The antioxidant defence system in the plant comprises several different enzymes. Superoxide dismutase (SOD), glutathione peroxidase (GPX) and catalase are the key enzymatic antioxidants of defense system by which the free radicals that are produced during metabolic reactions are removed.
These enzymes not only protect various components of the cells from damages, but also play an important role in plant growth and development by modulating cellular– sub-cellular processes such as mitosis, cell elongation, senescence and cell death.
Enzymatic antioxidants are mainly involved in either preventing the Haber-Weiss reaction or the Foyer– Halliwell–Asada pathway, which reduces the H2O2 and utilises the reducing potential of NADPH.
Superoxide dismutase (SOD) is characterized by the ability of converting the super oxide radical (O¯2) into hydrogen peroxide (H2O2) and oxygen in most aerobic organisms in addition to some non-aerobic organisms. Because of the super oxide radical is the first free radical produced as a result of reducing oxygen formed in many biological systems, SOD is considered the first defense line against the poisoning of oxidative stresses in plants and reducing them.
The enzyme SOD has three variants. The predominant copper-zinc containing enzymes are found in the cytoplasm while manganese SOD is located in the mitochondria. A third type is present extracellularly.
Catalase enzyme (CAT) is found almost in all plant and animal cells as well as bacteria. It prevents harmful hydrogen peroxide from aggregation, so it acts as a catalyst for the conversion of hydrogen peroxide to oxygen and water. It nullifies the effect of hydrogen peroxide that is present intracellularly.
Peroxidase enzyme (POD) enzyme is one of Oxidoreductase and it is common in plant and animal, addition to microorganism. Peroxidase enzyme is characterized by its stimulation capability to convert hydrogen peroxide into water, so it is one of the plant defense enzymes against the oxidative stresses.
Glutathione peroxidase and glutathione reductase are enzymes that act as antioxidants. The reduced form of glutathione is defensive in nature. The oxidized form is not protective. Reduced glutathione helps to neutralize hydrogen peroxide produced inside the cell. These enzymes are key players in preventing increased levels of oxidative stress. This repeated oxidization and reduction of glutathione makes it a free radicals scavenger.
Enzymatic antioxidants
