4-Hydroxyproline, or hydroxyproline, is an amino acid, the chemical structure of which is:

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Hydroxyproline is produced by hydroxylation of the amino acid proline in the body. It is not coded for by DNA, however, and is hydroxylated in situ after protein synthesis. Along with proline, it is one of two cyclic amino acids found in proteins.

Hydroxyproline is a major component of the protein collagen. It helps provide stability to the triple-helical structure of collagen by forming hydrogen bonds. Hydroxyproline is rarely found outside collagen.

Proline hydroxylation needs ascorbic acid. Most effects of absence of ascorbic acid in man come from the resulting defect in hydroxylation of proline residues of collagen , with reduced stability of the collagen molecule