Actinobacteria as Microbial Cell Factories and Biocatalysts in The Synthesis of Chiral Intermediates and Bioactive Molecules; Insights and Applications

Actinobacteria are one of the most intriguing bacterial phyla in terms of chemical diversity and bioactivities of their reported biomolecules and natural products, including various types of chiral molecules. Actinobacterial genera such as Detzia, Mycobacterium, and Streptomyces are among the microbial sources targeted for selective reactions such as asymmetric biocatalysis catalyzed by whole cells or enzymes induced in their cell niche. Remarkably, stereoselective reactions catalyzed by actinobacterial whole cells or their enzymes include stereoselective oxidation, stereoselective reduction, kinetic resolution, asymmetric hydrolysis, and selective transamination, among others. Species of actinobacteria function with high chemo-, regio-, and enantio-selectivity under benign conditions, which could help current industrial processing. Numerous selective enzymes were either isolated from actinobacteria or expressed from actinobacteria in other microbes and hence exploited in the production of pure organic compounds difficult to obtain chemically. In addition, different species of actinobacteria, especially Streptomyces species, function as natural producers of chiral molecules of therapeutic importance. Herein, we discuss some of the most outstanding contributions of actinobacteria to asymmetric biocatalysis, which are important in the organic and/or pharmaceutical industries. In addition, we highlight the role of actinobacteria as microbial cell factories for chiral natural products with insights into their various biological potentialities.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

Chiral Alcohols from Alkenes and Water: Directed Evolution of a Styrene Hydratase

Enantioselective synthesis of chiral alcohols through asymmetric addition of water across an unactivated alkene is a highly sought-after transformation and a big challenge in catalysis. Herein we report the identification and directed evolution of a fatty acid hydratase from Marinitoga hydrogenitolerans for the highly enantioselective hydration of styrenes to yield chiral 1-arylethanols. While directed evolution for styrene hydration was performed in the presence of heptanoic acid to mimic fatty acid binding, the engineered enzyme displayed remarkable asymmetric styrene hydration activity in the absence of the small molecule activator. The evolved styrene hydratase provided access to chiral alcohols from simple alkenes and water with high enantioselectivity (>99 : 1 e.r.) and could be applied on a preparative scale.

Revisiting the Burden Borne by Fumarase: Enzymatic Hydration of an Olefin

Fumarate hydratase (FH) is a remarkable catalyst that decreases the free energy of the catalyzed reaction by 30 kcal mol^-1, much larger than most exceptional enzymes with extraordinary catalytic rates. Two classes of FH are observed in nature: class-I and class-II, which have different folds, yet catalyze the same reversible hydration/dehydration reaction of the dicarboxylic acids fumarate/malate, with equal efficiencies. Using class-I FH from the hyperthermophilic archaeon Methanocaldococcus jannaschii (Mj) as a model along with comparative analysis with the only other available class-I FH structure from Leishmania major (Lm), we provide insights into the molecular mechanism of catalysis in this class of enzymes. The structure of MjFH apo-protein has been determined, revealing that large intersubunit rearrangements occur across apo- and holo-protein forms, with a largely preorganized active site for substrate binding. Site-directed mutagenesis of active site residues, kinetic analysis, and computational studies, including density functional theory (DFT) and natural population analysis, together show that residues interacting with the carboxylate group of the substrate play a pivotal role in catalysis. Our study establishes that an electrostatic network at the active site of class-I FH polarizes the substrate fumarate through interactions with its carboxylate groups, thereby permitting an easier addition of a water molecule across the olefinic bond. We propose a mechanism of catalysis in FH that occurs through transition-state stabilization involving the distortion of the electronic structure of the substrate olefinic bond mediated by the charge polarization of the bound substrate at the enzyme active site.

Biocatalytic fumarate synthesis from pyruvate and CO2 as a feedstock

The biocatalytic synthesis of fumarate from CO2 and pyruvate vial-malate as an intermediate in an aqueous medium using a biocatalytic system consisting of malate dehydrogenase and fumarase in the presence of NADH is developed.

