**3.1 Protein (colagens)**

Collagen belongs to the endogenous protein group, is found in connective tissues of vertebrates (skin, scales, bones, articular cartilage, blood vessels, and tendons), and contains basal components consisting of glycine, proline, alanine, and hydroxyproline. Glycine plays as the helical center in the structure of collagen. Collagen possesses a molecular weight of about 100,000 daltons, crosses intermolecular bonds, and the repeated glycine-proline-hydroxyproline chains [16]. Tiago *et al.* noticed that the scientists identify at least 28 collagen types, but the main of types I (bones, skin, tendons, and organs), II (cartilage), and III (reticular fibers, blood, and skin). The most prevalent invertebrates contain I to IV types collagens [17]. The structural stability of collagen depends mainly on the ratio of glycine and hydroxyproline that is different between animal species, even in a species. Various structural, chemicals, and amino acid content in glycoproteins lead to the difference. The collagen of fish is similar to mammalian on amino acid composition. Collagen having a molecular weight up to 3–10 kDa possesses antioxidant activity higher than other fractions. The antioxidant activity of collagen in fish is higher than that one in animals [18]. Differences in the structure and the molecular weight of acid-soluble collagens extracting from rainbow trout skins that have grown in the sea and freshwater do not occur. In rainbow trout skins, glycine content is the highest, following alanine, proline, and hydroxyproline with the various chains such as α1-, α2-, and β [19]. Denaturation temperature of fish collagen (25–30°C) is low in comparison to mammalian collagen (39–40°C) [20].
