Hierarchical Structure of Collagen

Collagen exhibits a complex hierarchical structure that is essential for its functionality. Its primary structure consists of a repeating (Gly-X-Y)n sequence, enriched with glycine, proline, and hydroxyproline, crucial for maintaining the stability of its triple helix conformation. The secondary structure features a left-handed polyproline II (PPII) helix, resulting from the repetitive sequence and restricted rotation around proline and hydroxyproline bonds. In its tertiary structure, collagen forms a triple helix stabilized by hydrogen bonds, hydration networks, and side-chain interactions. These triple helices further assemble into microfibrils that display a quarter-staggered arrangement and an axial D periodicity of approximately 67 nm. Across different tissues, collagen demonstrates distinct structural patterns, such as the highly organized, parallel fibrils found in tendons. These variations in collagen organization and composition highlight its crucial role in maintaining the structural integrity and functional capabilities of connective tissues.