(2S,4R)-4-Hydroxyproline, or L-hydroxyproline (C5H9O3N), is a common proteinogenic 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, the most notable ones being 2,3-cis-, 3,4-trans-, and 3,4-dihydroxyproline, which occurs in diatom cell wallsNakajima, T. and Volcani, B.E. (1969). 3,4-Dihydroxyproline: a new amino aci… (More on Hydroxyproline)