Selin Kara

RECENT TRENDS AND NOVEL CONCEPTS IN COFACTOR- DEPENDENT BIOTRANSFORMATIONS

Cofactor-dependent enzymes catalyze a broad range of synthetically useful transformations. However, the cofactor requirement also poses economic and practical challenges for the application of these biocatalysts. For three decades, considerable research effort has been devoted to the development of reliable in situ regeneration methods for the most commonly employed cofactors, particularly NADH and NADPH. Today, researchers can choose from a plethora of options, and oxidoreductases are routinely employed even on industrial scale. Nevertheless, more efficient cofactor regeneration methods are still being developed, with the aim of achieving better atom economy, simpler reaction setups, and higher productivities. Besides, cofactor dependence has been recognized as an opportunity to confer novel reactivity upon enzymes by engineering their cofactors, and to couple (redox) biotransformations in multi-enzyme cascade systems. These novel concepts will help to further establish cofactor-dependent biotransformations as an attractive option for the synthesis of biologically active compounds, chiral building blocks, and bio-based platform molecules.

Enantioselective Oxidation of Aldehydes Catalyzed by Alcohol Dehydrogenase

Teaching old dogs new tricks: Alcohol dehydrogenases (ADHs) may be established redox biocatalysts but they still are good for a few surprises. ADHs can be used to oxidize aldehydes, and this was demonstrated by the oxidative dynamic kinetic resolution of profens. In the presence of a suitable cofactor regeneration system, this reaction can occur with high selectivity.

One-pot combination of enzyme and Pd nanoparticle catalysis for the synthesis of enantiomerically pure 1,2-amino alcohols

One-pot combinations of sequential catalytic reactions can offer practical and ecological advantages over classical multi-step synthesis schemes. In this context, the integration of enzymatic and chemo-catalytic transformations holds particular potential for efficient and selective reaction sequences that would not be 10 possible using either method alone. Here we report the one-pot combination of alcohol dehydrogenasecatalysed asymmetric reduction of 2-azido ketones and Pd nanoparticle-catalysed hydrogenation of the resulting azido alcohols, which gives access to both enantiomers of aromatic 1,2-amino alcohols in high yields and excellent optical purity (ee >99%). Furthermore, we demonstrate the incorporation of an upstream azidolysis and a downstream acylation step into the one-pot system, thus establishing a highly 15 integrated synthesis of the antiviral natural product (S)-tembamide in 73% yield (ee >99%) over 4 steps. Avoiding the purification and isolation of intermediates in this synthetic sequence leads to an unprecedentedly low ecological footprint, as quantified by E-factor and solvent demand.

Expanding the Scope of Laccase‐Mediator Systems

The laccase‐mediator system (LMS) for the regeneration of oxidised nicotinamide co‐factors was revisited to broaden the mediator scope. Among the 18 mediators screened, acetosyringone, syringaldehyde and caffeic acid excelled with respect to activity and stability under process conditions. The LMS based on the laccase from Myceliophthora thermophila and acetosyringone was further investigated and applied to promote the nicotinamide adenine dinucleotide (NAD+)‐dependent oxidation of glucose as well as the oxidative lactonisation of 1,4‐butanediol to the corresponding γ‐butyrolactone.

A Bi-enzymatic Convergent Cascade for epsilon-Caprolactone Synthesis Employing 1,6-Hexanediol as a "Double-Smart Cosubstrate'

A bi‐enzymatic cascade consisting of a Baeyer–Villiger monooxygenase and an alcohol dehydrogenase (ADH) was designed in a convergent fashion to utilise two molar equivalents of cyclohexanone (CHO) and one equivalent of 1,6‐hexanediol as a ‘double‐smart cosubstrate’ to produce ε‐caprolactone (ECL) with water as sole by‐product. The convergent enzymatic cascade reaction reported herein, is performed at ambient conditions in water, is self‐sufficient with respect to cofactor, and incorporates all starting materials into the desired product, ECL. Among different enzymes explored, the reaction catalysed by cyclohexanone monooxygenase from Acinetobacter sp. NCIMB 9871 coupled with ADH from Thermoanaerobacter ethanolicus showed the best results, reaching 91 % conversion of CHO after 24 h with a product titre of 2 g L−1. Scale‐up of the coupled system (50 mL) performed better than the small‐scale reactions and >99 % conversion of CHO and ECL concentration of 20 mM were achieved within 18 h.

Bioreductions Catalyzed by an Alcohol Dehydrogenase in Non‐aqueous Media

Highly productive biocatalytic reductions were established using an isolated alcohol dehydrogenase (ADH) under water‐deficient conditions. First, a solvent‐free system was evaluated for the reduction of 2‐butanone catalyzed by ADH evo‐1.1.200 promoted by the “smart cosubstrate” 1,4‐butanediol. ADH evo‐1.1.200 excelled by its activity and stability under high reagent concentrations and hence was the enzyme of choice. However, conversion of 2‐butanone was limited to <1 % in 10 days under the solvent‐free conditions. Therefore, water‐immiscible organic solvents were evaluated whereby the highest conversions were achieved in MTBE and toluene. MTBE was chosen as its different boiling point compared to other reaction components (e.g., 2‐butanone, 2‐butanol, diol cosubstrate, and lactone coproduct) would simplify the downstream processing. Further on, by tuning substrate loading, the productivity of the ADH evo‐1.1.200 was successfully increased to a turnover number (TON) of 64 000.

