4-Hydroxyproline, or hydroxyproline(C5H9O3N), is an uncommon amino acid, abbreviated as HYP, e.g., in Protein Data Bank.
Hydroxyproline is produced by hydroxylation of the amino acid proline. It is not coded for by DNA, however, and is hydroxylated 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 found in few proteins other than collagen.
Proline hydroxylation requires ascorbic acid. Most effects of absence of ascorbic acid in humans come from the resulting defect in hydroxylation of proline residues of collagen, with reduced stability of the collagen molecule.