Water as a monomer: synthesis of an aliphatic polyethersulfone from divinyl sulfone and water

Using water as a monomer in polymerization reactions presents a unique and exquisite strategy towards more sustainable chemistry. Herein, the feasibility thereof is demonstrated by the introduction of the oxa-Michael polyaddition of water and divinyl sulfone. Upon nucleophilic or base catalysis, the corresponding aliphatic polyethersulfone is obtained in an interfacial polymerization at room temperature in high yield (>97%) within an hour. The polyethersulfone is characterized by relatively high molar mass averages and a dispersity around 2.5. The polymer was tested as a solid polymer electrolyte with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the salt. Free-standing amorphous membranes were prepared by a melt process in a solvent-free manner. The polymer electrolyte containing 15 wt% LiTFSI featured an oxidative stability of up to 5.5 V vs. Li/Li⁺ at 45 °C and a conductivity of 1.45 × 10⁻⁸ S cm⁻¹ at room temperature.

This study describes the first example of the polymerization of water as one of two monomers. The obtained polymer allows for a solvent-free preparation of polymer electrolyte membranes exhibiting a high oxidative stability.

Semi-rational Engineering of a Promiscuous Fatty Acid Hydratase for Alteration of Regioselectivity

Fatty acid hydratases (FAHs) catalyze regio- and stereo-selective hydration of unsaturated fatty acids to produce hydroxy fatty acids. Fatty acid hydratase-1 (FA-HY1) from Lactobacillus Acidophilus is the most promiscuous and regiodiverse FAH identified so far. Here, we engineered binding site residues of FA-HY1 (S393, S395, S218 and P380) by semi-rational protein engineering to alter regioselectivity. Although it was not possible to obtain a completely new type of regioselectivity with our mutant libraries, a significant shift of regioselectivity was observed towards cis-5, cis-8, cis-11, cis-14, cis-17-eicosapentaenoic acid (EPA). We identified mutants (S393/S395 mutants) with excellent regioselectivity, generating a single hydroxy fatty acid product from EPA (15-OH product), which is advantageous from application perspective. This result is impressive given that wild-type FA-HY1 produces a mixture of 12-OH and 15-OH products at 63 : 37 ratio (12-OH : 15-OH). Moreover, our results indicate that native FA-HY1 is at its limit in terms of promiscuity and regiospecificity, thus it may not be possible to diversify its product portfolio with active site engineering. This behavior of FA-HY1 is unlike its orthologue, fatty acid hydratase-2 (FA-HY2; 58 % sequence identity to FA-HY1), which has been shown earlier to exhibit significant promiscuity and regioselectivity changes by a few active site mutations. Our reverse engineering from FA-HY1 to FA-HY2 further demonstrates this conclusion.

Anti-Markovnikov hydro(amino)alkylation of vinylarenes via photoredox catalysis

Photoredox catalysis is a powerful means to generate odd-electron species under mild reaction conditions from a wide array of radical precursors. Herein, we present the application of this powerful catalytic manifold to address the hydroalkylation and hydroaminoalkylation of electronically diverse vinylarenes. This reaction allows for generalized alkene hydroalkylation leveraging common alkyl radical precursors, such as organotrifluoroborate salts and carboxylic acids. Furthermore, utilizing easily accessible α-silyl amine reagents or tertiary amines directly, secondary and tertiary amine moieties can be installed onto monoaryl and diaryl alkenes to access valuable products, including γ,γ-diarylamines pharmacophores. Thus, under a unified system, both hydroalkylation and hydroaminoalkylation of alkenes are achieved. The substrate scope is evaluated through 57 examples, the synthetic utility of the method is demonstrated, and preliminary mechanistic insights are presented.

Many useful chemical scaffolds include carbon or nitrogen substitutions at two or three atoms away from benzene. Here, the authors show a unified hydroalkylation and hydroaminoalkylation protocol to access these structures via a regioselective photocatalytic addition to simple styrenes.

Copper-Promoted Conjugate Addition of Carboxylic Acids to Ethenesulfonyl Fluoride (ESF) for Constructing Aliphatic Sulfonyl Fluorides

A CuO-promoted direct hydrocarboxylation of ethenesulfonyl fluoride (ESF) was developed using carboxylic acid as a nucleophile under mild conditions. A variety of molecules containing both ester group and aliphatic sulfonyl fluoride moiety exhibit great potential in medicinal chemistry and chemical biology. Furthermore, the modification of the known drugs Ibuprofen and Aspirin was also demonstrated.

