# Cell Response to Oxidative Stress in Antarctic Filamentous Fungi
## 5 Oxidative Stress and Antioxidant Defence
### Antioxidants and Antioxidant Defence
#### 5.2.1 Enzymatic Antioxidants  
The antioxidant enzymes SOD, CAT, and GPx play a crucial role in the ability of biological systems to fend off attacks from the free radicals. SOD (**EC** 1.15.1.1.) eliminates  
Figure 7: Classification of antioxidants. \({}^{\bullet}\)O\({}^{-}\)\({}_{2}\) radicals in a \"ping-pong\" mechanism. The enzyme catalyzes the dismutation of two molecules of \({}^{\bullet}\)O\({}^{-}\)\({}_{2}\) to hydrogen H\({}_{2}\)O\({}_{2}\) and O\({}_{2}\), which limits \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\) availability [Reaction 3]. This situation led to the reduction of iron and a decrease in the amount of H\({}_{2}\)O\({}_{2}\) formed by the Haber-Weiss/Fenton reaction [127, 128]. \({}^{\bullet}\)O\({}_{2}^{-}+{}^{\bullet}\)O\({}_{2}^{-}+2\)H\({}^{+}\)\(\overset{\text{SOD}}{\rightarrow}\) H\({}_{2}\)O\({}_{2}\) + O\({}_{2}\) [Reaction 3]  
The enzyme was discovered in 1969 by [PERSON] and [PERSON] and has since been the subject of much research. Initially, three types of SOD were identified, and now there are a known four. The first and most obvious similarity between these enzymes is that they all contain redox-active metal ions in the active center: Fe\({}^{2+/3+}\) in Fe-SOD, Mn\({}^{2+/3+}\) in Mn-SOD, Cu\({}^{1+/2+}\) in Cu/Zn-SOD, and Ni\({}^{2+/3+}\) in Ni-SOD [128, 129, 130, 131, 132, 133]. Manganese- and Fe-SOD are evolutionarily older than Cu/Zn-SOD and are derived from the same precursor enzyme [134]. In later stages of phylogenetic development, they diverged, and their amino acid sequences formed two distinct groups. The evolution of the Cu/Zn SOD family proceeded independently, resulting in a significant departure from the characteristics of the initial two enzymes. The distribution of SOD isoenzymes across various organisms is indicative of both the developmental stage of the organism and the specific cellular organelles with which these enzymes are associated [135, 136]. Cu/Zn-SOD is predominantly present in eukaryotic organisms, while Fe-SOD is typically found in prokaryotic organisms. Mn-SOD is observed in both prokaryotic and eukaryotic organisms, and Ni-SOD is specifically located in cyanobacteria and aquatic eukaryotes. Additionally, extracellular SOD (EC-SOD) is produced in the tissues of higher eukaryotes, playing a crucial role in the detoxification of superoxide radicals generated from external sources [137]. These isoenzymes exhibit distinct localization within cellular components [138]. Cu/Zn-SOD is found in the cell wall, cytosol, chloroplasts, and peroxisomes; Fe-SOD is present in the cytosol of prokaryotic organisms and within the chloroplasts of plants; Mn-SOD is located in mitochondria and peroxisomes. Fe-SOD is the most ancient SOD metalloenzyme with a high degree of homology with Mn-SOD. There are two recognized categories of Fe-SOD: homodimeric protein structures with an approximate molecular weight of 20 kDa, which incorporate 1-2 g-atoms of iron in their active site, and tetrameric forms with a molecular weight ranging from 80 to 90 kDa, containing 2-4 g-atoms of iron in their active site. In bacterial cells, manganese superoxide dismutase (MnSOD) is localized in the cytoplasm, whereas in eukaryotic cells, it is found within the mitochondria. MnSOD can exist as either a homotetramer or a homodimer, with molecular weights of approximately 96 kDa and 48 kDa, respectively. Each form comprises four or two identical subunits, with each subunit containing a single manganese atom. Cu/Zn-SOD is predominantly found in the cytosolic compartment of eukaryotic cells [139]. However, exceptions exist, as Cu/Zn-SOD has been identified in certain prokaryotic organisms and within the intermembrane space of mitochondria, albeit in considerably lower concentrations than its iron- and manganese-containing counterparts. In prokaryotes, it is frequently observed as periplasmic SOD, indicating its potential function as a defensive mechanism for pathogenic bacteria against oxidative stress generated by mammalian macrophages.