Ene-Reductase: A Multifaceted Biocatalyst in Organic Synthesis

Tailored Metal-Based Catalysts: A New Platform for Targeted Anticancer Therapies

Innovative strategies for targeted anticancer therapies have gained significant momentum, with metal complexes emerging as tunable catalysts for more effective and safer treatments. Rational design and engineering of metal complexes enable the development of tailored molecular structures optimized for precision oncology. The strategic incorporation of metal complex catalysts within combinatorial therapies amplifies their anticancer properties. This perspective highlights the advancements in synthetic strategies and rational design since 2019, showing how tailored metal catalysts are optimized by designing structures to release or in situ synthesize active drugs, leveraging the target-specific characteristics to develop more precise cancer therapies. This review explores metal-based catalysts, including those conjugated with biomolecules, nanostructures, and metal-organic frameworks (MOFs), highlighting their catalytic activity in biological environments and their in vitro/in vivo performance. To sum up, the potential of metal complexes as catalysts to reshape the landscape of anticancer therapies and foster novel avenues for therapeutic advancement is emphasized.

BioLindlar Catalyst: Ene-Reductase-Promoted Selective Bioreduction of Cyanoalkynes to Give (Z)-Cyanoalkenes

The direct synthesis of alkenes from alkynes usually requires the use of transition-metal catalysts. Unfortunately, efficient biocatalytic alternatives for this transformation have yet to be discovered. Herein, the selective bioreduction of electron-deficient alkynes to alkenes catalysed by ene-reductases (EREDs) is described. Alkynes bearing ketone, aldehyde, ester, and nitrile moieties have been effectively reduced with excellent conversions and stereoselectivities, observing clear trends for the E/Z ratios depending on the nature of the electron-withdrawing group. In the case of cyanoalkynes, (Z)-alkenes were obtained as the major product, and the reaction scope was expanded to a wide variety of aromatic substrates (up to >99 % conversion, and Z/E stereoselectivities of up to >99/1). Other alkynes containing aldehyde, ketone, or ester functionalities also proved to be excellent substrates, and interestingly gave the corresponding (E)-alkenes. Preparative biotransformations were performed on a 0.4 mmol scale, producing the desired (Z)-cyanoalkenes with good to excellent isolated yields (63-97 %). This novel reactivity has been rationalised through molecular docking by predicting the binding poses of key molecules in the ERED-pu-0006 active site.

Repurposing Visible-Light-Excited Ene-Reductases for Diastereo- and Enantioselective Lactones Synthesis

Repurposing enzymes to catalyze non-natural asymmetric transformations that are difficult to achieve using traditional chemical methods is of significant importance. Although radical C-O bond formation has emerged as a powerful approach for constructing oxygen-containing compounds, controlling the stereochemistry poses a great challenge. Here we present the development of a dual bio-/photo-catalytic system comprising an ene-reductase and an organic dye for achieving stereoselective lactonizations. By integrating directed evolution and photoinduced single electron oxidation, we repurposed engineered ene-reductases to steer non-natural radical C-O formations (one C-O bond for hydrolactonizations and lactonization-alkylations while two C-O bonds for lactonization-oxygenations). This dual catalysis gave a new approach to a diverse array of enantioenhanced 5- and 6-membered lactones with vicinal stereocenters, part of which bears a quaternary stereocenter (up to 99 % enantiomeric excess, up to 12.9 : 1 diastereomeric ratio). Detailed mechanistic studies, including computational simulations, uncovered the synergistic effect of the enzyme and the externally added organophotoredox catalyst Rh6G.

Solvent concentration at 50% protein unfolding may reform enzyme stability ranking and process window identification

As water miscible organic co-solvents are often required for enzyme reactions to improve e.g., the solubility of the substrate in the aqueous medium, an enzyme is required which displays high stability in the presence of this co-solvent. Consequently, it is of utmost importance to identify the most suitable enzyme or the appropriate reaction conditions. Until now, the melting temperature is used in general as a measure for stability of enzymes. The experiments here show, that the melting temperature does not correlate to the activity observed in the presence of the solvent. As an alternative parameter, the concentration of the co-solvent at the point of 50% protein unfolding at a specific temperature T in shortc U 50 T is introduced. Analyzing a set of ene reductases,c U 50 T is shown to indicate the concentration of the co-solvent where also the activity of the enzyme drops fastest. Comparing possible rankings of enzymes according to melting temperature andc U 50 T reveals a clearly diverging outcome also depending on the specific solvent used. Additionally, plots ofc U 50 versus temperature enable a fast identification of possible reaction windows to deduce tolerated solvent concentrations and temperature.

