Trans-4-hydroxy-L-proline

Trans-4-hydroxy-L-proline is a stereochemically specified hydroxyproline, with a hydroxyl substituent at ring position 4 and the configuration designated $(2S,4R)$. Its formula is ${\mathrm{C}}_5{\mathrm{H}}_9\mathrm{N}{\mathrm{O}}_3$. The name specifies more than the presence of an extra oxygen atom: it identifies the position and spatial arrangement of the substitution.^[ref]

In collagen maturation, prolyl 4-hydroxylase forms this residue by modifying selected proline residues within a polypeptide. That post-translational modification contributes to the collagen triple helix.^[ref]

The chemical distinction suggests two different measurements. A mass increase compatible with addition of oxygen supports hydroxylation, while establishing this particular stereoisomer requires further structural evidence. Substitution at another position can preserve the same formula without producing the same compound.

Consequently, an encoded protein sequence and the completed chemical structure of its residues need not be identical descriptions. The enzyme acts on a chain that already exists, so detecting the unmodified sequence alone does not establish that the mature collagen-related modification has occurred.