Direct electrical contact of NAD+/NADH-dependent dehydrogenase on electrode surface enabled by non-native solid-binding peptide as a molecular binder

NAD+/NADH-dependent redox enzymes constitute most known oxidoreductases, but their inherent complexity due to their reliance on the diffusional nature of soluble cofactors limits their application in bioelectrocatalytic systems. Herein, a gold-binding peptide (gbp) is genetically introduced at the NAD+/NADH-dependent formate dehydrogenase (FDH) to add a non-native gold-binding activity to directly wiring the enzyme to the electrode surface. Our gold-binding kinetics studies on the native and synthetic FDHs revealed that the gold-binding properties of the fused enzymes are highly dependent on the fusion site and that the tertiary structure of the fusion enzyme controls the efficiency of gold-binding domain display fusion. As such, the highest gold-binding activity was observed with the fusion of gbp at the C-terminus (FDHgbpC), whereas binding at both termini FDHgbpNC and FDHgbpN appeared to be less active. Moreover, the presence of gbp increased the stability of an integrated enzyme electrode in the bioelectrocatalytic reactions occurring at the enzyme-electrode interface. Direct electrochemical NADH oxidation produced by the enzymatic reaction in the presence of formate and NAD+ was observed at a low overpotential range of −0.45 to −0.15 V vs Ag/Ag+ for all types of enzyme-electrodes, which indicates direct-electrical contact between the cofactor binding site and the electrode surface. The enhanced electron transfer kinetics could be explained by the shorter distance between the cofactor-binding site and electrode surface. Furthermore, the addition of NADH and CO2 increased the reductive catalytic current, which suggested the enzymatic CO2 reduction to formate using a hydride of NADH and a subsequent reduction of the generated NAD+. We demonstrated that biotechnology can directly contact enzymes onto the electrode surface, which has proven to be a reliable method for bio-electronic applications, especially involving NAD+/NADH-dependent enzymes.