Which residues require hydroxylation for collagen synthesis, a process impaired by vitamin C deficiency?

• A. Glycine
• B. Lysine
• C. Proline
• D. Proline and lysine

Answer: (D)

Hydroxylation of specific amino acids in collagen is essential for its proper folding and stability. The enzymes prolyl hydroxylase and lysyl hydroxylase add hydroxyl groups to certain residues in the procollagen chains, and vitamin C is required to keep those enzymes’ iron in the active form. This modification creates hydroxyproline and hydroxylysine, which are needed for a stable triple-helix structure and for subsequent cross-linking and glycosylation that give collagen its strength. Without this hydroxylation, collagen is weak and fracture-prone, as seen in vitamin C deficiency. Therefore, the residues that must be hydroxylated are proline and lysine.