Experimental and Theoretical Insights into Bienzymatic Cascade for Mediatorless Bioelectrochemical Ethanol Oxidation with Alcohol and Aldehyde Dehydrogenases

The efficient utilization of biomass fuels is a critical component of a sustainable energy economy. Via respiration, acetic acid bacteria can oxidize biomass ethanol into acetic acid using membrane-bound alcohol and aldehyde dehydrogenases (ADH and AlDH, respectively). Focusing on the ability of these enzymes to interact directly and electrically with electrode materials, we constructed a mediatorless bioanode for ethanol oxidation based on a direct electron transfer (DET)-type bienzymatic cascade by ADH and AlDH. The three-dimensional structural data of ADH and AlDH elucidated by cryo-electron microscopy were valuable for effectively designing electrode platforms with multi-walled carbon nanotubes (MWCNTs) and pyrene derivatives. DET-type bioelectrocatalysis by ADH and AlDH was improved by using 1-pyrene carboxylic acid-functionalized MWCNT. The catalytic current densities for bienzymatic ethanol oxidation were recorded at the bioanodes modified by various ADH/AlDH ratios. The reaction model was constructed by focusing on the competitive ad-sorption of two enzymes on the electrode surface and the collection efficiency of the intermediately produced acetaldehyde. The power output of an ethanol/air biofuel cell using the bienzymatic bioanode reached 0.48 ± 0.01 mW cm–2, which is the highest value reported for ethanol biofuel cells. In addition, the Faraday efficiency of acetate production by the cell reached 100 ± 4%. This study will lead to efficient conversion of biomass fuels based on a multi-catalytic cascade system.