Direct electron transfer-type bioelectrocatalytic interconversion of carbon dioxide/formate and NAD+/NADH redox couples with tungsten-containing formate dehydrogenase

Abstract

Tungsten-containing formate dehydrogenase (FoDH1) with a molecular mass of 170kDa from Methylobacterium extorquens AM1 catalyzes the oxidation of formate (HCOO−) to carbon dioxide (CO2) with NAD+ as a natural electron acceptor in solution. FoDH1 does not produce any direct electron transfer (DET)-type bioelectrocatalytic wave at planar electrodes, but can adsorb on and communicate with mesoporous carbon electrodes. The curvature effect of mesoporous structures seems to increase the number of enzymes with orientations suitable for electrochemical communication. However, adsorption proceeds slowly on Ketjen Black-modified electrode and the catalytic current density remains low. Most probably, the size of the mesopores is too small to effectively trap FoDH1. The adsorbed FoDH1 catalyzes DET-type bioelectrocatalytic interconversion of the CO2/HCOO− and NAD+/NADH redox couples. Most probably, one of the iron–sulfur clusters located near the enzyme surface communicates with mesoporous electrodes. When the communication proceeds effectively, FoDH1 behaves as a novel bidirectional catalyst for the substrates, since FoDH1 can realize fast uphill intramolecular electron transfer. The non-covalently bound flavin mononucleotide (FMN) cofactor in FoDH1 is dissociated from some FoDH1 molecules and adsorbs on the mesoporous electrode to give a symmetrical surface-confined redox wave. Although adsorbed FMN cannot participate in mediated electron transfer (MET)-type bioelectrocatalysis, dissociated FMN in solution works as a mediator for MET-type bioelectrocatalysis of the HCOO− oxidation at planar electrodes.