Hydroxyproline !

Hydroxyproline is a proline with an added OH group inside collagen; without vitamin C it is not formed and collagen falls apart.

Updated 2026-11-01

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Key facts

It has no tRNA of its own: it forms after translation, when prolyl 4-hydroxylase hydroxylates prolines already built into collagen.
The enzyme needs vitamin C (keeps Fe²⁺ reduced), O₂ and α-ketoglutarate.
It nearly always sits at the Y position of Gly–X–Y and stabilises the triple helix at body temperature.
It is almost unique to collagen (and to a lesser degree elastin), so its urinary loss reflects collagen turnover.

The pyrrolidine ring of proline with a hydroxyl on carbon 4 (trans configuration).
The OH orients the ring in a way that favours triple-helix geometry.

Scurvy: without hydroxyproline the chains cannot form a stable triple helix at 37 °C and are degraded: fragile vessels and poor wound healing.
Urinary hydroxyproline was used as a bone-resorption marker (Paget disease, metastases); collagen telopeptides are preferred today.
Dietary hydroxyproline is not reused to build collagen: it is broken down.

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How is collagen hydroxyproline formed?

By hydroxylating prolines already in the chain (prolyl 4-hydroxylase with vitamin C, Fe²⁺, O₂ and α-ketoglutarate); there is no tRNA for it.

Why is hydroxyproline important for collagen?

It stabilises the triple helix; without it collagen denatures at body temperature.

What does urinary hydroxyproline reflect?

Collagen breakdown, mainly from bone; it rises in Paget disease and other states of high bone resorption.

Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: L-Hydroxyproline (CID 5810) (PubChem, US National Library of Medicine, 2026); Collagen biosynthesis and modifying enzymes (Reactome, 2026); Scurvy: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.