Molecular oxygen also has roles in antimicrobial defense1,2. Specifically macrophages use O2 to create cascades of reactive nitrogen intermediates (RNI) and reactive oxygen intermediates (ROI; aka ROS) — these compounds have antimicrobial effects.
The RNI cascade starts with nitric oxide (NO) and is catalyzed by a nitric oxide synthase, which converts arginine and O2 into citrulline and NO.
The ROI cascade starts with superoxide (O2•-) and is catalyzed by a NADPH oxidase, which transfers an electron from NADPH to O2 creating superoxide.
Figure 1 from2 gives an overview of these reactions:
Legend for figure 1:
ROI and RNI production in mammalian cells via phox and NOS: parallel but connecting paths. Nitroxyl anion (NO−), a one-electron reduction product of nitric oxide (•NO), is unlikely to arise from •NO under physiologic conditions, but is considered by some investigators to be a primary and more toxic product of NOS. Reaction of RNI with cysteine sulfhydryls can lead either to S-nitrosylation or to oxidation to the sulfenic acid, as well as to disulfide bond formation (not shown), all of which are potentially reversible. Peroxynitrite anion (OONO−) and peroxynitrous acid (OONOH) have distinct patterns of reactivity, but for simplicity, the text refers to both as peroxynitrite. OONOH spontaneously decomposes via species resembling the reactive radicals, hydroxyl (OH•) and/or nitrogen dioxide (•NO2). When L-arginine is limiting, NOS can produce superoxide (O2⨪) along with •NO, favoring the formation of peroxynitrite.
1: Weiss, G., & Schaible, U. E. (2015). Macrophage defense mechanisms against intracellular bacteria. Immunological reviews, 264(1), 182-203.
2: Nathan, C., & Shiloh, M. U. (2000). Reactive oxygen and nitrogen intermediates in the relationship between mammalian hosts and microbial pathogens. Proceedings of the National Academy of Sciences, 97(16), 8841-8848.