Novel oleate hydratases and potential biotechnological applications

Abstract

Oleate hydratase catalyses the addition of water to the CC double bond of oleic acid to produce (R)-10-hydroxystearic acid. The enzyme requires an FAD cofactor that functions to optimise the active site structure. A wide range of unsaturated fatty acids can be hydrated at the C10 and in some cases the C13 position. The substrate scope can be expanded using ‘decoy’ small carboxylic acids to convert small chain alkenes to secondary alcohols, albeit at low conversion rates. Systematic protein engineering and directed evolution to widen the substrate scope and increase the conversion rate is possible, supported by new high throughput screening assays that have been developed. Multi-enzyme cascades allow the formation of a wide range of products including keto-fatty acids, secondary alcohols, secondary amines and α,ω-dicarboxylic acids.

Key points

• Phylogenetically distinct oleate hydratases may exhibit mechanistic differences.

• Protein engineering to improve productivity and substrate scope is possible.

• Multi-enzymatic cascades greatly widen the product portfolio.

A Bi-Enzymatic Cascade Pathway towards Optically Pure FAHFAs*

Recently discovered endogenous mammalian lipids, fatty acid esters of hydroxy fatty acids (FAHFAs), have been proved to have anti-inflammatory and anti-diabetic effects. Due to their extremely low abundancies in vivo, forging a feasible scenario for FAHFA synthesis is critical for their use in uncovering biological mechanisms or in clinical trials. Here, we showcase a fully enzymatic approach, a novel in vitro bi-enzymatic cascade system, enabling an effective conversion of nature-abundant fatty acids into FAHFAs. Two hydratases from Lactobacillus acidophilus were used for converting unsaturated fatty acids to various enantiomeric hydroxy fatty acids, followed by esterification with another fatty acid catalyzed by Candida antarctica lipase A (CALA). Various FAHFAs were synthesized in a semi-preparative scale using this bi-enzymatic approach in a one-pot two-step operation mode. In all, we demonstrate that the hydratase-CALA system offers a promising route for the synthesis of optically pure structure-diverse FAHFAs.

Protein Fe-S Centers as a Molecular Target of Toxicity of a Complex Transition Metal Oxide Nanomaterial with Downstream Impacts on Metabolism and Growth

Oxidative stress is frequently identified as a mechanism of toxicity of nanomaterials. However, rarely have the specific underlying molecular targets responsible for these impacts been identified. We previously demonstrated significant negative impacts of transition metal oxide (TMO) lithium-ion battery cathode nanomaterial, lithium cobalt oxide (LCO), on the growth, development, hemoglobin, and heme synthesis gene expression in the larvae of a model sediment invertebrate Chironomus riparius. Here, we propose that alteration of the Fe-S protein function by LCO is a molecular initiating event leading to these changes. A 10 mg/L LCO exposure causes significant oxidation of the aconitase 4Fe-4S center after 7 d as determined from the electron paramagnetic resonance spectroscopy measurements of intact larvae and a significant reduction in the aconitase activity of larval protein after 48 h (p < 0.05). Next-generation RNA sequencing identified significant changes in the expression of genes involved in 4Fe-4S center binding, Fe-S center synthesis, iron ion binding, and metabolism for 10 mg/L LCO at 48 h (FDR-adjusted, p < 0.1). We propose an adverse outcome pathway, where the oxidation of metabolic and regulatory Fe-S centers of proteins by LCO disrupts metabolic homeostasis, which negatively impacts the growth and development, a mechanism that may apply for these conserved proteins across species and for other TMO nanomaterials.

