Sequence‐Based In‐silico Discovery, Characterisation, and Biocatalytic Application of a Set of Imine Reductases

Imine reductases (IREDs) have recently become a primary focus of research in biocatalysis, complementing other classes of amine‐forming enzymes such as transaminases and amine dehydrogenases. Following in the footsteps of other research groups, we have established a set of IRED biocatalysts by sequence‐based in silico enzyme discovery. In this study, we present basic characterisation data for these novel IREDs and explore their activity and stereoselectivity using a panel of structurally diverse cyclic imines as substrates. Specific activities of >1 U/mg and excellent stereoselectivities (ee>99 %) were observed in many cases, and the enzymes proved surprisingly tolerant towards elevated substrate loadings. Co‐expression of the IREDs with an alcohol dehydrogenase for cofactor regeneration led to whole‐cell biocatalysts capable of efficiently reducing imines at 100 mM initial concentration with no need for the addition of extracellular nicotinamide cofactor. Preparative biotransformations on gram scale using these ‘designer cells’ afforded chiral amines in good yield and excellent optical purity. All‐in‐one: A set of 14 imine reductases (IREDs) has been characterised and used for the reduction of a panel of structurally diverse imines. Co‐expression of the IREDs with an alcohol dehydrogenase afforded a convenient ‘all‐in‐one’ biocatalyst that reduced imines at the expense of 2‐propanol as co‐substrate and furnished chiral amines with excellent productivity and stereoselectivity.