In situ NADH regeneration coupled to enzymatic CO2 reduction: From fundamental concepts to recent advances

Biocatalytic CO₂ reduction using enzymes is among the most effective methods for producing hydrocarbon fuels from CO₂ with high selectivity. Many of these enzymes rely stoichiometrically on the NADH cofactor to provide the necessary reducing equivalents. However, the high cost and significant demand for the cofactor hinder the industrial feasibility of this method. NADH regeneration allows for the recycling of NADH at catalytic concentrations, thereby eliminating the stoichiometric requirement. Consequently, biocatalytic CO₂ reduction should be viewed as a two-part cooperative process involving an NADH regenerating unit and an enzymatic CO₂ reducing unit. This comprehensive review outlines the concepts and mechanisms underlying NADH regeneration and CO₂ reduction. In the first part of the review, various methods of NADH regeneration and related research are discussed. The second part addresses the basic concepts regarding enzymatic CO₂ reduction, followed by a review of research on CO₂ reduction coupled with NADH regeneration. This includes a review of single and multi-enzyme cascade systems for the production of primarily three products: formate, formaldehyde, and methanol. Various strategies to maximize the synergy between the two units and maintain their stability, such as compartmentalization, immobilization, and encapsulation of the multiple reaction partners, are examined. Toward the end, nature-inspired artificial photosynthetic systems are discussed. These systems facilitate the conversion of solar energy into fuels through CO₂ reduction, the carbon source, and water splitting, the electron source. This review highlights the advancements and challenges in the field of biocatalyzed CO₂ reduction, particularly concerning hybrid cofactor utilization systems.