Strategic enzymatic enantioselective desymmetrization of prochiral cyclohexa-2,5-dienones

Asymmetric desymmetrization through the selective reduction of one double bond of prochiral 2,5-cyclohexadienones is highly challenging. A novel method has been developed for synthesizing chiral cyclohexenones by employing an ene-reductase (Bacillus subtilis YqjM) enzyme that belongs to the OYE family. Our strategy demonstrates high substrate scope and enantioselectivity towards substrates containing all-carbon as well as heteroatom (O, N)-containing quaternary centers. The mechanistic studies (kH/D = ∼1.8) indicate that hydride transfer is probably the rate-limiting step. Mutation of several active site residues did not affect the stereochemical outcomes. This work provides a convenient way of synthesizing various enantioselective γ,γ-disubstituted cyclohexanones using enzymes.

From Ground-State to Excited-State Activation Modes: Flavin-Dependent "Ene"-Reductases Catalyzed Non-natural Radical Reactions

Enzymes are desired catalysts for chemical synthesis, because they can be engineered to provide unparalleled levels of efficiency and selectivity. Yet, despite the astonishing array of reactions catalyzed by natural enzymes, many reactivity patterns found in small molecule catalysts have no counterpart in the living world. With a detailed understanding of the mechanisms utilized by small molecule catalysts, we can identify existing enzymes with the potential to catalyze reactions that are currently unknown in nature. Over the past eight years, our group has demonstrated that flavin-dependent "ene"-reductases (EREDs) can catalyze various radical-mediated reactions with unparalleled levels of selectivity, solving long-standing challenges in asymmetric synthesis.This Account presents our development of EREDs as general catalysts for asymmetric radical reactions. While we have developed multiple mechanisms for generating radicals within protein active sites, this account will focus on examples where flavin mononucleotide hydroquinone (FMNhq) serves as an electron transfer radical initiator. While our initial mechanistic hypotheses were rooted in electron-transfer-based radical initiation mechanisms commonly used by synthetic organic chemists, we ultimately uncovered emergent mechanisms of radical initiation that are unique to the protein active site. We will begin by covering intramolecular reactions and discussing how the protein activates the substrate for reduction by altering the redox-potential of alkyl halides and templating the charge transfer complex between the substrate and flavin-cofactor. Protein engineering has been used to modify the fundamental photophysics of these reactions, highlighting the opportunity to tune these systems further by using directed evolution. This section highlights the range of coupling partners and radical termination mechanisms available to intramolecular reactions.The next section will focus on intermolecular reactions and the role of enzyme-templated ternary charge transfer complexes among the cofactor, alkyl halide, and coupling partner in gating electron transfer to ensure that it only occurs when both substrates are bound within the protein active site. We will highlight the synthetic applications available to this activation mode, including olefin hydroalkylation, carbohydroxylation, arene functionalization, and nitronate alkylation. This section also discusses how the protein can favor mechanistic steps that are elusive in solution for the asymmetric reductive coupling of alkyl halides and nitroalkanes. We are aware of several recent EREDs-catalyzed photoenzymatic transformations from other groups. We will discuss results from these papers in the context of understanding the nuances of radical initiation with various substrates.These biocatalytic asymmetric radical reactions often complement the state-of-the-art small-molecule-catalyzed reactions, making EREDs a valuable addition to a chemist's synthetic toolbox. Moreover, the underlying principles studied with these systems are potentially operative with other cofactor-dependent proteins, opening the door to different types of enzyme-catalyzed radical reactions. We anticipate that this Account will serve as a guide and inspire broad interest in repurposing existing enzymes to access new transformations.

