prolyl4hydroxylase

Prolyl 4-hydroxylase (P4H) refers to a family of enzymes that catalyze the stereospecific hydroxylation of proline residues to 4-hydroxyproline in proteins. The best-known members are collagen prolyl 4-hydroxylases (C-P4H), which modify procollagen in the endoplasmic reticulum, and the hypoxia-inducible factor prolyl hydroxylases (PHD1–3), which regulate the cellular response to oxygen by hydroxylating HIF-α subunits.

In collagen-producing cells, C-P4H forms a functional α2β2 tetramer, comprising two catalytic α-subunits (P4HA1, P4HA2, or P4HA3) and two β-subunits (P4HB), a protein disulfide isomerase–like chaperone. The enzyme operates in the endoplasmic reticulum and requires iron(II) as a cofactor, molecular oxygen, 2-oxoglutarate (α-ketoglutarate), and ascorbate (vitamin C) to maintain the catalytic cycle and prevent oxidation of the iron.

Mechanistically, P4H hydroxylates proline residues to 4-hydroxyproline, a modification that stabilizes the collagen triple helix and is essential for proper collagen folding, maturation, and secretion. A related set of enzymes, the HIF prolyl hydroxylases, uses a similar mechanism to regulate the stability of hypoxia-inducible factor (HIF-α): under normoxic conditions they hydroxylate HIF-α, targeting it for proteasomal degradation, whereas under low oxygen their activity declines, allowing HIF-α activity and hypoxic gene expression.

Clinical and physiological relevance includes the dependence of collagen synthesis on vitamin C, with deficiency causing scurvy, and the involvement of P4H and PHD activity in fibrotic disease, wound healing, and responses to hypoxia.