Tropocollagen is the basic structural unit of collagen fibers, formed by the assembly of three polypeptide chains called alpha chains. These alpha chains are composed of repeating amino acid sequences, predominantly glycine, proline, and hydroxyproline, which form a unique triple-helix structure. Each alpha chain adopts an extended left-handed helical conformation, and the three chains intertwine to form a right-handed superhelix, or triple helix, in the tropocollagen molecule.
The specific amino acid sequence and the presence of glycine, proline, and hydroxyproline residues contribute to the stability of the triple helix structure. Glycine, being the smallest amino acid, fits perfectly into the tight spaces within the triple helix, while proline and hydroxyproline contribute to the formation of hydrogen bonds that stabilize the structure.
In the process of collagen fiber formation, tropocollagen molecules are secreted into the extracellular matrix as procollagen, a precursor molecule with additional peptide sequences called propeptides at both ends. Enzymatic reactions remove these propeptides, and the resulting tropocollagen molecules spontaneously self-assemble into collagen fibrils. These fibrils then align and cross-link with one another, forming mature collagen fibers.
Collagen fibers provide strength, flexibility, and resilience to various connective tissues in the body, such as skin, tendons, ligaments, and cartilage. The assembly of tropocollagen molecules into collagen fibers is a crucial step in maintaining the structural integrity and function of these tissues.