The Reversible Electrochemical Interconversion of Formate and CO₂ by Formate Dehydrogenase from Cupriavidus necator

The bacterial molybdenum (Mo)-containing formate dehydrogenase (FdsDABG) from Cupriavidus necator is a soluble NAD⁺-dependent enzyme belonging to the DMSO reductase family. The holoenzyme is complex and possesses nine redox-active cofactors including a bis(molybdopterin guanine dinucleotide) (bis-MGD) active site, seven iron-sulfur clusters, and 1 equiv of flavin mononucleotide (FMN). FdsDABG catalyzes the two-electron oxidation of HCOO^- (formate) to CO₂ and reversibly reduces CO₂ to HCOO^- under physiological conditions close to its thermodynamic redox potential. Here we develop an electrocatalytically active formate oxidation/CO₂ reduction system by immobilizing FdsDABG on a glassy carbon electrode in the presence of coadsorbents such as chitosan and glutaraldehyde. The reversible enzymatic interconversion between HCOO^- and CO₂ by FdsDABG has been realized with cyclic voltammetry using a range of artificial electron transfer mediators, with methylene blue (MB) and phenazine methosulfate (PMS) being particularly effective as electron acceptors for FdsDABG in formate oxidation. Methyl viologen (MV) acts as both an electron acceptor (MV²⁺) in formate oxidation and an electron donor (MV^+•) for CO₂ reduction. The catalytic voltammetry was reproduced by electrochemical simulation across a range of sweep rates and concentrations of formate and mediators to provide new insights into the kinetics of the FdsDABG catalytic mechanism.