Oxygen-confined photooxidation represents a novel form of oxidative protein damage

Oxidative modifications can disrupt protein folds and functions, and are strongly associated with human aging and diseases. Conventional oxidation pathways typically involve the free diffusion of reactive oxygen species (ROS), which primarily attack the protein surface. Yet, it remains unclear whether and how internal protein folds capable of trapping oxygen (O2) contribute to oxidative damage. Here, we report a novel pathway of protein damage, which we refer to asO2-confined photooxidation. In this process, O2 is captured in protein cavities and subsequently converted into multiple ROS, primarily mediated by tryptophan residues under blue light irradiation. The generated ROS then attack the protein interior through constrained diffusion, causing protein damage. The effects of this photooxidative reaction appear to be extensive, impacting a wide range of cellular proteins,as supported by whole-cell proteomic analysis. This photooxidative mechanism may represent a latent oxidationpathway in human tissuesdirectly exposed to visible light, such as skin and eyes.