Structure,Activity and Stereoselectivity of NADPH‐Dependent Oxidoreductases Catalysing the S‐Selective Reduction of the Imine Substrate 2‐Methylpyrroline

Oxidoreductases from Streptomyces sp. GF3546 3546‐IRED], Bacillus cereus BAG3X2 (BcIRED) and Nocardiopsis halophila (NhIRED) each reduce prochiral 2‐methylpyrroline (2MPN) to (S)‐2‐methylpyrrolidine with >95 % ee and also a number of other imine substrates with good selectivity. Structures of BcIRED and NhIRED have helped to identify conserved active site residues within this subgroup of imine reductases that have S selectivity towards 2MPN, including a tyrosine residue that has a possible role in catalysis and superimposes with an aspartate in related enzymes that display R selectivity towards the same substrate. Mutation of this tyrosine residue—Tyr169—in 3546‐IRED to Phe resulted in a mutant of negligible activity. The data together provide structural evidence for the location and significance of the Tyr residue in this group of imine reductases, and permit a comparison of the active sites of enzymes that reduce 2MPN with either R or S selectivity.