Exploring the Substrate Specificity and Enantioselectivity of a Baeyer–Villiger Monooxygenase from Dietzia sp. D5: Oxidation of Sulfides and Aldehydes

Baeyer–Villiger monooxygenases (BVMOs) are valuable enzymes for specific oxyfunctionalization chemistry. They catalyze the oxidation of ketones to esters, but are also capable of oxidizing other chemical functions, namely aldehydes and heteroatoms such as sulfur, nitrogen, selenium and boron. The oxidation specificity and enantioselectivity of a newly characterized BVMO (BVMO4) from a strain of Dietzia towards sulfide- and aldehyde substrates have been studied. BVMO4 could react with sulfides containing an aromatic group. The presence of a substituent on the aromatic group was tolerated when they were in the meta- and para position and the oxidations yielded predominantly the (R)-sulfoxides. Similarly, BVMO4 displayed a higher activity for aldehydes containing a phenyl group, but long aliphatic aldehydes, namely octanal and decanal, were also accepted as substrate by this enzyme. The major oxidation products of the aldehyde substrates were the respective carboxylic acids in contrast to formate ester that was obtained in most of the previous reports. The Baeyer–Villiger oxidation of the substrate 2-phenylpropionaldehyde was studied in further detail and the corresponding acid product was obtained with good regio- and enantioselectivity. This is a unique feature for BVMO4 and is of great interest for further exploration of an alternative biocatalytic process.

Kinetic insights into ϵ-caprolactone synthesis: Improvement of an enzymatic cascade reaction

A computational approach for the simulation and prediction of a linear three‐step enzymatic cascade for the synthesis of ϵ‐caprolactone (ECL) coupling an alcohol dehydrogenase (ADH), a cyclohexanone monooxygenase (CHMO), and a lipase for the subsequent hydrolysis of ECL to 6‐hydroxyhexanoic acid (6‐HHA). A kinetic model was developed with an accuracy of prediction for a fed‐batch mode of 37% for substrate cyclohexanol (CHL), 90% for ECL, and >99% for the final product 6‐HHA. Due to a severe inhibition of the CHMO by CHL, a batch synthesis was shown to be less efficient than a fed‐batch approach. In the fed‐batch synthesis, full conversion of 100 mM CHL was 28% faster with an analytical yield of 98% compared to 49% in case of the batch synthesis. The lipase‐catalyzed hydrolysis of ECL to 6‐HHA circumvents the inhibition of the CHMO by ECL enabling a 24% higher product concentration of 6‐HHA compared to ECL in case of the fed‐batch synthesis without lipase. Biotechnol. Bioeng. 2017;114: 1215–1221.

A Fed‐Batch Synthetic Strategy for a Three‐Step Enzymatic Synthesis of Poly‐ϵ‐caprolactone

A three‐step enzymatic reaction sequence for the synthesis of poly‐ϵ‐caprolactone (PCL) was designed running in a fed‐batch operation. The first part of the cascade consisted of two oxidation steps starting with alcohol dehydrogenase catalyzed oxidation from cyclohexanol to cyclohexanone and further oxidation to ϵ‐caprolactone (ECL) by means of a Baeyer–Villiger monooxygenase. As a third step, lipase‐catalyzed hydrolysis of the lactone to 6‐hydroxyhexanoic acid (6‐HHA) was designed. With this biocatalytic multistep process reported herein, severe substrate surplus and product inhibition could be circumvented by the fed‐batch operation by adding the cyclohexanol substrate and by in situ product removal of ECL by hydrolysis, respectively. Up to 283 mm product concentration of 6‐HHA was reached in the fed‐batch operated process without loss in productivity within 20 h. After extraction and subsequent polymerization catalyzed by Candida antarctica lipase B, analysis of the unfractionated polymer revealed a bimodal distribution of the polymer population, which reached a mass average molar mass (Mw) value of approximately 63 000 g mol−1 and a dispersity (Mw/Mn) of 1.1 for the higher molecular weight population, which thus revealed an alternative route to the conventional synthesis of PCL.

Influence of reaction conditions on the enantioselectivity of biocatalyzed C–C bond formations under high pressure conditions

Benzoylformate decarboxylase (BFD, EC 4.1.1.7) is a homotetrameric thiamine diphosphate (ThDP)-dependent enzyme which catalyzes the synthesis of chiral 2-hydroxyketones accepting a broad range of aldehydes as substrates. In this study the synthesis of 2-hydroxypropiophenone (2-HPP) from benzaldehyde and acetaldehyde was catalyzed by three BFD variants namely BFD F464I, BFD A460I and BFD A460I-F464I. This paper reports the effect of hydrostatic pressure up to 290 MPa when the reactions were carried out at different benzaldehyde concentrations (5-40 mM) as well as at different pH values (7.0-8.5). Acetaldehyde concentration was fixed at 400 mM in all biotransformations. Reactions performed at high benzaldehyde concentrations and at high hydrostatic pressures showed an increase in (R)-2-HPP formation catalyzed by all BFD variants. For BFD A460I-F464I we observed an increase in the ee of (R)-2-HPP up to 80%, whereas at atmospheric conditions this variant synthesizes (R)-2-HPP with an ee of only 50%. Alkaline conditions (up to pH 8.5) and high hydrostatic pressures resulted in an increase of (R)-2-HPP synthesis, especially in the case of BFD A460I and BFD F464I.