Deracemization of 2-Methyl-1,2,3,4-Tetrahydroquinoline Using Mutant Cyclohexylamine Oxidase Obtained by Iterative Saturation Mutagenesis

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Abstract

The current toolkit of biocatalysts for the production of enantiomerically pure chiral amines is largely restricted to amine transaminases, ammonia lyases, or the genetic variants of monoamine oxidase N(MAO-N) from Aspergillus niger. Flavin-dependent amine oxidases have the apparent advantage of using molecular oxygen as a stoichiometric oxidant and their reactions are irreversible. To expand the toolkit and increase the substrate spectrum of a bacterial and flavin-dependent cyclohexylamine oxidase (CHAO) to enable deracemization of secondary amines, saturation mutagenesis of 11 amino acid residues located around the cyclohexanone substrate within a distance of 5 Å, followed by iterative saturation mutagenesis of four beneficial mutants, were performed. Screening with 2-methyl-1,2,3,4-tetrahydroquinoline as the substrate generated two improved CHAO variants, T198FL199S and T198FL199SM226F, that exhibited up to 406 times higher catalytic efficiency than the wild-type CHAO. Besides, high stereoselectivity for 2-methyl-1,2,3,4-tetrahydroquinoline and other 2-substituted-1,2,3,4-tetrahydroquinolines was demonstrated. In particular, deracemization of 2-methyl-1,2,3,4-tetrahydroquinoline by Escherichia coli whole cells expressing CHAO mutant T198FL199SM226F led to the production of (R)-2-methyl-1,2,3,4-tetrahydroquinoline with high yield (76%) and enantiomeric excess (ee, 98%). Tetrahydroquinolines are important building blocks of natural and synthetic products useful in the pharmaceutical and agrochemical industries.

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