A Proline-Based Artificial Enzyme That Favors Aldol Condensation Enables Facile Synthesis of Aliphatic Ketones via Tandem Catalysis

A proline-based artificial enzyme is prepared by grafting the l-proline moieties onto the surface of bovine serum albumin (BSA) protein through atom transfer radical polymerization (ATRP). The artificial enzyme, the BSA-PolyProline conjugate, prefers to catalyze the formation of unsaturated ketones rather than β-hydroxy ketones in the reaction between acetone and aldehydes, which is difficult to achieve in free-proline catalysis. The altered reaction selectivity is ascribed to the locally concentrated l-proline moieties surrounding the BSA molecule, indicating a microenvironmental effect-induced switching of the reaction mechanism. Taking advantage of this selectivity, we used this artificial enzyme in conjunction with a natural enzyme, old yellow enzyme 1 (OYE1), to demonstrate a simple synthesis of different aliphatic ketones from acetone and aldehydes via tandem catalysis.

Loop 6 and the β-hairpin flap are structural hotspots that determine cofactor specificity in the FMN-dependent family of ene-reductases

Flavin mononucleotide (FMN)-dependent ene-reductases constitute a large family of oxidoreductases that catalyze the enantiospecific reduction of carbon-carbon double bonds. The reducing equivalents required for substrate reduction are obtained from reduced nicotinamide by hydride transfer. Most ene-reductases significantly prefer, or exclusively accept, either NADPH or NADH. Despite their usefulness in biocatalytic applications, the structural determinants for cofactor preference remain elusive. We employed the NADPH-preferring 12-oxophytodienoic acid reductase 3 from Solanum lycopersicum (SlOPR3) as a model enzyme of the ene-reductase family and applied computational and structural methods to investigate the binding specificity of the reducing coenzymes. Initial docking results indicated that the arginine triad R283, R343, and R366 residing on and close to a critical loop at the active site (loop 6) are the main contributors to NADPH binding. In contrast, NADH binds unfavorably in the opposite direction toward the β-hairpin flap within a largely hydrophobic region. Notably, the crystal structures of SlOPR3 in complex with either NADPH4 or NADH4 corroborated these different binding modes. Molecular dynamics simulations confirmed NADH binding near the β-hairpin flap and provided structural explanations for the low binding affinity of NADH to SlOPR3. We postulate that cofactor specificity is determined by the arginine triad/loop 6 and the residue(s) controlling access to a hydrophobic cleft formed by the β-hairpin flap. Thus, NADPH preference depends on a properly positioned arginine triad, whereas granting access to the hydrophobic cleft at the β-hairpin flap favors NADH binding.

De Novo Synthesis of Dihydro-β-ionone through Metabolic Engineering and Bacterium-Yeast Coculture

Dihydro-β-ionone is a common type of ionone used in the flavor and fragrance industries because of its characteristic scent. The production of flavors in microbial cell factories offers a sustainable and environmentally friendly approach to accessing them, independent of extraction from natural sources. However, the native pathway of dihydro-β-ionone remains unclear, hindering heterologous biosynthesis in microbial hosts. Herein, we devised a microbial platform for de novo syntheses of dihydro-β-ionone from a simple carbon source with glycerol. The complete dihydro-β-ionone pathway was reconstructed in Escherichia coli with multiple metabolic engineering strategies to generate a strain capable of producing 8 mg/L of dihydro-β-ionone, although this was accompanied by a surplus precursor β-ionone in culture. To overcome this issue, Saccharomyces cerevisiae was identified as having a conversion rate for transforming β-ionone to dihydro-β-ionone that was higher than that of E. coli via whole-cell catalysis. Consequently, the titer of dihydro-β-ionone was increased using the E. coli-S. cerevisiae coculture to 27 mg/L. Our study offers an efficient platform for biobased dihydro-β-ionone production and extends coculture engineering to overproducing target molecules in extended metabolic pathways.

Remote stereocontrol with azaarenes via enzymatic hydrogen atom transfer

Strategies for achieving asymmetric catalysis with azaarenes have traditionally fallen short of accomplishing remote stereocontrol, which would greatly enhance accessibility to distinct azaarenes with remote chiral centres. The primary obstacle to achieving superior enantioselectivity for remote stereocontrol has been the inherent rigidity of the azaarene ring structure. Here we introduce an ene-reductase system capable of modulating the enantioselectivity of remote carbon-centred radicals on azaarenes through a mechanism of chiral hydrogen atom transfer. This photoenzymatic process effectively directs prochiral radical centres located more than six chemical bonds, or over 6 Å, from the nitrogen atom in azaarenes, thereby enabling the production of a broad array of azaarenes possessing a remote γ-stereocentre. Results from our integrated computational and experimental investigations underscore that the hydrogen bonding and steric effects of key amino acid residues are important for achieving such high stereoselectivities.

