Last time I posted I told you about why oxygen is a great terminal electron acceptor in aerobic (oxygen-using) organisms. It is very electron hungry, meaning it releases large amounts of energy when it takes electrons. Remember that this is good because we can then harvest all of that energy and use it to live. If oxygen weren’t so electron hungry, we would get less bang for our buck so to speak.
At the same time, oxygen is a strange molecule- it naturally has a kinetic barrier to accepting an electron pair. Consequently, oxygen can be used in a more controlled way and doesn’t go around randomly exploding.
However, electron *pair* is a key point. Although oxygen needs two electrons and it is barred from taking an electron pair, sometimes oxygen does grab one electron at a time. Remember that when you take electrons you are called an oxidant. When you are an oxidant and you take an electron, you become “reduced.” The thing you take the electron from is “oxidized,” because it has been used and abused by an oxidant.
When oxygen grabs just one electron, it turns from Mr. Hyde into Dr. Jekyll – because the partial reduction product (oxygen is only partially reduced since it still needs another electron) of oxygen and the molecules that it produces are the Reactive Oxygen Species, ROS for short, and these are a chemical weapon.
Meet the ROS
When oxygen grabs an electron, it turns into a molecule called SUPEROXIDE (see picture). That is a cool name. Superoxide is a free radical. Before I started this project, I kept hearing about free radicals and how bad they are, but I never had the faintest idea what they were or why they are bad. What IS a free radical – and why is everyone obsessed with those things called “antioxidants”??
A free radical is a molecule that has one unpaired electron. A molecule with one unpaired electron will do almost anything to get another one to match. Molecules do NOT like unpaired electrons. So free radicals are often dangerous because they can and will react with almost anything to get another electron.
And when free radicals react with another molecule, stealing an electron, they not only damage that molecule, sometimes irreparably, but they often just go and create another free radical since the victim of electron theft is usually left with an unpaired electron itself. This is how a free radical chain reaction begins.
The chain reaction only ends when two free radicals combine (resulting in one complete electron pair) or when the free radical meets an enzyme or molecule called an “antioxidant.” Antioxidants quell free radicals simply because they either have an electron to give, or facilitate the donation of an electron to the free radical.
So what happens to superoxide, these free radicals that oxygen can produce? Well, superoxide in turn generates other ROS. It spontaneously and catalytically (catalyzed by a special enzyme that is) dismutates into hydrogen peroxide and water.
Hydrogen peroxide, while not a free radical (see picture), is still extremely reactive and cytotoxic (toxic to cells). Have you ever cut yourself and cleaned the wound with hydrogen peroxide? That is because it kills microbes and thereby prevents infection. This is why organisms make ROS – to kill pathogens or invaders like endophytes – and this is what we call an oxidative burst.
The wild thing is, we humans do this too. And I don’t just mean by pouring peroxide on a cut; I mean we do it internally. Our white blood cells use almost the exact same oxidative burst defense to kill pathogens. The machinery may be a little different, but the products and the goal are exactly the same.
ROS are formed in small amounts all the time in living organisms, as oxygen is accidentally reduced near sites of electron transport. Oxygen likes to live on the wild side. However, our bodies have evolved ways of controlling the damage caused by oxygen’s recklessness.
Incidentally, the failure to control ROS damage contributes to many human diseases, like Parkinson’s disease, Alzheimer’s disease, and heart failure. It also causes oxygen toxicity which is a concern for scuba divers who dive deep.
However, ROS are not simply toxic byproducts of metabolism. Evolution is way too amazing for anything to be that simple. ROS are also important cellular messengers; they can act as signals to induce changes in cells.
And as you now know ROS can even be created on purpose. You know that organisms sort of shoot these molecules out of their cells to destroy microbes and invading pathogens. But you’ll have to wait until next week to find out why and how I’m studying this defense in Antarctic macroalgae…