Application of Imine Reductases (IREDs) in Micro-Aqueous Reaction Systems

Here we present the applicability of different imine reductases (IREDs) in micro-aqueous reaction systems. Subjects of the study were the IREDs from Streptomyces aurantiacus (SaIR), Streptomyces sp. GF3587 (RGF3587IR), Streptomyces kanamyceticus (SkIR), Streptomyces ipomoeae 91-03 (SiIR), Streptomyces sp. GF3546 (SGF3546IR), and Paenibacillus elgii B69 (PeIR). The IREDs were overexpressed in Escherichia coli (E. coli) cells and used directly after lyophilization. Several organic solvents and buffer amounts were screened for the reduction of the two substrates beta-carboline harmane and 1-methyl-3,4-dihydroisoquinoline to the corresponding amines. Cyclopentyl methyl ether (CPME) proved to be the best solvent choice for the envisaged reduction. In addition, CPME is currently referred to as an environmentally benign solvent. Optimized reaction conditions were applied to 20 mM of the hardly water soluble substrates, leading to good conversions (up to 96%) and excellent enantiomeric excesses (>99%) in the best cases. The use of micro-aqueous reaction systems opens the way to further applications of IREDs with hardly water soluble substrates.