Oxygen is the vital requirement for living beings. This is required for the complex metabolic pathways. The paradox of oxygen requirement is the high reactivity of the oxygen molecule. These reactive oxygen molecules damage living organisms by producing reactive oxygen species.
Reactive oxygen species (ROS)
The reactive oxygen species produced in cells include:
- Hydrogen peroxide (H2O2)
- Hypochlorous acid (HOCl) & hypochlorite radical
- Free radicals such as the hydroxyl radical (·OH)
- Superoxide anion (O2−)
- nitric oxide radical
- singlet oxygen
- lipid peroxides
These can react with membrane lipids, nucleic acids, proteins and enzymes, and other small molecules.
Why are free radicals unstable?
These free radicals contain an unpaired electron. This makes them unstable and they seek out and capture electrons from other substances in order to neutralize themselves. This initially stabilizes the free radical but generates another in the process. Soon a chain reaction begins and thousands of free radical reactions can occur within a few seconds on the primary reaction.
The hydroxyl radical
The hydroxyl radical is particularly unstable and will react rapidly and non-specifically with most biological molecules. This species is produced from hydrogen peroxide in metal-catalyzed redox reactions such as the Fenton reaction. These reactions lead to lipid peroxidation and oxidizing DNA or proteins to mediate cell damage.
The superoxide anion
The superoxide anion is produced as a by-product of several steps in the electron transport chain. There is the reduction of coenzyme Q in complex III. This forms a highly reactive free radical as an intermediate called the (Q·−). This intermediate leads to electron "leakage", when electrons jump directly to oxygen and form the superoxide anion. Peroxide is also produced from the oxidation of reduced flavoproteins.
Free radicals in plants
In plants, as well as algae, reactive oxygen species are also produced during photosynthesis, particularly under conditions of high light intensity. This is prevented by carotenoids in photoinhibition. These antioxidants react with over-reduced forms of the photosynthetic reaction centres to prevent the production of ROS.
Each of the living organisms thus have a complex network of antioxidant metabolites and enzymes that act together to prevent oxidative damage to cellular components such as DNA, proteins and lipids. These systems of antioxidants prevent these reactive species from being formed or remove them before they can damage vital components of the cell.
Oxidative stress, imbalance and disease
Despite an effective antioxidant network there are several diseases that can result from an imbalance between pro-oxidants and antioxidants. The term “oxidative stress” has been coined to represent a shift towards the pro-oxidants. This stress can be due to several environmental factors such as exposure to pollutants, alcohol, medications, infections, poor diet, toxins, radiation etc.
Oxidative damage to DNA, proteins, and other macromolecules may lead to a wide range of human diseases most notably heart disease, lung diseases like asthma, and cancer.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)