Photoirradiation Generates an Ultrastable 8-Formyl FAD Semiquinone Radical with Unusual Properties in Formate Oxidase

Formate oxidase (FOX) was previously shown to contain a noncovalently bound 8-formyl FAD (8-fFAD) cofactor. However, both the absorption spectra and the kinetic parameters previously reported for FOX are inconsistent with more recent reports. The ultraviolet visible (UV-vis) absorption spectrum reported in early studies closely resembles the spectra observed for protein-bound 8-formyl flavin semiquinone species, thus suggesting FOX may be photosensitive. Therefore, the properties of dark and light-exposed FOX were investigated using steady-state kinetics and site-directed mutagenesis analysis along with inductively coupled plasma optical emission spectroscopy, UV-vis absorption spectroscopy, circular dichroism spectroscopy, liquid chromatography and mass spectrometry, and electron paramagnetic resonance (EPR) spectroscopy. Surprisingly, these experimental results demonstrate that FOX is deactivated in the presence of light through generation of an oxygen stable, anionic (red) 8-fFAD semiquinone radical capable of persisting either in an aerobic environment for multiple weeks or in the presence of a strong reducing agent like sodium dithionite. Herein, we study the photoinduced formation of the 8-fFAD semiquinone radical in FOX and report the first EPR spectrum of this radical species. The stability of the 8-fFAD semiquinone radical suggests FOX to be a model enzyme for probing the structural and mechanistic features involved in stabilizing flavin semiquinone radicals. It is likely that the photoinduced formation of a stable 8-fFAD semiquinone radical is a defining characteristic of 8-formyl flavin-dependent enzymes. Additionally, a better understanding of the radical stabilization process may yield a FOX enzyme with more robust activity and broader industrial usefulness.