Figure 2.

Metabolism of reactive oxygen species. Activated NADPH oxidase catalyzes the transfer of electrons from NADPH to molecular oxygen generating superoxide anions (${\text{O}}_2^{-}$) as the primary product. To minimize damage, cells are equipped with antioxidant scavenging enzymes, such as superoxide dismutase (SOD), which dismutates ${\text{O}}_2^{-}$ to non-radical species hydrogen peroxide (H₂O₂), and catalase. SOD and glutathione peroxidase can further convert these species into water, which limit damages to the host. On the other hand, ${\text{O}}_2^{-}$ can be converted to other reactive oxygen species that can damage nucleic acids, proteins, and cell membranes. Granule-localized myeloperoxidase (MPO) can convert H₂O₂ to hypochlorous acid (HOCl), which can enhance clearance of invading pathogens. MPO can also directly convert ${\text{O}}_2^{-}$ into singlet oxygen (¹O₂^*). In addition, ferric iron can convert ${\text{O}}_2^{-}$ and H₂O₂ into hydroxyl radical (OH). Components of the NADPH oxidase: gp91^phox (green), gp22phox (light green), regulatory factors (purple).