Inverting the stereoselectivity of an NADH-dependent imine reductase variant

Imine reductases (IREDs) offer biocatalytic routes to chiral amines and have a natural preference for the NADPH cofactor. In previous work, we reported enzyme engineering of the (R)-selective IRED from Myxococcus stipitatus (NADH-IRED-Ms) yielding a NADH-dependent variant with high catalytic efficiency. However, no IRED with NADH specificity and (S)-selectivity in asymmetric reductions has yet been reported. Herein, we applied semi-rational enzyme engineering to switch the selectivity of NADH-IRED-Ms. The quintuple variant A241V/H242Y/N243D/V244Y/A245L showed reverse stereopreference in the reduction of the cyclic imine 2- methylpyrroline compared to the wild-type and afforded the (S)- amine product with >99% conversion and 91% enantiomeric excess. We also report the crystal-structures of the NADPH-dependent (R)- IRED-Ms wild-type enzyme and the NADH-dependent NADH-IREDMs variant and molecular dynamics (MD) simulations to rationalize the inverted stereoselectivity of the quintuple variant.