Bidirectional electro-enzymatic reaction of coenzyme F₄₂₀ using benzyl viologen and F₄₂₀-dependent sulfite reductase

Coenzyme F₄₂₀ is recognized as a crucial electron carrier in methane-generating metabolism but, beyond this, has garnered significant attention for its role in diverse microbial physiologies and relevance in industrial, medical, and environmental applications. However, one limitation of current application of F₄₂₀ is the necessity of chemical electron donors for its reduction. In this study, an electrochemical reaction system was designed to facilitate electron transfer between the electrode and F₄₂₀ using F₄₂₀-dependent sulfite reductase (Fsr) as the catalyst and benzyl viologen (BV) as the redox mediator. Photometric analysis and cyclic potential scanning demonstrated that the occurrence of bidirectional (reversible) electrochemical oxidation and reduction of F₄₂₀ in this system depended on the electrode potential. The formal redox potential of F₄₂₀ in this system was -540 mV vs. Ag|AgCl|sat. KCl, which aligned with values previously determined using biochemical assays.