Molecular oxygen-carrying proteins (like hemoglobin) in our blood transfer the oxygen molecules (O2) to cells - where the O2 diffuses inside at relatively low concentrations. O2 can for example, oxidize -SH groups that may be present (important sulfur components of proteins) to form S-S interactions; however, our cells and other aerobic organisms' cells have lots of different enzymes which re-reduce S-S (disulfides) back to -SH. There are other enzymes which also can maintain reduced components which may have been oxidized by O2. Therefore, while the presence of O2 can be harmful - it is just that over evolution, many mechanisms have been developed which protect cellular constituents from the potentially harmful effects of O2. There are other things going on as well.

All air-breathing organisms get much of their energy to sustain life by "directing" electrons gotten from certain "food" molecules to "land" on O2 (called aerobic respiration). When electrons land on O2, the O2 picks up some hydrogen atoms as well, and becomes the substance we all know and love - water... H2O. The energy of the electrons "falling" down an energy hill to land on O2 - the fall occurs via what is called the electron-transport system - allows cells to make a very important substance called ATP.... the "fuel" which drives the reactions that allow life. Now, we can tolerate O2 in our cells primarily because O2 has trouble picking up the first of the two electrons required for reduction to H2O. However, once that first e- is associated, O2 is converted to a _very_ dangerous negatively-charged free-radical molecule called superoxide anion (O2-). This superoxide anion will then take 3 additional e- from something if at all possible... and thus this form of oxygen can oxidize almost anything... - us included.....

Therefore, as the electrons fall down the hill via the electron transport system, at the last step where e- are being accepted by O2 to be reduced to H2O, there are actually _two_ molecules of O2 held there _very_ tightly by the "cytochrome a" component of the enzyme cytochrome oxidase (one of the important enzymes which transfers electrons down the hill). It is _only_ when each of these two tightly-held O2 have picked up 2e- each (total of 4 electrons), that the two O2 molecules are released into the cell. Each of these O2 molecules then immediately acquires 2 protons (positively-charged hydrogen ions), and 2 molecules of H2O are generated. There are additional safeguards:

The superoxide anion mentioned above (O2-), if available, can be converted by an enzyme called superoxide dismutase to hydrogen peroxide (H2O2) and molecular oxygen (O2) by the following reaction:
2O2- + 2H+ => O2 + H2O2
Obligate aerobes - organisms which absolutely require O2 (many, many microorganisms, including bacteria, and a bunch of macroorganisms - like us) have this enzyme. As it turns out, H2O2 is also toxic - bummer! - get rid of one thing and another one pops up! But - cells are "smart." While H2O2 is dangerous, this stuff is significantly less toxic than superoxide anions. But, cells may also have yet another enzyme (our cells do - as do obligate aerobes) called catalase. This enzyme converts H2O2 to H20 and O2 in the following reaction:
2H2O2 => 2H2O + O2
So, all strict aerobes have _both_ of these enzymes.. and although some bacteria do not have catalase, most _will_ have superoxide dismutase. If they don't have superoxide dismutase, then they'll have an enzyme which uses some metal ion (like manganese or some such thing) which destroys the superoxide anions. Therefore, O2 is necessary for all aerobes, but certain forms of O2 are dangerous as well. Consequently, there are very sophisticated systems which protect cells from the potentially toxic effects of this critical life-giving substance.

As an aside, it is the presence of superoxide anions and hydrogen peroxide in cells which ascorbic acid (Vitamin C) is supposed to help get rid of - it is for this reason - the harmful oxidizing properties of O2 - that some people feel that taking a bunch of Vitamin C could help diminish the potential harm to our cells, since Vitamin C is a reducing agent. This belief remains controversial - that is, "the jury is still out" on the use of Vitamin C for this purpose. It is best NOT to take mega-anything without first consulting with your physician - some important, necessary substances are fine in tiny amounts, but can be exceedingly dangerous in large amounts. So - always check with your physician - get as much accurate information as you can before taking larger-than-normally-recommended amounts of anything.

Book: Don't Touch That Doorknob!

Copyright John C. (Jack) Brown, February, 1996