Ultrafast Dynamics of Photoinduced Electron Transfer and Decarboxylation in the Flavoenzyme Lactate Monooxygenase

Lactate monooxygenase (LMO) is a flavoenzyme that utilizes its flavin mononucleotide (FMN) cofactor for the oxidation of lactates. Without the substrate, the LMO shows a unique histidine-to-FMN electron transfer (ET) when the cofactor is excited by blue light. Besides its native catalytic functions, this enzyme binds various carboxylates and catalyzes photodecarboxylation via an ET mechanism. However, the ET dynamics in the LMO have not been fully characterized. The link between the substrate structure and photodecarboxylation efficiency is unclear. Here, we applied site-directed mutagenesis and femtosecond-resolved transient absorption spectroscopy to investigate the ultrafast dynamics of the histidine-FMN* ET and the photodecarboxylation of different carboxylates. Global analysis of the transient data gives the absorption spectra of vibrationally hot FMN^•-†/HisH^•+ and other ET intermediates. Reaction dynamics with different substrates suggest that the backward electron transfer rate, rather than the forward electron transfer rate, greatly impacts the catalytic efficiency.