# Cell Response to Oxidative Stress in Antarctic Filamentous Fungi
## 5 Oxidative Stress and Antioxidant Defence
### Antioxidants and Antioxidant Defence  
According to [PERSON] [121], antioxidants are \"any substance that delays prevents or removes oxidative damage to a target molecule\". They can also be defined as sufficiently stable molecules capable of accepting or donating an electron to any free radical and eliminating its unpaired state (Figure 6) [122]. Antioxidants cover a wide range of substances produced within cells or supplied by food substrates. Their function includes reducing the potential of ROS to cause damage, or interacting with ROS and preventing the chain reaction of their generation before damage occurs to molecules vital to the cell.  
Antioxidant activity can be found in the cellular defense system at different levels, depending on the strategy used: prevention, scavenging and destroying radicals, repairing damage, and adaptation. They can also be classified as natural and synthetic, enzymatic and non-enzymatic, low molecular and high molecular, intracellular and extracellular, water-soluble and lipid-soluble (Figure 7) [96; 123].  
Figure 6: Antioxidant action against free radicals.  
Enzymatic antioxidants are the first line of defense and are the most effective antioxidants in aerobic cells. They are classified as primary and secondary. Primary enzymes include superoxide dismutase (SOD) and catalase (CAT). Depending on the developmental stage of the organism, they are joined by glutathione peroxidase (GPx). SOD is responsible for the dismutation reaction in which \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\) is converted to H\({}_{2}\)O\({}_{2}\). In subsequent reactions catalyzed by CAT or GPx, H\({}_{2}\)O\({}_{2}\) is converted to water and oxygen. Glutathione reductase and glucose-6-phosphate dehydrogenase are part of the secondary enzymatic defense. Glutathione reductase converts glutathione from its oxidized to its reduced state to recycle glutathione and scavenge more free radicals. Nicotinamide adenine dinucleotide phosphate, a coenzyme involved in anabolic processes, is renewed by glucose-6-phosphate (NADP), resulting in a reducing environment [124]. These two enzymes do not directly eliminate free radicals but rather provide support to the other endogenous antioxidants.  
Third-line antioxidants recognize, degrade, and remove oxidized or damaged proteins, DNA, and lipids to prevent their accumulation, which can be toxic to cells. These include DNA repair enzyme systems (polymerases, glycosyles, and nucleases) and proteolytic enzymes (proteinases, proteases, and peptidases) found in both the cytosol and mitochondria of cells. The third lineage includes direct and indirect repair systems [125,126].  
The fourth line of defense is the adaptive response to oxidative stress. This line uses the signals the cell receives about the formation or action of ROS and transports the appropriate antioxidants to the right place. The adaptive response depends on changes in gene expression or increased translation of a pre-existing mRNA to induce the synthesis of antioxidant enzymes. This leads to increased resistance to the stressors [126].