Which protein forms the main structural component of connective tissue extracellular matrix?

• A. Collagen
• B. Elastin
• C. Keratin
• D. Fibrin

Answer: (A)

Collagen forms the main structural component of connective tissue extracellular matrix. It organizes into long, rope-like fibrils that give tissues their tensile strength, anchoring cells and providing a supportive scaffold in skin, bone, tendon, and many other tissues. The collagen molecule is a triple helix built from repeating glycine-proline-hydroxyproline sequences, which allows tight packing and strong fibril formation. Its stability is enhanced by cross-links formed through lysyl oxidase and by posttranslational hydroxylation of proline and lysine residues, a process that requires vitamin C; without enough vitamin C, collagen becomes weak, as seen in scurvy. Elastin, by contrast, grants elasticity to tissues such as arteries and lungs, while keratin is the main structural protein inside epithelial cells (hair, nails, skin) and fibrin is involved in blood clotting, not the connective tissue ECM.