Asymmetric α-benzylation of cyclic ketones enabled by concurrent chemical aldol condensation and biocatalytic reduction

Chemoenzymatic cascade catalysis has emerged as a revolutionary tool for streamlining traditional retrosynthetic disconnections, creating new possibilities for the asymmetric synthesis of valuable chiral compounds. Here we construct a one-pot concurrent chemoenzymatic cascade by integrating organobismuth-catalyzed aldol condensation with ene-reductase (ER)-catalyzed enantioselective reduction, enabling the formal asymmetric α-benzylation of cyclic ketones. To achieve this, we develop a pair of enantiocomplementary ERs capable of reducing α-arylidene cyclic ketones, lactams, and lactones. Our engineered mutants exhibit significantly higher activity, up to 37-fold, and broader substrate specificity compared to the parent enzyme. The key to success is due to the well-tuned hydride attack distance/angle and, more importantly, to the synergistic proton-delivery triade of Tyr28-Tyr69-Tyr169. Molecular docking and density functional theory (DFT) studies provide important insights into the bioreduction mechanisms. Furthermore, we demonstrate the synthetic utility of the best mutants in the asymmetric synthesis of several key chiral synthons.

Experimental Evolution Reveals a Novel Ene Reductase That Detoxifies α,β-Unsaturated Aldehydes in Listeria monocytogenes

The plant essential oil component trans-cinnamaldehyde (t-CIN) exhibits antibacterial activity against a broad range of foodborne pathogenic bacteria, including L. monocytogenes, but its mode of action is not fully understood. In this study, several independent mutants of L. monocytogenes with increased t-CIN tolerance were obtained via experimental evolution. Whole-genome sequencing (WGS) analysis revealed single-nucleotide-variation mutations in the yhfK gene, encoding an oxidoreductase of the short-chain dehydrogenases/reductases superfamily, in each mutant. The deletion of yhfK conferred increased sensitivity to t-CIN and several other α,β-unsaturated aldehydes, including trans-2-hexenal, citral, and 4-hydroxy-2-nonenal. The t-CIN tolerance of the deletion mutant was restored via genetic complementation with yhfK. Based on a gas chromatography-mass spectrometry (GC-MS) analysis of the culture supernatants, it is proposed that YhfK is an ene reductase that converts t-CIN to 3-phenylpropanal by reducing the C=C double bond of the α,β-unsaturated aldehyde moiety. YhfK homologs are widely distributed in Bacteria, and the deletion of the corresponding homolog in Bacillus subtilis also caused increased sensitivity to t-CIN and trans-2-hexenal, suggesting that this protein may have a conserved function to protect bacteria against toxic α,β-unsaturated aldehydes in their environments. IMPORTANCE While bacterial resistance against clinically used antibiotics has been well studied, less is known about resistance against other antimicrobials, such as natural compounds that could replace traditional food preservatives. In this work, we report that the food pathogen Listeria monocytogenes can rapidly develop an elevated tolerance against t-cinnamaldehyde, a natural antimicrobial from cinnamon, by single base pair changes in the yhfK gene. The enzyme encoded by this gene is an oxidoreductase, but its substrates and precise role were hitherto unknown. We demonstrate that the enzyme reduces the double bond in t-cinnamaldehyde and thereby abolishes its antibacterial activity. Furthermore, the mutations linked to t-CIN tolerance increased bacterial sensitivity to a related compound, suggesting that they modify the substrate specificity of the enzyme. Since the family of oxidoreductases to which YhfK belongs is of great interest in the mediation of stereospecific reactions in biocatalysis, our work may also have unanticipated application potential in this field.

The Evolving Nature of Biocatalysis in Pharmaceutical Research and Development

Biocatalysis is a highly valued enabling technology for pharmaceutical research and development as it can unlock synthetic routes to complex chiral motifs with unparalleled selectivity and efficiency. This perspective aims to review recent advances in the pharmaceutical implementation of biocatalysis across early and late-stage development with a focus on the implementation of processes for preparative-scale syntheses.

