| Section2 = | Section3 = }} 4-Hydroxyproline, or hydroxyproline (C5H9O3N), is an uncommon amino acid, abbreviated as HYP, for example, 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… (More on Hydroxyproline)