2.3.8—Collagen structure

Collagen is a fibrous protein built from three polypeptide chains that wind together as a triple helix, also called a tropocollagen molecule. Its repeated sequence and layered stabilisation make the structure strong and organised.

• Three-chain assembly: Each polypeptide chain has a helix shape, and three chains are held closely together by hydrogen bonds to form the triple helix.
• Repetitive sequence: Glycine occurs at about every third position. Its small R group allows the three chains to pack closely inside the helix.
• Within the helix: Many hydrogen bonds between the chains stabilise the triple-helix arrangement.
• Between molecules: Parallel triple helices form covalent cross-links between amino-acid R groups, holding collagen molecules together into fibrils.

The sequence supports close packing, close packing enables the three-chain helix, and hydrogen bonds stabilise that helix. Covalent cross-links then connect neighbouring triple helices into larger fibrils, so stability is built at more than one structural level.

Keep the levels distinct: three polypeptide chains form one triple-helix collagen molecule; cross-links between parallel molecules help form fibrils. This card explains collagen structure and stabilisation, while the next card handles the full structure-to-tissue-function account.