Biocatalytic reduction of a- or b-alkyl-barylnitroalkenes provides a convenient and efficient method to prepare chiral substituted nitroalkanes. Pentaerythritol tetranitrate reductase (PETN reductase) from Enterobacter cloacae st. PB2 catalyses the reduction of nitroolefins such as 1-nitrocyclohexene (1) with steady state and rapid reaction kinetics comparable to other old yellow enzyme homologues. Furthermore, it reduces 2-aryl-1-nitropropenes (4aa??d) to their equivalent (S)-nitropropanes 9aa??d. The enzyme shows a preference for the (Z)-isomer of substrates 4aa??d, providing almost pure enantiomeric products 9aa??d (ees up to>99 ) in quantitative yield, whereas the respective (E)-isomers are reduced with lower enantioselectivity (63a??89 ee) and lower product yields. 1-Aryl-2-nitropropenes (5a, b) are also reduced efficiently, but the products (R)-10 have lower optical purities. The structure of the enzyme complex with 1-nitrocyclohexene (1) was determined by X-ray crystallography, revealing two substrate-binding modes, with only one compatible with hydride transfer. Models of nitropropenes 4 and 5 in the active site of PETN reductase predicted that the enantioselectivity of the reaction was dependent on the orientation of binding of the (E)- and (Z)- substrates. This work provides a structural basis for understanding the mechanism of asymmetric bioreduction of nitroalkenes by PETN reductase.
Toogood, H. S., Fryszkowska, A., Hare, V., Fisher, K., Roujeinikova, A., Leys, D., Gardiner, J. M., Stephens, G. M., & Scrutton, N. S. (2008). Structure-based insight into the asymmetric bioreduction of the C=C double bond of alpha,beta-unsaturated nitroalkenes by pentaerythritol tetranitrate reductase. Advanced Synthesis and Catalysis, 350(17), 2789 - 2803. https://doi.org/10.1002/adsc.200800561