Process Development and Protein Engineering Enhanced Nitroreductase-Catalyzed Reduction of 2-Methyl-5-nitro-pyridine

Reduction of aromatic nitro compounds to anilines is of great interest to the chemical industry. Biocatalytic reduction of nitroarenes has made it possible to effectively produce anilines by applying nitroreductase enzymes (NR) in combination with vanadium pentoxide. Herein, the NR-catalyzed reduction of 2-methyl-5-nitro-pyridine (2) to give the desired aniline (1) was studied as a model reaction. It demonstrates the importance of process development and enzyme engineering as key approaches to overcome scale-up issues and improve yield and productivity. Moving to fed-batch allowed controlling the feeding rate of 2 to prevent the accumulation of intermediates and formation of undesired side products. Starting with a substrate (2) concentration of 200 mM (28 g/L) and enzyme loading of 5 mg/mL (18% w/w), it was possible to achieve complete conversion and 1 in 95% yield by high-performance liquid chromatography (89.1% isolated yield) over 18 h, whereas, with 500 mM (69 g/L) 2 and an enzyme loading of 10 mg/mL (14.5% w/w), the same conversion and yield were achieved in 26 h. A rational engineering of NR-4 yielded faster variants, including NR-5, in only one round. The improved rate of the new variants allowed increasing the feeding rate of 2 to shorten the reaction time to less than a day as well as decreasing the enzyme loading to 3.6%.