José A. Hernández Cortés (Scientist Researcher, CEBAS-CSIC, Murcia)
As we indicated in the previous chapter, the use of the molecular oxygen (O2) as final electron acceptor in the mitochondrial respiratory chain, supposed a huge benefit to the life evolution in the earth. However, the consequence of the O2 transformation into water (H2O) in the cellular metabolism is the formation of Reactive Oxygen Species (ROS), very toxic intermediates to the cellular components. O2 is a free radical, and it has two impaired electrons with the same spin quantum number. This spin restriction makes O2 to accept its electrons in univalent steps, leading to the so-called ROS, sometimes also referred as AOS (activated oxygen species) or ROI (reactive oxygen intermediaries).
But, who are they and what effects do they have?
Four electrons (e–) and four protons (H+) are required for the full reduction of dioxygen to water, but it can be produced in single steps, giving rise to superoxide radicals (O2.-), hydrogen peroxide (H2O2) and hydroxyl radicals (.OH). All these intermediates are chemically reactive and biologically toxic. This toxicity is reflected by their short half-life for reacting with cellular components or molecules. In plant tissues, about 1-2% of oxygen consumption leads to the generation of ROS under normal conditions. So, ROS are an inevitable consequence of aerobic respiration. This percentage is higher in environmental stress conditions, being ROS involved in the cellular damages produced in such situations.
Thus, most of the ROS are formed by e– transfer to O2. However, others ROS, such as the singlet oxygen (1O2) is generated by the energy transfer from triplet chlorophyll to O2. As general damages, the ROS cause the inhibition of some enzymes, chlorophylls degradation, produce damages in membranes and in the DNA (can give room for unusual mutations), protein oxidation, etc.
In human beings, the ROS are implicated in different diseases such as rheumatoid arthritis, hepatitis, Alzheimer’s, Parkinson’s, muscular dystrophy, multiple sclerosis, cataracts, macular degeneration, autoimmune diseases, etc.
However, not everything is bad. ROS have also positive effects and they are involved in other processes such as cell signaling, lignin biosynthesis, seed germination, the cell wall polysaccharide metabolism, hypersensitive response etc, and in response to stressful situations. Under normal conditions, a balance between ROS production and scavenging take place. However, if an imbalance occurs, an oxidative stress is produced.
More information about ROS
The O2.- is the first reduction product of the O2. It can be transformed spontaneously, or by enzyme action, in H2O2:
O2.- + O2.- + 2 H+ = H2O2 + O2
In this reaction, an O2.- radical is reduced (to H2O2) and the another O2.- radical is oxidised (to O2). This process is known as dismutation, which implies that the same molecule suffers reduction and oxidation at the same reaction.
The superoxide can be protonated (to accept an H+) to form the perhydroxyl radical (HO2.-), which can cross membranes and act as a signaling molecule.
The H2O2, although it is a ROS, it is not a free radical (it has paired electrons). It also can cross membranes and it has longer half-life than other ROS. These two characteristics (longer half-life and their high permeability across membranes make it to be accepted as a second messenger) make that H2O2 can be considered as a signaling molecule. However, the true toxicity of O2.- and H2O2 is its capacity to generate .OH in presence of transition metals (such as the Fe2+ or the Cu2+):
Fe2+ + H2O2 = .OH + OH– + Fe3+ (Fenton’s reaction)
Next, the O2.- reduces the ferrous ion (Fe3+) to produce Fe2+ and allows that the previous reaction can continue:
O2.- + Fe3+ = O2 + Fe2+
The sum of both reactions is known as “Haber-Weiss reaction”:
O2.- + H2O2 = .OH + OH– + O2
The hydroxyl radical (.OH) is the most powerful oxidant known in the biological systems, which joined to its very low lifetime (1 ns), makes itself very toxic. It can react with any biological molecule at the same time it is formed. It causes exceptional mutations in the DNA, attacks to the membranes (membrane lipid peroxidation) and protein oxidation. An excess of .OH, ultimately, leads to the cell death.
In the next chapter, we will talk about the defence mechanisms which plants have designed to cope with ROS.
Halliwell B, Gutteridge JMC (2003) Free Radicals in Biology and Medicine. Third Edition, Oxford University Press Inc, New York, ISBN 0 19 850044 0.