Electrode with Immobilized Electron Mediator for Nicotinamide Adenine Dinucleotide Phosphate Reduction

Biocatalysts are widely used in various industrial fields, but the use of oxidoreductases has been limited because of the requirement for electrons in their enzymatic function. The reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) is a known cofactor that supplies electrons for oxidoreductases, but it is inactivated to the oxidized form NADP⁺ after use. The complex three-dimensional structure of NADP⁺ makes it difficult to reduce (i.e., supply with electrons so that can again be used as an electron donor) through simple electrochemical methods. Although adding an electron mediator is a potential method to facilitate NADP⁺ reduction, it is not practical for industrial use. Here, we demonstrate that an electrode coated with 1-butanethiol-1'-methyl-4,4'-bipyridine (BMB) effectively transports electrons to NADP⁺, leading to NADPH formation. The presence of BMB immobilized on a gold electrode was confirmed through contact angle, cyclic voltammetry, and electrochemical quartz crystal microbalance measurements. BMB was immobilized on the gold electrode in an amount of 12.69 ng/cm². Applying a voltage to an NADP⁺ solution using the BMB-coated gold electrode led to increase in absorbance at 340 nm. These findings indicate that NADP⁺ can be reduced to NADPH using a BMB-coated gold electrode without free electron mediators, which is promising for industrial NADPH regeneration.