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

Abstract 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 reveal a novel pathway of protein damage, which we refer to as O2-confined photooxidation. In this process, O2 is captured in protein cavities and subsequently converted to singlet oxygen (1O2), a highly reactive ROS, under blue light irradiation. The generated 1O2 then attacks the protein interior through constrained diffusion, causing protein damage. The effects of this photooxidative reaction are extensive, affecting a wide range of cellular proteins, as supported by whole-cell proteomic analysis. This photooxidative mechanism may represent a latent oxidation pathway in human tissues directly exposed to visible light, such as skin and eyes.