UV Accelerated Assemblies Constructed Using Calixpyridinium in Aqueous Solution

In this work, an irregular calixpyridinium-suramin sodium supramolecular assembly was constructed by the strong host-guest electrostatic interactions. More interestingly, a novel regular spherical supramolecular assembly was also fabricated by the hydrogen bonding interactions between suramin sodium and the UV accelerated addition product of deprotonated calixpyridinium in water. The same principle was also applied to construct a UV accelerated regular spherical self-assembly by the addition product of deprotonated calixpyridinium in water. Compared with the complicated and irreversible covalent connection of the light-responsive groups to the building block, which is one of the common means of obtaining light-responsive supramolecular systems, this finding not only provides a smart, facile, and universally applicable method to construct deprotonated calixpyridinium-based light-responsive host-guest systems but also provides a new idea for the development of other novel light-responsive building blocks.

Photobiocatalytic synthesis of chiral secondary fatty alcohols from renewable unsaturated fatty acids

En route to a bio-based chemical industry, the conversion of fatty acids into building blocks is of particular interest. Enzymatic routes, occurring under mild conditions and excelling by intrinsic selectivity, are particularly attractive. Here we report photoenzymatic cascade reactions to transform unsaturated fatty acids into enantiomerically pure secondary fatty alcohols. In a first step the C=C-double bond is stereoselectively hydrated using oleate hydratases from Lactobacillus reuteri or Stenotrophomonas maltophilia. Also, dihydroxylation mediated by the 5,8-diol synthase from Aspergillus nidulans is demonstrated. The second step comprises decarboxylation of the intermediate hydroxy acids by the photoactivated decarboxylase from Chlorella variabilis NC64A. A broad range of (poly)unsaturated fatty acids can be transformed into enantiomerically pure fatty alcohols in a simple one-pot approach.

Natural fatty acids are important starting materials in bio-based chemical production. Here, the authors developed a two-enzyme cascade to produce enantiomerically pure secondary fatty alcohols from natural unsaturated fatty acids in one pot.

Exploring the abundance of oleate hydratases in the genus Rhodococcus-discovery of novel enzymes with complementary substrate scope

Oleate hydratases (Ohys, EC 4.2.1.53) are a class of enzymes capable of selective water addition reactions to a broad range of unsaturated fatty acids leading to the respective chiral alcohols. Much research was dedicated to improving the applications of existing Ohys as well as to the identification of undescribed Ohys with potentially novel properties. This study focuses on the latter by exploring the genus Rhodococcus for its plenitude of oleate hydratases. Three different Rhodococcus clades showed the presence of oleate hydratases whereby each clade was represented by a specific oleate hydratase family (HFam). Phylogenetic and sequence analyses revealed HFam-specific patterns amongst conserved amino acids. Oleate hydratases from two Rhodococcus strains (HFam 2 and 3) were heterologously expressed in Escherichia coli and their substrate scope investigated. Here, both enzymes showed a complementary behaviour towards sterically demanding and multiple unsaturated fatty acids. Furthermore, this study includes the characterisation of the newly discovered Rhodococcus pyridinivorans Ohy. The steady-state kinetics of R. pyridinivorans Ohy was measured using a novel coupled assay based on the alcohol dehydrogenase and NAD⁺-dependent oxidation of 10-hydroxystearic acid.

Sated by a Zero-Calorie Sweetener: Wastewater Bacteria Can Feed on Acesulfame

The widely used artificial sweetener acesulfame K has long been considered recalcitrant in biological wastewater treatment. Due to its persistence and mobility in the aquatic environment, acesulfame has been used as marker substance for wastewater input in surface water and groundwater. However, recent studies indicated that the potential to remove this xenobiotic compound is emerging in wastewater treatment plants worldwide, leading to decreasing mass loads in receiving waters despite unchanged human consumption patterns. Here we show evidence that acesulfame can be mineralized in a catabolic process and used as sole carbon source by bacterial pure strains isolated from activated sludge and identified as Bosea sp. and Chelatococcus sp. The strains mineralize 1 g/L acesulfame K within 8–9 days. We discuss the potential degradation pathway and how this novel catabolic trait confirms the “principle of microbial infallibility.” Once the enzymes involved in acesulfame degradation and their genes are identified, it will be possible to survey diverse environments and trace back the evolutionary origin as well as the mechanisms of global distribution and establishment of such a new catabolic trait.

