Explain how the formation of different levels of protein structure is responsible for the specific shape and function of a named fibrous protein.

Collagen is a fibrous protein. Its primary structure – the linear sequence of amino acids linked by peptide bonds – determines which side‑chain interactions can occur. In collagen the repeating Gly–X–Y motif allows the polypeptide chains to pack tightly, and the backbone amide and carbonyl groups form hydrogen bonds that stabilise a left‑handed α‑helix. The helices then associate into a triple helix (quaternary structure) through inter‑chain hydrogen bonds and covalent cross‑links, giving collagen a rigid, inextensible filament that provides tensile strength to connective tissues.

Examiner tips: 1) Name a specific fibrous protein. 2) Link primary structure to possible interactions. 3) Explain secondary structure via H‑bonding. 4) Describe quaternary assembly and its mechanical role.

Common mistakes: 1) Confusing secondary with tertiary structure. 2) Forgetting the Gly–X–Y repeat in collagen. 3) Not linking structure to function (strength/flexibility).