Vitamin C is a water-soluble enzyme in the body. It acts as a potent antioxidant, protecting against damage by reactive oxygen species. Vitamin C also has a role in making cross links between amino acids in collagen.

Collagen is unusual in its widespread modification of proline to hydroxyproline and lysine to hydroxylysine. Most of the hydrogen bonds between chains in the triple helix are from amide protons to carbonyl oxygens, but the OH groups of hydroxyproline also seem to participate in stabilizing the structure. Hydroxylysine residues in collagen serve to form attachment sites for polysaccharides.

The hydroxylation reactions in collagen involve vitamin C. A symptom of extreme vitamin C deficiency, called scurvy, is the weakening of collagen fibers caused by the failure to hydroxylate proline and lysine. Consequences are as might be expected: Lesions develop in skin and gums, and blood vessels weaken. The condition quickly improves with administration of vitamin C.