# 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 molecular weight of Cu/Zn-SOD is approximately 32 kDa, consisting of two identical subunits (homodimer), with each subunit incorporating one atom of copper and one atom of zinc [137]. The coordination of these metal ions at the active site is facilitated by the histidine residue at position 61. The inclusion of zinc ions is likely crucial for maintaining the structural integrity of the enzyme. In periplasmicbacterial SOD, both monomeric and dimeric forms are present, with the dimeric forms exhibiting distinct characteristics compared to those found in eukaryotic systems. The presence of Ni-SOD can be attributed to evolutionary adaptations that enable the utilization of nickel as an essential redox metal. This adaptation arose in response to the diminished availability of iron in oceanic environments, a consequence of photosynthetic processes. Within the SOD family, this particular enzyme is distinctive, characterized by its unique amino acid sequence and specific spectroscopic properties. The enzyme is structured as a homohexamer, consisting of six identical subunits, each formed from monomers comprising 120 amino acids, resulting in a molecular mass of approximately 13 kDa, with each subunit containing one atom of nickel. EC-SOD shares homology with intracellular Cu/Zn-SOD and is characterized as a tetrameric glycoprotein that incorporates a Cu/Zn prosthetic group [137]. This enzyme is released into the interstitial spaces of tissues as well as extracellular fluids, including plasma, lymph, and synovial fluid, in both humans and animals. It exhibits a binding affinity for the plasma membrane and heparin-associated components of the extracellular matrix. Notably, EC-SOD has been identified in certain instances within plants and prokaryotic organisms. Its primary role is to catalyze the dismutation of superoxide radicals that are generated extracellularly. Catalase (EC 1.11.6) is an enzyme present in nearly all aerobic organisms and a variety of microaerophilic microorganisms, such as lactic acid bacteria, which thrive in low-oxygen environments [140; 141]. This enzyme serves a crucial protective function for aerobic organisms by decomposing H\({}_{2}\)O\({}_{2}\) generated within cells before it can escape into the surrounding environment. Although CAT may not be critical for every cell type, it significantly enhances the organism's resilience against various oxidants. Historically, it was believed that CAT was primarily located in peroxisomes; however, recent findings indicate its presence in mitochondria, cytosol, and chloroplasts as well. CAT found in peroxisomes originates from the nucleus and requires transport to reach these organelles. Within the peroxisomes, the individual monomers assemble into tetramers, and the heme group is incorporated during this process. CAT is an enzyme characterized by the presence of porphyrin and is composed of four identical subunits, forming a homotetrameric structure. Each subunit has a molecular weight of 60 kDa, resulting in a total molecular mass of 240 kDa for the complete enzyme, and comprises approximately 500 amino acids. This enzyme is recognized for its high catalytic efficiency. It facilitates the breakdown of H\({}_{2}\)O\({}_{2}\) into water and oxygen, while also interacting with hydrogen donors such as methanol, ethanol, and phenol, utilizing one mole of peroxide in the process. The active site of the enzyme contains either manganese or iron ions. There are four types of CATs, categorized based on the characteristics of their active sites:  
* Compound I (Fe+
- Por+ = 0) + H2O [Reaction 4] (ferryl porphyrin) Compound I (Fe+
- Por+ = 0) + H2O2
- Enzyme (Fe+Por) + H2O + 02 [Reaction 5]
* The second category of enzymes, known as catalase-peroxidases, demonstrates a bifunctional nature, possessing both catalase and peroxidase activities. However, their activity levels are significantly lower, by two to three orders of magnitude, compared to conventional catalases.