineered for academic and industrial purposes.

Role of Conformational Dynamics in the Evolution of Retro-Aldolase Activity

Enzymes exist as ensembles of conformations that are important for function. Tuning these populations of conformational states through mutation enables evolution toward additional activities. Here we computationally evaluate the population shifts induced by distal and active site mutations in a family of computationally designed and experimentally optimized retro-aldolases. The conformational landscape of these enzymes was significantly altered during evolution, as pre-existing catalytically active conformational substates became major states in the most evolved variants. We further demonstrate that key residues responsible for these substate conversions can be predicted computationally. Significantly, the identified residues coincide with those positions mutated in the laboratory evolution experiments. This study establishes that distal mutations that affect enzyme catalytic activity can be predicted computationally and thus provides the enzyme (re)design field with a rational strategy to determine promising sites for enhancing activity through mutation.

Biocatalytic Friedel-Crafts Acylation and Fries Reaction

The Friedel-Crafts acylation is commonly used for the synthesis of aryl ketones, and a biocatalytic version, which may benefit from the chemo- and regioselectivity of enzymes, has not yet been introduced. Described here is a bacterial acyltransferase which can catalyze Friedel-Crafts C-acylation of phenolic substrates in buffer without the need of CoA-activated reagents. Conversions reach up to >99 %, and various C- or O-acyl donors, such as DAPG or isopropenyl acetate, are accepted by this enzyme. Furthermore the enzyme enables a Fries rearrangement-like reaction of resorcinol derivatives. These findings open an avenue for the development of alternative and selective C-C bond formation methods.

Biocatalysis in the Pharmaceutical Industry: The Need for Speed

The use of biocatalysis in the pharmaceutical industry continues to expand as a result of increased access to enzymes and the ability to engineer those enzymes to meet the demands of industrial processes. However, we are still just scratching the surface of potential biocatalytic applications. The time pressures present in pharmaceutical process development are incompatible with the long lead times required for engineering a suitable biocatalyst. Dramatic increases in the speed of protein engineering are needed to deliver on the ever increasing opportunities for industrial biocatalytic processes.

Chemoselective Henry Condensations Catalyzed by Artificial Carboligases

The promiscuity of de novo designed enzymes provides a privileged platform for diverse abiological reactions. In this work, we report the first example of a nitroolefin synthase that catalyzes the Henry condensation between aromatic aldehydes and nitromethane. Significant catalytic activity was discovered in the computationally designed and evolved carboligase RA95.5-8, and mutations around the active site were shown to improve the reaction rate, demonstrating the potential to optimize the enzyme by directed evolution. This novel nitroolefin synthase could participate in complex biological cascades, whereby the highly tunable chemoselectivity could afford useful synthetic building blocks.