Cholesterol oxidase from Streptomyces lividans (SlChOx) has been investigated by spectroelectrochemistry in the pH range between pH 5 and pH 9. SlChOx is a monomeric flavoenzyme with a molar mass of about 58 kDa that contains a non‐covalently bound flavin adenine dinucleotide (FAD) cofactor. Spectroelectrochemical measurements were carried out in the presence of different one and two electron redox mediators and methylene blue only, as methylene blue does not contribute significantly to the spectral changes between 350 nm and 500 nm. During the stepwise reduction and reoxidation, the presence of a stable singly reduced flavosemiquinone radical anion was observed. The pH dependent midpoint potentials (Em, pH7≈0 mV) and the redox potentials for the first and second transition Fl(quinone)/Fl(semiquinone rad anion) and Fl(semiquinone rad anion)/Fl(hydroquinone anion) were determined. A pH dependency of about −30 mV and −50 mV per pH unit was determined for the first and second reduction, respectively.

The Cover Feature shows the gold capillary working electrode used for the spectroelectrochemical investigation of cholesterol oxidase (ChOx). The spectral changes collected during the ongoing reduction of ChOx (by mediated electron transfer, MET) in the presence of methylene blue as redox mediator are shown. Furthermore the titration curves at 374 and 454 nm for the reduction and reoxidation as well as the possible redox states of the FAD cofactor are depicted. More information can be found in the Article by M. Heinelt et al. on page 2174 in Issue 8, 2019 (DOI: 10.1002/celc.201801416).