Hidden route of protein damage through confined oxygen gas

Abstract Oxidative modifications can severely impair protein structure, fold, and function, closely linked to human aging and diseases. Conventional oxidation pathways typically involve the free diffusion of reactive oxygen species (ROS), followed by chemical attacks on the protein surface. Here, we report a hidden route of protein oxidative damage, which we refer to as O 2 -confinement oxidation pathway. This pathway starts with the initial trapping of dissolved molecular oxygen (O 2 ) within protein cavity spaces, followed by interaction with photosensitizing tryptophan residues. The trapped O 2 is then converted to singlet oxygen ( 1 O 2 ), a powerful ROS, through spin-flip electron transfer mechanism under blue light. The generated 1 O 2 within the protein ultimately attacks the protein core residues through constrained diffusion, accelerating the oxidative damage. This alternative photooxidation pathway through the initial O 2 trapping would bypass the antioxidant defense systems which target freely-diffusing ROS, constituting an additional layer of protein oxidative damage in cells and tissues.