# Cell Response to Oxidative Stress in Antarctic Filamentous Fungi
## 5 Oxidative Stress and Antioxidant Defence
### Antioxidants and Antioxidant Defence
#### 5.2.2 Non-Enzymatic Antioxidants
The antioxidant properties of carotenoids are attributed to their polyene structure, which enables them to interact effectively with reactive oxygen species, including singlet oxygen [149]. Alterations to this polyene structure, such as modifications in the number of conjugated double bonds or the introduction of oxygen-containing functional groups, can significantly influence the reactivity of carotenoids. Polyphenols represent a significant category of secondary metabolites derived from plants, encompassing approximately 10,000 distinct structures. The increasing interest in polyphenols is attributed to their potential in mitigating oxidative stress and associated diseases. This group is classified into two primary categories: flavonoids and non-flavonoids [150]. Non-flavonoid polyphenols consist of phenolic acids, such as hydroxybenzoic and hydroxycinnamic acids, as well as stilbenes and lignans. Flavonoid polyphenols function as exogenous antioxidants and are characterized by a C6-C3-C6 structural framework, which consists of two aromatic rings linked by a three-carbon bridge. These metabolites play a crucial role in various cellular functions, influencing pigmentation, aroma, flavor, growth, and reproductive processes. The principal dietary flavonoids are categorized into six groups based on the configuration of the C ring: anthocyanidins, flavan-3-ols, flavanols, flavones, isoflav, and flavanones [143]. The process by which these antioxidants exert their effects involves the transfer of a hydrogen atom from a flavonoid to a free radical, resulting in the formation of a phenoxy radical. This phenoxy radical is characterized by its stability, which effectively halts the progression of the chain reaction. Subsequently, the phenoxy radical may engage with another free radical, leading to the production of a quinone. Alternatively, rather than donating a hydrogen atom, the flavonoid can also provide an electron to the free radical, resulting in the generation of a stable cation radical. Melatonin (N-acetyl-5-methoxyryptamine) is a hormone produced intracellularly by the pineal gland in mammals. Its antioxidant properties are realized through both direct and indirect mechanisms [151]. In the direct detoxification of reactive oxygen species, melatonin donates one or more electrons to free radicals (such as OH\(\bullet\)), facilitating their transformation into stable, non-radical forms. This action is particularly effective in curtailing lipid peroxidation and neutralizing singlet oxygen. The indirect antioxidant effects involve the activation of key antioxidant enzymes (GPx, glutathione S-transferase, and CAT, as well as the stimulation of glutathione production. Ubiquinones possess a 1,4-benzoquinone framework, with coenzyme Q10 being the most biologically active variant. This compound, resembling a vitamin, is fat-soluble and exhibits antioxidant properties while playing a crucial role in the electron transport chain involved in ATP synthesis. Its antioxidant mechanism is linked to the safeguarding of unsaturated fatty acids within cellular membranes from oxidative damage caused by peroxides, the neutralization of reactive carbonyl species, and its synergistic interactions with other antioxidants, such as vitamin E [152]. Glutathione is a tripeptide composed of three amino acids: cysteine, glutamine, and glycine. It functions primarily as an intracellular antioxidant, with approximately 90% of its presence in the reduced form and only 10% in the disulfide form. The antioxidant mechanism of glutathione involves its role as a substrate for various enzymes that facilitate the detoxification of reactive metabolites produced by the interaction of ROS with DNA, proteins, and other biomolecules. Additionally, certain enzymes utilize glutathione to neutralize potentially harmful substances, including estrogens and xenobiotics. Furthermore, glutathione serves as an endogenous antioxidant, effectively neutralizing free radicals.