Evolving the Promiscuity of Elizabethkingia meningoseptica Oleate Hydratase for the Regio- and Stereoselective Hydration of Oleic Acid Derivatives

The addition of water to non-activated carbon-carbon double bonds catalyzed by fatty acid hydratases (FAHYs) allows for highly regio- and stereoselective oxyfunctionalization of renewable oil feedstock. So far, the applicability of FAHYs has been limited to free fatty acids, mainly owing to the requirement of a carboxylate function for substrate recognition and binding. Herein, we describe for the first time the hydration of oleic acid (OA) derivatives lacking this free carboxylate by the oleate hydratase from Elizabethkingia meningoseptica (OhyA). Molecular docking of OA to the OhyA 3D-structure and a sequence alignment uncovered conserved amino acid residues at the entrance of the substrate channel as target positions for enzyme engineering. Exchange of selected amino acids gave rise to OhyA variants which showed up to an 18-fold improved conversion of OA derivatives, while retaining the excellent regio- and stereoselectivity in the olefin hydration reaction.

Modular DNA-based hybrid catalysts as a toolbox for enantioselective hydration of α,β-unsaturated ketones

The direct addition of water to a carbon-carbon double bond remains a challenge, but such a reaction is essential for the development of efficient catalysts that enable direct access to chiral alcohols. We now report on the enantioselective hydration of α,β-unsaturated ketones, catalyzed by modular DNA-based hybrid catalysts, affording β-hydroxy ketones with up to 87% enantiomeric excess. Oligonucleotides containing an intrastrand bipyridine ligand were readily synthesized by a straightforward process using an automated solid-phase synthesis. A library of DNA-based hybrid catalysts could be systematically generated based on the composition of nucleobases, and the incorporation of a binding ligand and a nonbinding steric moiety. This study demonstrates the potential of modular DNA-based hybrid catalysts as a toolbox to accomplish the challenging enantioselective hydration reaction.

Oxidative functionalization of a halimane diterpenoid achieved by fungal transformation

Regio and stereoselective activation of sp³ CH bonds remain one of the major advantages of biocatalysis over traditional chemocatalytic methods. Herein, we describe the oxy-functionalization of halimane diterpenoid 1 by whole cells of three filamentous fungi, aiming to obtain derivatives with desirable biological properties. After incubating 1 with Fusarium oxysporum, Myrothecium verrucaria, and Rhinocladiella similis at different concentrations and incubation times, four known (3, 5, 6, and 7) and three new (2, 4, and 8) halimane derivatives were obtained and characterized. F. oxysporum catalyzed the hydroxylation of positions C-2 (2) and C-7 (4), while R. similis simultaneously mediated the 2-oxo-functionalization and the hydration of 13,14-(CC)double bond belonging to an α,β-unsaturated carbonyl system (8). Compounds 1-7 were non-cytotoxic against HCT-116 and MCF-7 cancer cell lines at tested concentrations. However, substrate 1 displayed moderate reduction ability against biofilm produced by Staphylococcus epidermidis ATCC35984 (84% at 1.6 mM), and this effect was retained to some extent by derivatives 4 and 7. These results emphasize the prominent potential of filamentous fungi associated with the microbiota of medicinal plants as versatile catalysts for singularly useful reactions through their complex enzymatic machinery, as well as the high susceptibility of halimane-diterpenoid substrates.

Biocatalytic selective functionalisation of alkenes via single-step and one-pot multi-step reactions

Alkenes are excellent starting materials for organic synthesis due to the versatile reactivity of C[double bond, length as m-dash]C bonds and the easy availability of many unfunctionalised alkenes. Direct regio- and/or enantioselective conversion of alkenes into functionalised (chiral) compounds has enormous potential for industrial applications, and thus has attracted the attention of researchers for extensive development using chemo-catalysis over the past few years. On the other hand, many enzymes have also been employed for conversion of alkenes in a highly selective and much greener manner to offer valuable products. Herein, we review recent advances in seven well-known types of biocatalytic conversion of alkenes. Remarkably, recent mechanism-guided directed evolution and enzyme cascades have enabled the development of seven novel types of single-step and one-pot multi-step functionalisation of alkenes, some of which are even unattainable via chemo-catalysis. These new reactions are particularly highlighted in this feature article. Overall, we present an ever-expanding enzyme toolbox for various alkene functionalisations inspiring further research in this fast-developing theme.

