# Diversity of Collagen Proteins and Their Biomedical Applications in Drug Delivery
## 2 Diversity of Collagens
### Mammalian Collagen  
In the mammalian kingdom, collagens are a superfamily of triple helical proteins consisting of 29 different types with biological functions concerning the construction of cell scaffold, the interaction between cells and fibrils, and the immune response, among others. These vertebrate collagens are classified into different supra structures including fibril, FACIT, macrofibril, anchoring fibrils, transmembrane collagen, basement membrane collagen, and short chains collagen according to their functions (Table 2). For instance, types I, II, III, V, XI, XXIV, and XXVII belong to fibrils in which major fibrils are types I, II, and III. Type I collagen is the most common type of collagen, accounting for around ninety percent of the total collagen content [39]. Type II collagen, composed of unique triple helical chains with prominent advantages of flexibility, resilience, and mechanical stress, is mainly distributed in cartilaginous tissues and is generally used as the repair material for orthopedics and joint injury [40]. Type III collagen is in charge of creating the framework that supports the structural integrity and durability of skin, muscles, organs, and arteries. It is also known as reticular collagen. This reticular collagen can aid in wound healing by providing a three-dimensional structure for new tissue growth and promoting the regeneration process [41]. Collagen type III is unique in its ability to enhance skin elasticity and tautness while maintaining the skin's optimal health. Type IV collagen is composed of triple same alpha peptide chains and is the main component of the cell basement membrane [42]. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Organisms** & **Species** & **Tissue Sources** & **Collagen Content (\%)** & **References** \\ \hline Sea eel & _Muraenesox cinereus_ & Swim bladder & 93.7 & [31] \\ Megalonibea & _Megalonibea fusca_ & Swim bladder & 84.8 & [32] \\ Sea cucumber & _Holothuria cinearcascens_ & Body wall & 72.2 & [33] \\ Totoaba & _Totoaba macdonaldi_ & Swim bladder & 65 & [34] \\ Yellowfin tuna & _Thumus albacares_ & Skin & 61.26 & [35] \\ Asian sea bass & _Lates calcarifer_ & Skin & 59.31 & [35] \\ Mackerel & _Scomberomonas niphonius_ & Skin & 58.62 & [36] \\ Seer fish & _Scomberomonas commerson_ & Skin & 58.21 & [35] \\ Grass carp & _Ctenopharynxgodon idella_ & Swim bladder & 38.9 & [37] \\ Sturgeon & _Acipenser baeri_ & cartilage & 28.8 & [38] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Ten cases ranked by high collagen content of type I collagen obtained from different marine organisms. Based on the basic biological principle of congruence between structure and function, the triple helix of collagen is an ancient and conserved structure between animals that exists ubiquitously in the ECMs. Glycine residues are essential for maintaining the structural integrity of the triple helix. The mutation of glycine residue resulted in defective collagen molecules causing faulty fibrils, networks, and other assemblies. Therefore, there are dozens of genetic diseases in humans that are related to glycine mutation (Table 3).