Antioxidants are a class of molecules that help prevent the cell from oxidative stress due to build up of free radicals like ROS and RNS.
What is common between dark chocolate, green tea, and strawberries? They’re all rich in molecules called antioxidants. According to scientific professionals, the media and every green tea advertisement out there, antioxidants are great for your health. More specifically though, what do they do and what exactly are these mysterious substances?
Antioxidants, as the name suggests, are molecules that help prevent harmful oxidation in the body. To understand this further, let’s look at what oxidation is, where it occurs in the body, and how antioxidants come to the rescue.
The word oxidation implies the involvement of oxygen in a chemical/metabolic process. Initially, scientists referred to the reaction of adding oxygen to an element as “oxidation”. Reduction, on the other hand, earned the name because it led back to an element in its original state. For example, the oxidation of copper into copper oxide would be an oxidation reaction by the above definition, whereas a return to pure elemental copper would be a reduction reaction.
2Cu + O2 —–> 2CuO (oxidation)
CuO + H2 —–> Cu + H2O (reduction)
This definition changed when electrons were discovered. Chemists realized that oxidation and reduction were not due to oxygen or a lack of oxygen; rather, it was due to the transfer of electrons during a reaction between elements or molecules.
By this definition, oxidation is the loss of electrons and reduction is the gain of electrons. Therefore, each reaction is a redox reaction where one molecule is altered. In the above example of the oxidation of copper, copper would get oxidized by oxygen and lose electrons, while oxygen would get reduced by gaining electrons (those lost by copper).
2Cu + O2 —–> 2[Cu2+][O2-]
(Cu lost electrons, and therefore got oxidized)
(Oxygen gained electrons, and therefore got reduced)
Oxidation and reduction in biological systems:
All metabolic reactions are redox reactions. The most basic metabolic pathway—the breakdown of glucose (glycolysis) to yield energy—is a redox reaction. In this, glucose is broken down through its oxidation (the removal of electrons) and another molecule gets reduced, which then goes on to produce ATP. A molecule with more capability to be oxidized, i.e., a reduced substance, will yield more energy. However, the problem arises when oxidation occurs where you don’t want it to.
Oxygen is vital for aerobic life. Humans inhale to get a fresh dose of O2 and exhale to remove CO2, which we get from the utilization of O2. However, as a result of the many metabolic reactions occurring in the cell, certain byproducts called free radicals, or simply radicals, are formed. One of the chief offenders when it comes to creating free radicals is our cellular mitochondria. These radicals are most commonly linked to oxygen, and are called Reactive Oxygen Species (ROS), while nitrogen-derived radicals are called Reactive Nitrogen Species (RNS).
The different ROS are superoxide anion, peroxide radical, hydroxyl radical, and hydroxyl ion. ROS are unstable in themselves due to the unpaired electrons in their outer shells, which makes them highly reactive.
ROS can react with the molecules of the cell, such as proteins, enzymes, and the lipid membrane (lipid peroxidation), which would prevent these molecules from functioning and interacting in the way they were designed to. A massive load of ROS causes oxidative stress in the cell, leading to cell death if not controlled.
How the body deals with free radicals: Antioxidants
The body’s solution to free radicals are antioxidants, which are a class of molecules that can prevent free radicals from causing harm to the cell’s machinery. Some enzymes are antioxidants, such as superoxide dismutase and glutathione peroxidase, both of which neutralize the threat of ROS buildup and protect the cell from damage. Unfortunately, these enzymes can’t do everything.
There are also non-enzymatic antioxidants that we either get from our diet, such as vitamins E, C and A, as well as secondary plant metabolites (molecules produced from plant metabolism), such as flavonoids, polyphenols, carotenoids etc. There are also antioxidants that our body produces internally, such as glutathione. These chemicals serve to prevent the multitude of assaults that free radicals can wreak on the body. Different antioxidants react with different radicals, so no single antioxidant can completely protect the body from oxidative stress.
If a free radical causes damage to a molecule, it sets off a chain reaction. Non-enzymatic antioxidants, such as vitamin E, interrupt the chain reaction and prevent further damage. One such process is that of lipid peroxidation. Lipid peroxidation is a chain reaction where an ROS reacts with the double bond in the lipid molecule, thus changing the lipid’s chemical structure.
When the antioxidant stops the chain reaction, it becomes a radical itself, but is regenerated either by changing its chemical structure (through tautomerization) or by engaging with other antioxidants. For example, after vitamin E stops the chain reaction of lipid peroxidation, it then becomes a radical, but is regenerated by the activity of vitamin C and glutathione.
Antioxidants are important to help cells remain healthy and function properly. Without them, our cells would not be able to survive. Many wonderful foods, such as tea, dark chocolate, fruits and vegetables (especially berries) are rich sources of antioxidants.
With all the above being said, ROS aren’t entirely bad. There is solid research showing that ROS promote gene transcription and translation, as well as DNA replication (Source). ROS and RNS are also used by the immune system to kill pathogens that invade our body. Clearly, ROS can be as helpful as they are dangerous, so moderation is always recommended. That should give you something to think about the next time you are debating whether to brew a second pot of green tea in a single afternoon!