Enzymatic synthesis of enantiopure alcohols: current state and perspectives

Enantiomerically pure alcohols, as key intermediates, play an essential role in the pharmaceutical, agrochemical and chemical industries. Among the methods used for their production, biotechnological approaches are generally considered a green and effective alternative due to their mild reaction conditions and remarkable enantioselectivity. An increasing number of enzymatic strategies for the synthesis of these compounds has been developed over the years, among which seven primary methodologies can be distinguished as follows: (1) enantioselective water addition to alkenes, (2) enantioselective aldol addition, (3) enantioselective coupling of ketones with hydrogen cyanide, (4) asymmetric reduction of carbonyl compounds, (5) (dynamic) kinetic resolution of racemates, (6) enantioselective hydrolysis of epoxides, and (7) stereoselective hydroxylation of unactivated C-H bonds. Some recent reviews have examined these approaches separately; however, to date, no review has included all the above mentioned strategies. The aim of this mini-review is to provide an overview of all seven enzymatic strategies and draw conclusions on the effect of each approach.

Asymmetric Enzymatic Hydration of Unactivated, Aliphatic Alkenes

The direct enantioselective addition of water to unactivated alkenes could simplify the synthesis of chiral alcohols and solve a long‐standing challenge in catalysis. Here we report that an engineered fatty acid hydratase can catalyze the asymmetric hydration of various terminal and internal alkenes. In the presence of a carboxylic acid decoy molecule for activation of the oleate hydratase from E. meningoseptica, asymmetric hydration of unactivated alkenes was achieved with up to 93 % conversion, excellent selectivity (>99 % ee, >95 % regioselectivity), and on a preparative scale.

Oxa-Michael Addition to α,β-Unsaturated Nitriles: An Expedient Route to γ-Amino Alcohols and Derivatives

Water addition to α,β-unsaturated nitriles would give facile access to the β-hydroxy-nitriles, which in turn can be hydrogenated to the γ-amino alcohols. We have previously shown that alcohols readily add in 1,4-fashion to these substrates using Milstein's Ru(PNN) pincer complex as catalyst. However, attempted water addition to α,β-unsaturated nitriles gave the 3-hydroxynitriles in mediocre yields. On the other hand, addition of benzyl alcohol proceeded in excellent yields for a variety of β-substituted unsaturated nitriles. Subsequent treatment of the benzyl alcohol addition products with TMSCl/FeCl3 resulted in the formation of 3-hydroxy-alkylnitriles. The 3-benzyloxy-alkylnitriles obtained from oxa-Michael addition also could be hydrogenated directly in the presence of acid to give the amino alcohols as their HCl salts in excellent yields. Hydrogenation under neutral conditions gave a mixture of the secondary and tertiary amines. Hydrogenation in the presence of base and Boc-anhydride gave the orthogonally bis-protected amino alcohols, in which the benzyl ether can subsequently be cleaved to yield Boc-protected amino alcohols. Thus, a variety of molecular scaffolds with a 1,3-relationship between O- and N-functional group is accessible starting from oxa-Michael addition of benzyl alcohol to α,β-unsaturated nitriles.

On the current role of hydratases in biocatalysis

Water addition to carbon-carbon double bonds provides access to value-added products from inexpensive organic feedstock. This interesting but relatively little-studied reaction is catalysed by hydratases in a highly regio- and enantiospecific fashion with excellent atom economy. Considering that asymmetric hydration of (non-activated) carbon-carbon double bonds is virtually impossible with current organic chemistry, enzymatic hydration reactions are highly attractive for industrial applications. Hydratases have been known for several decades but their biocatalytic potential has only been explored over the past 15 years. As a result, a considerable amount of information on this enzyme group has become available, enabling their development for practical applications. This review focuses on hydratases catalysing water addition to non-activated carbon-carbon double bonds, and examines hydratases from a biochemical, structural and mechanistic angle. Current challenges and opportunities in hydration biocatalysis are discussed, and, ultimately, their potential for organic synthesis is highlighted.

