Vitamin C deficiency impairs collagen synthesis by preventing hydroxylation of which residues?

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

Answer: (C)

Vitamin C serves as a cofactor for the enzymes that modify collagen after it's synthesized: prolyl hydroxylase and lysyl hydroxylase. These enzymes add hydroxyl groups to specific amino acids in collagen, namely proline and lysine, converting them to hydroxyproline and hydroxylysine. This modification is crucial for the stability of the collagen triple helix and for subsequent cross-linking of collagen fibers. Without sufficient vitamin C, these residues aren’t properly hydroxylated, leading to weak, poorly cross-linked collagen and the connective-tissue symptoms seen in deficiency. Glycine is plentiful in collagen but is not the target of these hydroxylation steps, so it remains unaffected by vitamin C deficiency.