Bioelectrocatalysis based on direct electron transfer of fungal pyrroloquinoline quinone-dependent dehydrogenase lacking the cytochrome domain

Direct electron transfer (DET) between oxidoreductases and electrodes is crucial for understanding the fundamental features of redox proteins as well as developing mediator-free bioelectronics devices. Bioelectrocatalysis with pyrroloquinoline quinone (PQQ)-dependent dehydrogenases is promising for both biosensors and biofuel cells. However, a limited number of studies have reported DET between PQQ in the enzyme and the electrode. Here, the PQQ domain (containing only the PQQ cofactor) of the fungal PQQdependent pyranose dehydrogenase from Coprinopsis cinerea, was studied in DET with various electrodes. A 2-mercaptoethanol self-assembled monolayer (SAM)-coated polycrystalline gold electrode was found to give excellent DET for the PQQ domain, resulting in 10-times the catalytic current density compared to bare glassy carbon. The amount of immobilized PQQ domain was determined to be 8.2 +/- 0.4 pmol/cm(2) by using a 27 MHz quartz-crystal microbalance (QCM), suggesting an approximate protein monolayer formation on the SAM modified gold surface. To improve current density, gold nanoparticles (AuNPs) were modified on top of polycrystalline gold electrodes. Importantly, high catalytic current densities of 1.6 mA/cm(2) for the oxidation of L-fucose were achieved under optimized conditions. Together, these results demonstrate highly efficient DET to PQQ in the active site of the fungal PQQ-dependent dehydrogenase. Thus, our PQQ-domain/SAM/AuNPs coated polycrystalline gold electrode could serve as an exquisitely sensitive biosensor and a highly active sugar oxidizing anode for biofuel cells. (C) 2020 Elsevier Ltd. All rights reserved.