Oxygen as a signaling molecule

Molecular O2 is obviously essential for conserving energy in a form useable for aerobic life; however, its utilization comes at a cost: the production of reactive oxygen species (ROS). ROS can be highly damaging to a range of biological macromolecules, and in the past, it was thought that the over-production of these short-lived molecules in a variety of disease states was exclusively toxic to cells and tissues such as the lung. However, recent basic research has indicated that ROS production, in particular the production of hydrogen peroxide (H2O2), plays an important role in both intra- and extra- cellular signal transduction that involves diverse functions from vascular health to host defense. This review summarizes O2's capacity, acting through its reactive intermediates, to recruit the enzymatic anti-oxidant defenses, stimulate cell repair processes, and mitigate cellular damage.