Photosensitized Radical-Anion-Driven Metal-Free Selective Reduction of Aldehydes Using Graphene Oxide as an Electron Relay Mediator under Visible Light

Despite the modern boost, developing a new photocatalytic system for the reduction of aldehydes is still challenging due to their high negative reduction potential. Herein, we have used a metal-free photoinduced electron-transfer system based on a cheap and readily available organic dye eosin Y (EY), graphene oxide (GO), and ammonium oxalate (AO) for photocatalytic reduction of structurally diverse aldehydes under sustainable conditions. The protocol shows remarkable selectivity for the photocatalytic reduction of aldehydes over ketones. The decisive interaction of GO and AO with the various states of EY (ground, singlet, triplet, and radical anions), which are responsible for the commencement of the reaction, was examined by various theoretical, optical, electrochemical, and photo-electrochemical studies. The synergetic system of GO, EY, and AO is appropriate for enhancing the separation efficiency of visible-light-induced charge carriers. GO nanosheets act as an electron reservoir to accept and transport photogenerated electrons from the photocatalytic system to the reactant. The reduction of the GO during the process ruled out the back transfer of photoexcited charges. Control experiments explained that the reaction involves two stages: electron transfer and protonation. This process eliminates the necessity of precious-metal-based photocatalysts or detrimental sacrificial agents and overcomes the redox potential limitations for the photoreduction of aldehydes.

Mechanistic Insights into the Ene-Reductase-Catalyzed Promiscuous Reduction of Oximes to Amines

The biocatalytic reduction of the oxime moiety to the corresponding amine group has only recently been found to be a promiscuous activity of ene-reductases transforming α-oximo β-keto esters. However, the reaction pathway of this two-step reduction remained elusive. By studying the crystal structures of enzyme oxime complexes, analyzing molecular dynamics simulations, and investigating biocatalytic cascades and possible intermediates, we obtained evidence that the reaction proceeds via an imine intermediate and not via the hydroxylamine intermediate. The imine is reduced further by the ene-reductase to the amine product. Remarkably, a non-canonical tyrosine residue was found to contribute to the catalytic activity of the ene-reductase OPR3, protonating the hydroxyl group of the oxime in the first reduction step.

Biohydrogenation of 1,3-Butadiene to 1-Butene under Acetogenic Conditions by Acetobacterium wieringae

The environmental fate and transformation mechanism(s) of 1,3-butadiene (BD) under anoxic conditions remain largely unexplored. Anaerobic consortia that can biohydrogenate BD to stoichiometric amounts of 1-butene at a maximum rate of 205.7 ± 38.6 μM day^-1 were derived from freshwater river sediment. The formation of 1-butene occurred only in the presence of both H₂ and CO₂ with concomitant acetate production, suggesting the dependence of BD biohydrogenation on acetogenesis. The 16S rRNA gene-targeted amplicon sequencing revealed the enrichment and dominance of a novel Acetobacterium wieringae population, designated as strain N, in the BD-biohydrogenating community. Multiple genes encoding putative ene-reductases, candidate catalysts for the hydrogenation of the C═C bond in diene compounds, were annotated on the metagenome-assembled genome of strain N, and thus attributed the BD biohydrogenation activity to strain N. Our findings emphasize an essential but overlooked role of certain Acetobacterium members (e.g., strain N) contributing to the natural attenuation of BD in contaminated subsurface environments (e.g., sediment and groundwater). Future efforts to identify and characterize the ene-reductase(s) responsible for BD biohydrogenation in strain N hold promise for the development of industrial biocatalysts capable of stereoselective conversion of BD to 1-butene.

Chlamydomonas reinhardtii mutants deficient for Old Yellow Enzyme 3 exhibit increased photooxidative stress

Old Yellow Enzymes (OYEs) are flavin-containing ene-reductases that have been intensely studied with regard to their biotechnological potential for sustainable chemical syntheses. OYE-encoding genes are found throughout the domains of life, but their physiological role is mostly unknown, one reason for this being the promiscuity of most ene-reductases studied to date. The unicellular green alga Chlamydomonas reinhardtii possesses four genes coding for OYEs, three of which we have analyzed biochemically before. Ene-reductase CrOYE3 stood out in that it showed an unusually narrow substrate scope and converted N-methylmaleimide (NMI) with high rates. This was recapitulated in a C. reinhardtii croye3 mutant that, in contrast to the wild type, hardly degraded externally added NMI. Here we show that CrOYE3-mediated NMI conversion depends on electrons generated photosynthetically by photosystem II (PSII) and that the croye3 mutant exhibits slightly decreased photochemical quenching in high light. Non-photochemical quenching is strongly impaired in this mutant, and it shows enhanced oxidative stress. The phenotypes of the mutant suggest that C. reinhardtii CrOYE3 is involved in the protection against photooxidative stress, possibly by converting reactive carbonyl species derived from lipid peroxides or maleimides from tetrapyrrole degradation.

