Electrochemical HO - stat mode as reaction concept to improve the process performance of an unspecific peroxygenase.

The electroenzymatic hydroxylation of 4-ethylbenzoic acid catalyzed by the recombinant unspecific peroxygenase from the fungus Agrocybe aegerita (rAaeUPO) was performed in a gas diffusion electrode (GDE)-based system. Enzyme stability and productivity are significantly affected by the way the co-substrate hydrogen peroxide (HO) is supplied. In this study, two in-situ electrogeneration modes of HO were established and compared. Experiments under galvanostatic conditions (constant productivity of HO) were conducted at current densities spanning from 0.8 mA cm to 6.4 mA cm. For comparison, experiments under HO-stat mode (constant HO concentration) were performed. Here, four HO concentrations between 0.06 mM and 0.28 mM were tested. A maximum HO productivity of 5.5 µM min cm and productivity of 10.5 g L d were achieved under the galvanostatic condition at 6.4 mA cm. Meanwhile, the highest total turnover number (TTN) of 710,000 mol mol and turnover frequency (TOF) of 87.5 s were obtained under the HO-stat mode at concentration limits of 0.15 mM and 0.28 mM, respectively. The most favorable outcome in terms of maximum achievable TTN, TOF and productivity was found under the HO-stat mode at concentration limit of 0.2 mM. Here, a TTN of 655,000 mol mol, a TOF of 80.3 s and a productivity of 6.1 g L d were achieved. The electrochemical HO-stat mode not only offers a promising alternative reaction concept to the well-established galvanostatic mode but also enhances the process performance of unspecific peroxygenases.