Recent Progress and Perspectives on Electrochemical Regeneration of Reduced Nicotinamide Adenine Dinucleotide (NADH)

NAD is a cofactor that maintains cellular redox homeostasis and has immense industrial and biological significance. It acts as an enzymatic mediator in several biocatalytic electrochemical reactions and undergoes oxidation/reduction to form NAD⁺ or NADH, respectively. The NAD redox couple (NAD⁺ /NADH) mostly exists in enzyme-assisted metabolic reactions as a coenzyme during which electrons and protons are transferred. NADH shuttles these charges between the enzyme and the substrate. In order to understand such complex metabolic reactions, it is vital to study the bio-electrochemistry of NADH. In addition, the regeneration of NADH in industries has attracted significant attention due to its vast usage and high cost. To make biocatalysis economically viable, primary methods of NADH regeneration including enzymatic, chemical, photochemical and electrochemical methods are widely used. This review is mainly focused on the electrochemical reduction of NAD⁺ to NADH with specific details on the mechanism and kinetics of the reaction. It provides emphasis on the different routes (direct and mediated) to electrochemically regenerate NADH from NAD⁺ highlighting the NAD dimer formation. Also, it describes the electrocatalysts developed until now and the scope for development in this area of research.