Structure-Based Insight into the Asymmetric Bioreduction of the C=C Double Bond of alpha,beta-Unsaturated Nitroalkenes by Pentaerythritol Tetranitrate Reductase

Focused directed evolution of pentaerythritol tetranitrate reductase by using automated anaerobic kinetic screening of site-saturated libraries

This work describes the development of an automated robotic platform for the rapid screening of enzyme variants generated from directed evolution studies of pentraerythritol tetranitrate (PETN) reductase, a target for industrial biocatalysis. By using a 96-well format, near pure enzyme was recovered and was suitable for high throughput kinetic assays; this enabled rapid screening for improved and new activities from libraries of enzyme variants. Initial characterisation of several single site-saturation libraries targeted at active site residues of PETN reductase, are described. Two mutants (T26S and W102F) were shown to have switched in substrate enantiopreference against substrates (E)-2-aryl-1-nitropropene and α-methyl-trans-cinnamaldehyde, respectively, with an increase in ee (62 % (R) for W102F). In addition, the detection of mutants with weak activity against α,β-unsaturated carboxylic acid substrates showed progress in the expansion of the substrate range of PETN reductase. These methods can readily be adapted for rapid evolution of enzyme variants with other oxidoreductase enzymes.

Continuous two-phase flow miniaturised bioreactor for monitoring anaerobic biocatalysis by pentaerythritol tetranitrate reductase

A novel continuous recirculating two-phase flow miniaturised bioreactor was developed for biocatalytic transformations with the enzyme pentaerythritol tetranitrate reductase using on-chip spectroscopic detection of the organic and aqueous phases. A phase separation technique is described that uses electrostatic attraction to force charged droplets to merge back into the aqueous phase and thus allow the monitoring of both reaction phases during enzymatic turnover. We report an increased rate of enzyme catalysed reduction of trans-2-(2-nitrovinyl)thiophene, which was used as a model system to demonstrate the principles of the bioreactor design, compared to conventional macroscale experiments. Additional data obtained with ketoisophorone, trans-cinnamaldehyde and 2-methylmaleimide support our findings and provide a basis for improving the chemistry of biocatalysis.

A short, chemoenzymatic route to chiral beta-aryl-gamma-amino acids using reductases from anaerobic bacteria

A short chemoenzymatic synthesis of beta-aryl-gamma-aminobutyric acids has been developed, based on a highly enantioselective biocatalytic reduction of beta-aryl-beta-cyano-alpha,beta-unsaturated carboxylic acids.

Asymmetric Reduction of Activated Alkenes by Pentaerythritol Tetranitrate Reductase: Specificity and Control of Stereochemical Outcome by Reaction Optimisation

We show that pentaerythritol tetranitrate reductase (PETNR), a member of the 'ene' reductase old yellow enzyme family, catalyses the asymmetric reduction of a variety of industrially relevant activated alpha,beta-unsaturated alkenes including enones, enals, maleimides and nitroalkenes. We have rationalised the broad substrate specificity and stereochemical outcome of these reductions by reference to molecular models of enzyme-substrate complexes based on the crystal complex of the PETNR with 2-cyclohexenone 4a. The optical purity of products is variable (49-99% ee), depending on the substrate type and nature of substituents. Generally, high enantioselectivity was observed for reaction products with stereogenic centres at Cbeta (>99% ee). However, for the substrates existing in two isomeric forms (e.g., citral 11a or nitroalkenes 18-19a), an enantiodivergent course of the reduction of E/Z-forms may lead to lower enantiopurities of the products. We also demonstrate that the poor optical purity obtained for products with stereogenic centres at Calpha is due to non-enzymatic racemisation. In reactions with ketoisophorone 3a we show that product racemisation is prevented through reaction optimisation, specifically by shortening reaction time and through control of solution pH. We suggest this as a general strategy for improved recovery of optically pure products with other biocatalytic conversions where there is potential for product racemisation.