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I know that molecular oxygen (O2) serves as the electron acceptor in Complex IV of the electron transport chain, which maintains the proton gradient that produces ATP for, as far as I know, every cell in the human body.

Is this the only reason we need molecular oxygen for life, or are there other vital functions (i.e. necessary for life) that molecular oxygen serves within the human body?

Edit

For clarification, there are probably well over a million oxygen-containing compounds, from water all the way up to the largest proteins, which perform repeatable roles in basically every system of the body. But in cases where oxygen is exhausted, specifically consumed requiring a continuous supply of new oxygen, what roles does oxygen serve with the exception of serving as electron acceptor in Complex IV of the electron transport chain?

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  • $\begingroup$ Are you asking about molecular oxygen, rather than all the roles of oxygen containing compounds? I'm not sure why you're getting the downvotes, but maybe clarifying this will help you get a more favorable response ... $\endgroup$
    – tyersome
    Jan 19 '20 at 17:39
  • $\begingroup$ @tyersome The original intention was to capture purposes served that consume oxygen in a one-off process, rather than incorporating it into components that can be reused. The significance here is that the vast majority of the oxygen balance in the body will go into processes that consume oxygen, rather than incorporating it. Yes, oxygen-containing compounds will eventually degrade or be removed from the body and require replacement, but in studying oxygen balance, it is these consuming processes that hold the most sway, and I'd like to gain a better understanding of these processes. $\endgroup$ Jan 19 '20 at 18:59
  • $\begingroup$ So, would it be accurate to say that the question you want to ask is something like: 'Does molecular oxygen (O₂) have any roles other than as an electron acceptor for the ETC?' $\endgroup$
    – tyersome
    Jan 19 '20 at 19:27
  • $\begingroup$ @tyersome Not necessarily, because molecular oxygen can be transformed before it is consumed in a different form, so that would be just slightly too specific a phrasing $\endgroup$ Jan 19 '20 at 20:17
  • $\begingroup$ OK, but the question as written isn't specific enough since it would seem to include roles of water (and almost every biomolecule since they mostly contain oxygen) — i.e. all of biology! ——— How about: 'Is molecular oxygen (O₂) consumed in any process other than by acting as an electron acceptor for the ETC?' $\endgroup$
    – tyersome
    Jan 19 '20 at 20:28
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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: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC34021/figure/F1/

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.

References:

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.

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    $\begingroup$ This is very interesting. However the text under the figure makes the figure itself unreadable on my phone. Could you possibly separate the two? $\endgroup$
    – David
    Jan 20 '20 at 20:21
  • $\begingroup$ @David — Gladly. Does this work better? $\endgroup$
    – tyersome
    Jan 20 '20 at 20:48
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    $\begingroup$ Yes. That’s fine. Thanks. $\endgroup$
    – David
    Jan 20 '20 at 20:49

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