## 4 Natural Antioxidants and Their Role in Cosmetics
### Peptides, Amino Acids, Enzymes  
Peptides are chemical compounds made of amino acids connected by a peptide bond. Peptides are increasingly used in cosmetic preparations as a biologically active ingredient, especially in combination with copper ions Cu\({}^{2+}\). This combination affects skin reconstruction and increases its regeneration. Low-molecular transport peptides, due to their structural and spatial structure, are able to bind other substances, increase their solubility, bioavailability, and durability. Most often, these are water-soluble tripeptides, and the best-known transporting Cu\({}^{2+}\) ions are the tripeptide Gly-His-Lys (GHK) and Gly-Gly-His (GGH) [130]. GHK components are responsible for regenerative processes and maintaining proper skin density (Cu activates fibroblasts by increasing the production of mRNA and proteins for elastin, collagen, proteoglycans, and glycosaminoglycans). They stimulate the production of proteinases and metalloproteinases, which eliminate damaged collagen. Clinical studies have shown that copper peptides can increase collagen production by as much as 70% and reduce the number of wrinkles by 31.6% and their depth by 23.4% [131]. In addition, copper tripeptides have a number of scientifically proven properties: they support collagen synthesis; promote the production of glycosaminoglycans, which improves skin elasticity and firmness; activate the system responsible for removing damaged collagen and elastin from scar tissue and skin; smooth the skin; reduce wrinkles; lighten discolorations; support wound healing; reduce inflammation caused by free radicals; and protect skin cells against oxidative stress and the negative effects of UV radiation [132]. Glutathione (\(\gamma\)-glutamylglycineinylglycine) (Figure 5) is a water-soluble tripeptide containing cysteine. It is present in most plant organisms, tissues of all mammals, and microorganisms. It occurs in reduced (GSH) and oxidized (GSSG) forms. About 90% of GSH occurs in the cell cytosol, 10% in mitochondria, and a small amount in the endoplasmicreticulum. One of the most important functions of the reduced form of glutathione is the role of a storage and source of cysteine in the body, as well as maintaining the redox balance of the cell and counteracting the effects of oxidative stress. The highest concentration of GSH occurs in the liver [133]. In order to ensure an adequate level of GSH in the body, an appropriately maintained level of reducing equivalents is necessary. The most important reductant is NADPH, which is produced by the pentose pathway in the cell cytosol. The enzyme responsible for maintaining the proper concentration of NADPH is G6 PDH--glucose-6-phosphate dehydrogenase [134]. Glutathione is a powerful antioxidant composed of three amino acids: glutamate, cysteine, and glycine. It plays a crucial role in various biological processes, including skin lightening, through the following mechanisms: the inhibition of tyrosinase activity and shift in melanin production. Glutathione inhibits the enzyme tyrosinase, which is essential for melanin production. By blocking tyrosinase, glutathione reduces melanin synthesis, leading to lighter skin. Glutathione influences melanin types by shifting the production from eumelanin (responsible for dark brown or black pigmentation) to pheomelanin (responsible for yellow-red pigmentation) [135; 136]. The choice between fermentation and enzymatic synthesis for glutathione production depends on factors such as desired purity, production scale, and cost considerations. Advancements in biotechnological methods continue to enhance the efficiency and sustainability of glutathione production for cosmetic applications. Yeasts such as _Saccharomyces cerevisiae_ and _Candida utilis_ are commonly used for glutathione production. Metabolic engineering and fermentation optimization in bacterial systems have been investigated to improve glutathione yields. Enzymatic synthesis involves the direct enzymatic production of glutathione using its precursor amino acids [137; 138; 139].