Selective Redox Tuning Enables Potent Intracellular Reduction of Nicotinamide Cytosine Dinucleotide.

The ubiquitous nicotinamide adenine dinucleotide (NAD) engages in diverse biological processes, leading to non-selective energy transfer toward target synthetic reactions. To achieve selective energy transfer in complex biological systems, we previously constructed artificial systems mediated by the non-natural cofactor nicotinamide cytosine dinucleotide (NCD), which can be specifically recognized by engineered enzymes with minimal cross-talk with natural cofactors. For enhanced energy transfer and higher product yields, efficient conversion of NCD to NCDH is required to deliver reducing power in NCD-mediated biosynthetic pathways. Here, we established a comprehensive strategy for selective reduction in intracellular NCD. First, coupled enzymatic colorimetric assays with high specificity were validated for quantifying NAD, NADP, and NCD. With phosphite as the energy source, we selectively elevated the intracellular NCDH/NCD ratio with minimal perturbation to NADH/NAD and NADPH/NADP ratios in E. coli. To overcome the limitation of phosphite transmembrane transport, cell-free systems were constructed to confirm that phosphite could drive near-complete NCD reduction. Finally, cells were treated with polymyxin B, which promoted phosphite uptake and thereby enabled maximal reduction in intracellular NCD. An NCDH/NCD ratio of 47 was achieved, demonstrating that 98% of the intracellular NCD pool existed in the reduced form. This work demonstrates that NCD can function as an independent redox cofactor for selective regulation, providing viable strategies for artificial cofactor-driven biosynthesis.