(2S,4R)-4-Hydroxyproline, or L-hydroxyproline (593), is a common , abbreviated as HYP, e.g., in .
Hydroxyproline is a major component of the . Hydroxyproline and proline play key roles for collagen stability. They permit the sharp twisting of the collagen helix. In the canonical collagen Xaa-Yaa-Gly triad (where Xaa and Yaa are any amino acid), a proline occupying the Yaa position is hydroxylated to give a Xaa-Hyp-Gly sequence. This modification of the proline residue increases the stability of the collagen triple helix. It was initially proposed that the stabilization was due to water molecules forming a hydrogen bonding network linking the prolyl hydroxyl groups and the main-chain carbonyl groups. It was subsequently shown that the increase in stability is primarily through and that hydration of the hydroxyproline residues provides little or no additional stability.
Hydroxyproline is found in few proteins other than collagen. The only other protein that includes hydroxyproline is . For this reason, hydroxyproline content has been used as an indicator to determine and/or amount.