Hydroxyproline

4-Hydroxyproline, or hydroxyproline (C5H9O3N), is an uncommon amino acid, abbreviated as HYP, e.g., in Protein Data Bank.

Structure
Hydroxyproline differs from proline by the presence of a hydroxyl (OH) group attached to the C (gamma) atom.

Other hydroxyprolines also exist in nature, notably 2,3-cis-3,4-trans-3,4-dihydroxyproline which occurs in diatom cell walls, and is postulated to have a role in silica deposition. Hydroxyproline is also found in the walls of oomycetes, fungus-like protists related to diatoms.

Production and function
Hydroxyproline is produced by hydroxylation of the amino acid proline by the enzyme prolyl hydroxylase following protein synthesis (as a post-translational modification).

Hydroxyproline is a major component of the protein collagen. Hydroxyproline and proline play key roles for collagen stability. They permit the sharp twisting of the collagen helix. It helps provide stability to the triple-helical structure of collagen by forming hydrogen bonds. Hydroxyproline is found in few proteins other than collagen. The only other mammalian protein which includes hydroxyproline is elastin. For this reason, hydroxyproline content has been used as an indicator to determine collagen and/or gelatin amount.

Clinical significance
Proline hydroxylation requires ascorbic acid. The most obvious, first effects (gum and hair problems) 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 causing scurvy.

Additional images
Hydroxyprolin Hydroxyproline ヒドロキシプロリン Hydroksyprolina