Ultrafast photooxidation of semireduced flavin in fatty acid photodecarboxylase

The initial photoproduct of the natural photoenzyme fatty acid photodecarboxylase involves the flavin anion radical flavin adenine dinucleotide (FAD^•-). Using spectrally resolved ultrafast transient absorption spectroscopy, we demonstrate that FAD^•- photoexcitation in the absence of substrate leads to the formation of the oxidized flavin FAD_ox (the resting state in the catalytic cycle) within 100 femtoseconds. While this feature is similar to that occurring in flavoprotein oxidases, the ensuing photocycle is more complex. Upon excitation at the lowest-energy transition, the ejected electron is initially captured as a hydrated electron (_e^-_H) before transferring to a secondary acceptor in 2.5 picoseconds and returning to the flavin in 37 picoseconds. This implies that _e^-_H can be generated within a protein environment, an unprecedented finding. This assessment is supported by molecular dynamics simulations showing an expansion of the flavin-binding pocket without substrate, allowing water molecules to fill the void. Our results may pave the way to developing unconventional photocatalytic processes.