Electrochemical H$_2$O$_2$ - 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 (H$_2$O$_2$) is supplied. In this study, two in-situ electrogeneration modes of H$_2$O$_2$ were established and compared. Experiments under galvanostatic conditions (constant productivity of H$_2$O$_2$) were conducted at current densities spanning from 0.8 mA cm$^{−2}$ to 6.4 mA cm$^{−2}$. For comparison, experiments under H$_2$O$_2$-stat mode (constant H$_2$O$_2$ concentration) were performed. Here, four H$_2$O$_2$ concentrations between 0.06 mM and 0.28 mM were tested. A maximum H$_2$O$_2$ productivity of 5.5 µM min$^{−1}$ cm$^{−2}$ and productivity of 10.5 g L$^{−1}$ d$^{−1}$ were achieved under the galvanostatic condition at 6.4 mA cm$^{−2}$. Meanwhile, the highest total turnover number (TTN) of 710,000 mol mol$^{−1}$ and turnover frequency (TOF) of 87.5 s$^{−1}$ were obtained under the H$_2$O$_2$-stat mode at concentration limits of 0.15 mM and 0.28 mM, respectively. ... mehrThe most favorable outcome in terms of maximum achievable TTN, TOF and productivity was found under the H$_2$O$_2$-stat mode at concentration limit of 0.2 mM. Here, a TTN of 655,000 mol mol$^{−1}$, a TOF of 80.3 s$^{−1}$ and a productivity of 6.1 g L$^{−1}$ d$^{−1}$ were achieved. The electrochemical H$_2$O$_2$-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.