Cytochrome b$_{562}$ fusion to formaldehyde dehydrogenase enables increased direct electron transfer

Direct electron transfer (DET) between redox enzymes and electrodes is a crucial process in developing biosensors and cofactor-free bio electrosynthesis. However, due to unfavourable orientations, the absence of accessible redox centres, or long electron transfer distances, DET efficiency can be low. Here we present a systematic approach to better understand, evaluate and increase the DET capabilities of a formaldehyde dehydrogenase (F$_{ald}$DH). FaldDH catalyses the reversible oxidation of formaldehyde to formate and is part of the CO$_2$ to methanol enzyme cascade. F$_{ald}$DH from Burkholderia multivorans was fused to a DET capable domain, the soluble subunit of cytochrome b$_{562}$ from Escherichia coli. Fusion proteins with two different linker morphologies and various lengths were designed and biochemically and electrochemically characterised. The longest flexible linker had minor effects on biochemical constants and exhibited the highest increase in current density. We also identified an undesirable side-reaction between formaldehyde and basic amino acids to interfere with the electrochemical measurements and therefore normalised all currents to the percentage of basic amino acids on the solvent exposed surface area of the protein. ... mehrThis enabled us to present the first protein engineering approach to increase DET in this enzyme, resulting in a 1.6-fold increase in current density, compared to the wild-type enzyme.