Prolyl 4-hydroxylase

In collagen synthesis, prolyl 4-hydroxylase is the enzyme that converts selected peptide-bound proline residues into Trans-4-hydroxy-L-proline. The official name procollagen-proline 4-dioxygenase emphasizes both the protein substrate and oxygen-dependent reaction. It acts on procollagen and other proteins with collagen-like domains.^[ref]

The reaction couples hydroxylation of a proline residue to conversion of 2-Oxoglutarate into succinate and carbon dioxide, consuming molecular oxygen. The enzyme requires ferrous iron and vitamin C. It operates in the lumen of the endoplasmic reticulum and recognizes sequences containing $\mathrm{X}-\mathrm{Pro}-\mathrm{Gly}$, where $\mathrm{X}$ denotes a variable residue and Gly denotes glycine.^[ref]

This is a post-translational modification: the peptide chain exists before its proline side groups are hydroxylated. The modified residues support the collagen triple helix. In a simplified fully coupled assay, one mole of hydroxylated residues corresponds to one mole of succinate formed. Measuring a coproduct can therefore report reaction progress, provided appropriate controls establish that it comes from the coupled enzyme reaction rather than other sources.