Rhodococcus strains as source for ene-reductase activity

Rhodococcus strains are ubiquitous in nature and known to metabolise a wide variety of compounds. At the same time, asymmetric reduction of C=C bonds is important in the production of high-valued chiral building blocks. In order to evaluate if Rhodococci can be used for this task, we have probed several Rhodococcus rhodochrous and R. erythropolis strains for ene-reductase activity. A series of substrates including activated ketones, an aldehyde, an imide and nitro-compound were screened using whole cells of seven Rhodococcus strains. This revealed that whole cells of all Rhodococcus strains showed apparent (S)-selectivity towards ketoisophorone, while most other organisms show (R)-selectivity for this compound. Three putative ene-reductases from R. rhodochrous ATCC 17895 were heterologously expressed in Escherichia coli. One protein was purified and its biocatalytic and biochemical properties were characterised, showing typical (enantioselective) properties for class 3 ene-reductases of the old yellow enzyme family.

Biochemical characterization and essentiality of fumarate hydratase

Plasmodium falciparum (Pf), the causative agent of malaria, has an iron-sulfur cluster-containing class I fumarate hydratase (FH) that catalyzes the interconversion of fumarate to malate, a well-known reaction in the tricarboxylic acid cycle. In humans, the same reaction is catalyzed by class II FH that has no sequence or structural homology with the class I enzyme from Plasmodium Fumarate is generated in large quantities in the parasite as a by-product of AMP synthesis and is converted to malate by FH and then used in the generation of the key metabolites oxaloacetate, aspartate, and pyruvate. Previous studies have identified the FH reaction as being essential to P. falciparum, but biochemical characterization of PfFH that may provide leads for the development of specific inhibitors is lacking. Here, we report on the kinetic characterization of purified recombinant PfFH, functional complementation of fh deficiency in Escherichia coli, and mitochondrial localization in the parasite. We found that the substrate analog mercaptosuccinic acid is a potent PfFH inhibitor, with a K_i value in the nanomolar range. The fh gene could not be knocked out in Plasmodium berghei when transfectants were introduced into BALB/c mice; however, fh knockout was successful when C57BL/6 mice were used as host, suggesting that the essentiality of the fh gene to the parasite was mouse strain-dependent.

Recombinant expression, purification and biochemical characterization of kievitone hydratase from Nectria haematococca

Kievitone hydratase catalyzes the addition of water to the double bond of the prenyl moiety of plant isoflavonoid kievitone and, thereby, forms the tertiary alcohol hydroxy-kievitone. In nature, this conversion is associated with a defense mechanism of fungal pathogens against phytoalexins generated by host plants after infection. As of today, a gene sequence coding for kievitone hydratase activity has only been identified and characterized in Fusarium solani f. sp. phaseoli. Here, we report on the identification of a putative kievitone hydratase sequence in Nectria haematococca (NhKHS), the teleomorph state of F. solani, based on in silico sequence analyses. After heterologous expression of the enzyme in the methylotrophic yeast Pichia pastoris, we have confirmed its kievitone hydration activity and have assessed its biochemical properties and substrate specificity. Purified recombinant NhKHS is obviously a homodimeric glycoprotein. Due to its good activity for the readily available chalcone derivative xanthohumol (XN), this compound was selected as a model substrate for biochemical studies. The optimal pH and temperature for hydratase activity were 6.0 and 35°C, respectively, and apparent V_max and K_m values for hydration of XN were 7.16 μmol min^-1 mg^-1 and 0.98 ± 0.13 mM, respectively. Due to its catalytic properties and apparent substrate promiscuity, NhKHS is a promising enzyme for the biocatalytic production of tertiary alcohols.

