Capturing the proton relay intermediate— Unveiling the unified mechanism of light-state photocycles in BLUF domains

Introduction:

The photoreaction cycle of the blue-light receptor protein BLUF domains involves forward and reverse proton-coupled electron transfer (PCET) reactions, yet a unified cross-species mechanism is lacking. By capturing the key proton relay intermediates, for the first time we resolved the photoreaction mechanism of the BLUF domain light state using a unified kinetic model.

Article Interpretation:

Proton-coupled electron transfer reactions are crucial for biological and chemical processes. The BLUF domain is a widespread blue-light switching protein domain in nature, whose photoactivation involves complicated forward and reverse proton-coupled electron transfer reactions. The key reverse PCET step has not been resolved, and the elementary reactions are still unclear. Through femtosecond transient absorption spectroscopy and target data analysis, the authors have captured a key proton relay intermediate involving tyrosine in a across species manner, unveiling a unified mechanism of the light-state photoreaction cycle in BLUF domains. Specifically, the photoreaction cycle involves four elementary steps: a forward concerted electron-proton transfer (CEPT), proton rocking, and a reverse concerted proton-electron transfer (CPET). It is important to highlight that the final reverse CPET step is consistent in both light and dark-state photoreactions, the ultrafast timescale of which is crucial for the photoswitching functionality of the BLUF domain. This research will help to fully reveal the molecular mechanisms of photoactivation in wild-type BLUF domains.