Ferredoxin NADP⁺ reductase for NADPH and NADH regeneration in a flow bioelectrochemical reactor

Ferredoxin-NADP⁺ reductase (FNR) is an efficient and selective biocatalyst to continuously regenerate the NADPH cofactor consumed in biomolecular synthesis for the chemical and pharmaceutical sectors. In this work, FNR from Chlamydomonas reinhardtii was applied to electrochemical regeneration of the nicotinamide cofactors, by combining this enzymatic catalyst in a flow reactor with the oxidation of hydrogen, a clean source of electrons and protons. FNR was immobilized on the surface of oxidized multi-walled carbon nanotubes, which allowed maintaining its activity for over six days under high flow rate. Surprisingly, this modified FNR electrode was effective not only in regenerating NADPH but also NADH. The cofactor regeneration was then applied to the NADH-dependent production of lactate from pyruvate, using L-lactate dehydrogenase (LDH) in the presence of low NAD⁺ concentration (10 µM). Both FNR and LDH enzymes were immobilized in the bioelectrochemical system that achieved a remarkable total turnover number (TTN) of 10⁴ for the nicotinamide cofactor and a faradaic efficiency higher than 80 %.