One-Pot Biocatalytic Synthesis of Primary, Secondary, and Tertiary Amines with Two Stereocenters from α,β-Unsaturated Ketones Using Alkyl-Ammonium Formate

The efficient asymmetric catalytic synthesis of amines containing more than one stereogenic center is a current challenge. Here, we present a biocatalytic cascade that combines ene-reductases (EReds) with imine reductases/reductive aminases (IReds/RedAms) to enable the conversion of α,β-unsaturated ketones into primary, secondary, and tertiary amines containing two stereogenic centers in very high chemical purity (up to >99%), a diastereomeric ratio, and an enantiomeric ratio (up to >99.8:<0.2). Compared with previously reported strategies, our strategy could synthesize two, three, or even all four of the possible stereoisomers of the amine products while precluding the formation of side-products. Furthermore, ammonium or alkylammonium formate buffer could be used as the only additional reagent since it acted both as an amine donor and as a source of reducing equivalents. This was achieved through the implementation of an NADP-dependent formate dehydrogenase (FDH) for the in situ recycling of the NADPH coenzyme, thus leading to increased atom economy for this biocatalytic transformation. Finally, this dual-enzyme ERed/IRed cascade also exhibits a complementarity with the recently reported EneIRED enzymes for the synthesis of cyclic six-membered ring amines. The ERed/IRed method yielded trans-1,2 and cis-1,3 substituted cyclohexylamines in high optical purities, whereas the EneIRED method was reported to yield one cis-1,2 and one trans-1,3 enantiomer. As a proof of concept, when 3-methylcyclohex-2-en-1-one was converted into secondary and tertiary chiral amines with different amine donors, we could obtain all the four possible stereoisomer products. This result exemplifies the versatility of this method and its potential for future wider utilization in asymmetric synthesis by expanding the toolbox of currently available dehydrogenases via enzyme engineering and discovery.

Radical Termination via β-Scission Enables Photoenzymatic Allylic Alkylation Using "Ene"-Reductases

Allylations are practical transformations that forge C-C bonds while introducing an alkene for further chemical manipulations. Here, we report a photoenzymatic allylation of α-chloroamides with allyl silanes using flavin-dependent 'ene'-reductases (EREDs). An engineered ERED can catalyze annulative allylic alkylation to prepare 5, 6, and 7-membered lactams with high levels of enantioselectivity. Ultrafast transient absorption spectroscopy indicates that radical termination occurs via β-scission of the silyl group to afford a silyl radical, a distinct mechanism by comparison to traditional radical allylations involving allyl silanes. Moreover, this represents an alternative strategy for radical termination using EREDs. This mechanism was applied to intermolecular couplings involving allyl sulfones and silyl enol ethers. Overall, this method highlights the opportunity for EREDs to catalyze radical termination strategies beyond hydrogen atom transfer.

Immobilization of Ene Reductase in Polyvinyl Alcohol Hydrogel

In this study, ene reductase (ER) was entrapped in polyvinyl alcohol hydrogel, adsorbed on montmorillonite and immobilized covalently on glutaraldehyde activated 3-aminopropyl-functionalized silica gel. Although protein recovery yields were at least 85% for adsorption and covalent immobilization, only the encapsulated ER showed activity. The activity of free and entrapped ER preparations was measured by following NADPH-dependent reduction of 2-cyclohexen-1-one. The both protein recovery and activity recovery yields were calculated as 100% when 1 mg protein was used for immobilization. The both free and entrapped ER preparations showed the same optimum pH and temperature as 7.0 and 30 °C, respectively. The entrapped ER showed 34.4-fold more thermal stability than that of the free ER at 30 °C. Michaelis-Menten constant and maximum velocity values were 0.25 mM and 1.2 U/mg protein, respectively for the free ER towards 2-cyclohexen-1-one. The corresponding values were 1.5 mM and 0.9 U/mg protein for the entrapped ER. The results of time-course reduction of 2-cyclohexen-1-one showed that the entrapped ER catalyzed the reaction as effectively as the free ER. The entrapped ER remained 85% of its initial activity after 10 reused cycles.