An unexpected alternative viologen electron mediator site in tungsten-containing formate dehydrogenase.

The molecular mechanisms by which artificial electron mediators, such as ethyl viologen (EV), interact with tungsten-containing formate dehydrogenases (FDHs) during reversible CO2 reduction remain poorly understood. Here, we reveal an unexpected alternative mediator interaction site in FDH1 of Methylorubrum extorquens AM1. Removing the native flavin mononucleotide cofactor abolishes NAD+-dependent activity but preserves EV-driven catalysis. Through mutagenesis, kinetic analysis, and molecular docking, we identified a cooperative aromatic network-comprising residues F232, F471, and Y329-that stabilizes EV via stacking interactions near the proximal B1 iron-sulfur cluster. Disrupting these residues impairs EV-mediated electron transfer without destabilizing global structure. These findings reveal a dual interaction strategy for artificial mediators in tungsten FDHs, offering a structural framework to rationally engineer biocatalysts for CO2 conversion.