Mediated electron transfer with monooxygenases—Insight in interactions between reduced mediators and the co-substrate oxygen

One of the most important obstacles to overcome in biocatalysis with monooxygenases is the enzymeʹs dependency on the costly redox cofactor NAD(P)H. Electrochemical regeneration systems, in which an electrode serves as electron donor, provide an alternative route to enzymatic redox reactions. Mediators are often used to accelerate electron transfer between electrode and enzyme. We investigated the mediated bioelectrochemical conversion of p-xylene to 2,5-dimethylphenol (2,5-DMP) by a P450 BM3 variant and were able to produce 2,5-DMP electrochemically. Due to the fact that mediator reduction is limited by the electrode surface a scale-up was performed. However, increasing the electrode surface area to reactor volume ratio led to a drastic increase in cathodic oxygen reduction, causing a drop in product formation. It was shown that reduced cobalt sepulchrate reacts with the co-substrate oxygen. Furthermore, the reportedly oxygen stable mediator [Cp*Rh(I)(bpy)H]+ was compared to cobalt sepulchrate. While its turnover frequency is of comparable magnitude to cobalt sepulchrate when transferring the electrons between electrode and enzyme, using NADP+ as intermediary between the mediator and the enzyme significantly increased the mediatorʹs turnover frequency. The rhodium mediator [Cp*Rh(I)(bpy)H]+ does not appear to be significantly more oxygen stable.