Design of an enantioselective artificial metallo-hydratase enzyme containing an unnatural metal-binding amino acid

The design of artificial metalloenzymes is a challenging, yet ultimately highly rewarding objective because of the potential for accessing new-to-nature reactions. One of the main challenges is identifying catalytically active substrate-metal cofactor-host geometries. The advent of expanded genetic code methods for the in vivo incorporation of non-canonical metal-binding amino acids into proteins allow to address an important aspect of this challenge: the creation of a stable, well-defined metal-binding site. Here, we report a designed artificial metallohydratase, based on the transcriptional repressor lactococcal multidrug resistance regulator (LmrR), in which the non-canonical amino acid (2,2'-bipyridin-5yl)alanine is used to bind the catalytic Cu(ii) ion. Starting from a set of empirical pre-conditions, a combination of cluster model calculations (QM), protein-ligand docking and molecular dynamics simulations was used to propose metallohydratase variants, that were experimentally verified. The agreement observed between the computationally predicted and experimentally observed catalysis results demonstrates the power of the artificial metalloenzyme design approach presented here.

The Crystal Structure of a Bacterial l-Arabinonate Dehydratase Contains a [2Fe-2S] Cluster

We present a novel crystal structure of the IlvD/EDD family enzyme, l-arabinonate dehydratase from Rhizobium leguminosarum bv. trifolii (RlArDHT, EC 4.2.1.25), which catalyzes the conversion of l-arabinonate to 2-dehydro-3-deoxy-l-arabinonate. The enzyme is a tetramer consisting of a dimer of dimers, where each monomer is composed of two domains. The active site contains a catalytically important [2Fe-2S] cluster and Mg²⁺ ion and is buried between two domains, and also at the dimer interface. The active site Lys129 was found to be carbamylated. Ser480 and Thr482 were shown to be essential residues for catalysis, and the S480A mutant structure showed an unexpected open conformation in which the active site was more accessible for the substrate. This structure showed the partial binding of l-arabinonate, which allowed us to suggest that the alkoxide ion form of the Ser480 side chain functions as a base and the [2Fe-2S] cluster functions as a Lewis acid in the elimination reaction.

Catalytic glycerol hydrogenolysis to 1,3-propanediol in a gas–solid fluidized bed

We report the gas-phase hydrogenolysis of glycerol to 1,3-propanediol over Pt/WO₃/Al₂O₃in a fluidized bed operating above 240 °C and at ambient pressure.

X-ray structure of linalool dehydratase/isomerase from Castellaniella defragrans reveals enzymatic alkene synthesis

Linalool dehydratase/isomerase (Ldi), an enzyme of terpene degradation in Castellaniella defragrans, isomerizes the primary monoterpene alcohol geraniol into the tertiary alcohol (S)-linalool and dehydrates (S)-linalool to the alkene β-myrcene. Here we report on the crystal structures of Ldi with and without terpene substrates, revealing a cofactor-free homopentameric enzyme. The substrates were embedded inside a hydrophobic channel between two monomers of the (α,α)6 barrel fold class and flanked by three clusters of polar residues involved in acid-base catalysis. The detailed view into the active site will guide future biotechnological applications of Ldi, in particular, for industrial butadiene and isoprene production from renewable sources.

Stereochemistry of enzymatic water addition to C=C bonds

Water addition to carbon-carbon double bonds using hydratases is attracting great interest in biochemistry. Most of the known hydratases are involved in primary metabolism and to a lesser extent in secondary metabolism. New hydratases have recently been added to the toolbox, both from natural sources or artificial metalloenzymes. In order to comprehensively understand how the hydratases are able to catalyse the water addition to carbon-carbon double bonds, this review will highlight the mechanistic and stereochemical studies of the enzymatic water addition to carbon-carbon double bonds, focusing on the syn/anti-addition and stereochemistry of the reaction.

Task-specific ionic liquid and CO2-cocatalysed efficient hydration of propargylic alcohols to α-hydroxy ketones

The hydration of propargylic alcohols is a green route to synthesize α-hydroxy ketones. Herein a CO₂-reactive ionic liquid (IL), [Bu₄P][Im], was found to display high performance for catalyzing the hydration of propargylic alcohols in the presence of atmospheric CO₂, and a series of propargylic alcohols could be converted into the corresponding α-hydroxy ketones in good to excellent yields. In the IL/CO₂ reaction system, CO₂ served as a cocatalyst by forming α-alkylidene cyclic carbonates with propargylic alcohols, and was released via the rapid hydrolysis of the carbonates catalysed by the IL. This is the first example of the efficient hydration of propargylic alcohols under metal-free conditions.